For larger slabs, cut control joints into the surface to accommodate shrinkage and changes in temperature. When overlapping rebar, the length of the overlap should be at least 40 bar diameters. 9 gage wires spaced not less than 6 inches (152 mm) or one 1 / 4 inch (6. (f) Curtailment of bars in slabs Curtailment of bars is done according to the moment envelope. Reinforcing concrete slabs with rebar is an excellent way to prevent slab failure, as long as the slab is not over 4 inches thick. 668 pounds; Applications: Residential foundations, thin concrete structures, and masonry walls. If you prefer to calculate the number of bags of concrete needed, you can use the weight of the bags as a reference. All 3/4 inch diameter anchors must be a minimum of 6 inches away from any expansion seams, control joints or other inconsistencies in the concrete. 1. No, you do not need rebar for a 4-inch slab of concrete on grade. As per the present market rates, the average cost of concrete for a 400-square foot slab, the typical size for a two-car garage, ranges between $583 and $637 or between $1. Thickness of concrete slab depends on loads and size of the slab. For example, a size 4 rebar will have a diameter of 4/8” or ½ inches. #4 Rebar ; The #4 rebar is often used in residential and commercial projects to add strength in slabs and columns. Concrete Slab Rebar Spacing Guideline. Non-reinforced slabs – where no. 1 (a) (5) of ACI 318-19, nominal maximum size of coarse aggregate shall not exceed the least of (i), (ii), and (iii): (i) one-fifth the narrowest dimension between sides of forms. Rule of thumb is to have the minimum clear spacing between bars be 1. into an existing footer with a min. Provide Construction Joints (ie, transverse joint) @ 40'-0" intervals if you have a long driveway. The exact spacing for the expansion joints is dependent on the size and design of the slab, and should be determined by a professional engineer familiar. Use unwashed material that has a top size of 11/4 inches and includes smaller sizes right down to the fines; the irregular shapes and sizes interlock nicely when compacted. 5 inch line of your patio slab to keep the dowels in the upper portion of the slab. ( W) Width of the slab. Do You Need Rebar For A 4 Inch Driveway Slab? A 4 inch concrete driveway slab is too thin for rebar. 8 wire transversely). How to lay a concrete slab. So if you have, say a 3/4" max. What should be the size rebar for a 4-inch slab?. For example, if you’re calculating rebar for a slab, input the slab dimensions, reinforcement spacing, and desired rebar size. 11. 1. In slabs with an average thickness of less than 6 inches, the spacing should be at. This will provide adequate reinforcement for most applications. For a patio you can use a grid spacing of 24 inches. 26 inches (18" controls) #3 bar = 15. Concrete volume calculator plan view diagram. To calculate Total Area of a multi sided slab, first divide the shape into rectangles or squares. In that case, a 4-inch slab of higher strength concrete could typically support upwards of 15,000 PSF. In order to calculate this, you need to input the length and width of your slab. New concrete slabs must conform to the above stated properties before installation of. As per the present market rates, the average cost of concrete for a 400-square foot slab, the typical size for a two-car garage, ranges between $583 and $637 or between $1. Most homeowners can expect to pay between $4. 2-1/4 inch rebar high chair is designed to hold rebar in place during concrete applications. Most reinforcing bars bearing a Metric diameter mark Now, divide each measurement by the 14-inch spacing. Many other styles of supports are available and effective. New concrete slabs must conform to the above. This illustrates a common problem for the concre t e s u b c o n t ra c t o r : How do you place a l e v el slab of uniform thickness ove r steel beams that can va r y in eleva-MINIMUM VERTICAL REINFORCEMENT AND SPACING (INCHES) b, c: Soil classes and lateral soil load d (psf per foot below grade) GW, GP, SW and SP soils 30: GM, GC, SM, SM-SC and ML soils 45: SC, ML-CL and inorganic CL soils 60: 6 feet 8 inches: 4 feet (or less) #4 at 48 #4 at 48 #4 at 48: 5 feet #4 at 48 #4 at 48 #4 at 48: 6 feet 8 inches #4 at. For example, for a one-way solid slab with #5. The same slab without mesh can hold up to 25,400 lbs. Using the calculator, you would find that you need approximately 0. If the slab is expected to bear heavier loads, such as from. But if you feel that the slab is in a sensitive position and extra protection is needed, you can add rebar to it. For beams and walls, the spacing should be closer and is usually 3-4 inches apart. ACI 360R-10 (ACI 360), as referenced by ACI 318, as referenced by IBC, includes Figure 6. Vertical placement tolerance for reinforcement in slabs-on-ground is ± 3/4 inch from the specified location. Running Bar Points (Length - Left to Right): Running Bar Points (Width - Top to Bottom): Calculate Rebar Quantity and Mesh From Total Slab Area and Perimeter. The maximum weight of fly ash, other pozzolans, silica fume, slag or blended cements that is included in concrete mixtures for. The diameter of bar generally used in slabs are: 6 mm, 8 mm, 10 mm, 12 mm and 16 mm. thick slab, the maximum joint spacing should be. Provide 21" minimum rebar splice laps. Slabs for hot tubs generally are 8 feet by 8 feet. But to get two layers in an 8" slab-on-grade where the bottom cover is 3" would might violate the bar to bar spacing between layers. The size of the rebar depends on the specific application, but typically #4, #5, #6, and #8 rebar are used. (UBC), may require the use of rebar in 4-inch concrete slabs. inches thick and about 4 x 4 inches square. The ACI 318-19 provides a number of requirements regarding slab thickness, concrete cover, and reinforcement ratio which facilitate the design process. For flat slabs like driveways, a grid spacing of 18 inches is recommended, with the edge rebar placed at the same distance from each side. It's not a problem to get two layers in on an interior 8" elevated slab where the bottom cover is 3/4". 5 times the slab thickness in inches. Rebar is recommended on concrete measuring 5 – 6 inches thick. I was thinking of the following: basement wall 8 inches thick 10 feet high backfilled up to 7 to 8 feet. The main reinforcement of the slab panel “A” is marked as (1). (This convention begins to break down for bar diameters above #8. With rebar this thick, a four-inch slab will be more durable and last longer. All 3/4 inch diameter anchors must be a minimum of 6 inches away from any expansion seams, control joints or other inconsistencies in the concrete. 5% reinforcing steel in the top 2 inches of the slab in both directions can allow you to eliminate joints. Highway bridgework may require more stringent. For a 4-inch slab, #3 rebar should be spaced 12 inches apart on center. See STRIPS. Finally, reinforcing mesh can replace bars if needed, but it must be spaced according to manufacturer’s guidelines. The installation of the rebar is critical for the success of the overall project. Construction joints also have this spacing for strip placement of the concrete. Rebar helps prevent cracks from growing wider largely by preventing cracked slabs from moving apart. Generally, lighter-duty applications require a rebar spacing of 8-12 inches (20-30 cm), while heavier-duty applications require a closer spacing of 6-8 inches (15-20 cm). To calculate the amount of rebar per square foot, you will need to convert the 6 foot length to a linear foot, and the 50 pounds to pounds per linear foot. 15 per cent of the gross sectional area. 4" is plenty, be sure the base is good - compacted, free draining, etc use a relatively tight mix, ie low water to cement ration, slump in the 3 to 3. 2: Repeat and continue to draw complete (closed) shape. Find the number of concrete bags in the most popular sizes in the chart below: Bag Size. The spacing and quantity of the rebar will depend on factors such as the load the slab needs to support and local building codes. 0. #6 bars and larger – 2 inches #5 bars or smaller – 1½ inches. Use 2” rebar chairs to support the rebar in the center strata of the pad. construction projects. 4 bar at 48 inches (1220 mm) on center horizontally grouted in place. 6 feet is equal to 72 inches, which is divided by 12 to get the 6. approximately the bar diameter in eighths of an inch [whole millimeters], used to designate the reinforcing bar size. Do you require gravel under concrete? The minimum spacing of rebar in slabs varies depending on the application, with typical spacing distances between 5 and 6 inches on center. I used no. Place the holes at the 2. transverse wire spacing with smooth No. The evaluation of rebar spacing results shows that the maximum standard deviation for rebar spacing is 0. 19 rebars for the slab. How to calculate the rebar quantity from concrete? Convert your longitude estimation into inches: 15 x 12 = 180. Again, without mesh this drops to 34,300 lbs. In other words, for a 4-inch slab, space the joints 8 to 12 feet apart. of Main bars = (3000)/150 + 1. We always recommend that you have a system in place to strengthen the concrete, though, to ensure the best results. To do this, concrete pros prop up the rebar. So if you have, say a 3/4" max. For a 4" slab on grade house, what would you say about #3 vs #4 rebar, and the spacing? I was going to do #4 at 12" - Answered by a verified Structural Engineer. Why Rebar Spacing is Crucial. Anyhow, the rebar chairs are relatively cheap. the calculator will give you the default spacing values (for rebar – rebar it’s 40cm, and for edge – grid it’s 8cm). A few rules of thumb for control joints: Space them 2 to 3 times in feet the thickness of the slab in inches (4" slab = 8' to 12' max); Joints should create nearly square sections; joints should be struck or sawn to a depth of 1/4 the slab thickness; Sawn joints need to be cut before shrinkage cracks occur, no more than 12 hours. Its rebar size in inches is 0. Tie two bands of 1/2-inch steel reinforcing rods (rebar) to stakes set about four inches from the perimeter of the forms. Calculate the rebar size, spacing, and number: Use the structure’s specifications to determine the rebar size needed for the concrete. If you have a 5 inch slab and multiply it by 24 to 32 you will find that your joints should be placed every 120 to 180 inches or 10 to 15 feet; Or another way to figure this out is to times it by 2 or 3 in feet. 12% for HYSD bars and 0. 10” or so around the edge and 4” everywhere else. Whatever you do, please don’t pour a gigantic slab of concrete with no. 5 inches of thickness at the edge of the pool. The spacing of the rebar is typically 12” on center for most residential applications. Place concrete at a minimum thickness of 4 inches. With 3-inch screws, attach the two-by-fours to the stakes: A 12-foot two-by-four on each short side. Lay out and dig the footings. 8" - 3/4" (top cover) - 2" (4-#4 bars) - 3" (bottom. Proper and even spacing is also important. Horizontal rebar starting at 12 inches and at 18 inch intervals after to the top. There are three different sizes of rebar typically used for home construction projects. area = 25′ × 25′. 4. 24-foot joint spacing. In general, 6 inch (150mm) slab thickness is considered for residential and commercial buildings with reinforcement details as per design. I just got off the phone with a local contractor after picking his brain a bit and he typically will use a #4 bar @ 48" o/c. The main reinforcement of the slab panel “A” is marked as (1). Slabs for septic tanks may require the use. No, you do not need rebar for a 4-inch slab of concrete on grade. Generally, rebar has an immense impact on concrete. 23 inches For a 4" thick slab with GR60 bars I get the following max. What size rebar for 4 inch slab. If you’re making a slab 4 inches tall, use Grade 40 or Grade 50 rebar. You don’t need an 8mm Rebar on your 4” thick slab. 125" clear between bars. In order to calculate this, you need to input the length and width of your slab. For aboveground forms, space stakes 16 in. 1. 4. It has a foot print of four tires about 10 inches by 10 inches time 4. For an unreinforced slab, control joints should be spaced in feet 2 to 3 times the slab thickness in inches. There should be a minimum of 3 inches of clear cover from the fi nished surface of the slab. 78. Driveway : The total cost of a concrete driveway will depend on size. For corrosion protection, however, most specifications require a minimum depth of concrete cover that usually ranges from 1 inch to 2 inches. The typical depth for rebar placement for slabs is 2-4 inches. 6 rebar spaced 4 inches on center is P = 8. 15 percent of total cross-sectional area. Maximum spacing of main bar is restricted to 3 times effective depth or 300 mm whichever is. aggregate, you'd need 1. For a 4 inches slab, rebar is not necessary. Add rebar for reinforcement, Ralston places rebar every 16” on center. As well as that, the calculator will also tell you. Rebar is used t o help improve concrete’s resistance to loads andThe maximum diameter of bar used in slab should not exceed 1/8 of the total thickness of slab. 1. For exam-ple, for a one-way solid slab, with #5 temperature-shrinkage bars, high chairs are used at 4 feet on center; for #4 bars, high chairs would be placed 3 feet on center to prevent sagging. For garage slabs, two rows of #4 rebar are recommended around the perimeter, one above the other. 60 per square foot. Sawcut Joints. 561 kg/m. The size of the rebar depends on the specific application, but typically #4, #5, #6, and #8 rebar are used. Determine the weight of the patio cover, gazebo, or other decorative structures you plan to install. From 3” to 12 1/4” in height, each chair has two cover heights. 2 only if the steel reinforcing is greater than 0. You'll also need vertical rebar hooks protruding at least 6” into the footing and out at least 18” every 48”, or less. Joint distance (D) or tank diameter in feet based on Steel Schedule Minimum end area (As) of steel (in2) for slab thickness, t= 5 inch *Anchor bolts are located a minimum of 1 3 / 4 inches (45 mm) from the edge of the concrete parallel to the length of the wood sill plate. 43/30 = 0. Concrete needs rebar to be as strong as possible; however, if poured thin enough and used only for walking, you can quickly go without rebar. basement foundations). For example, 4 inches patio slab will need rebar. MATERIALS The materials used to construct the dowel bar shall meet the following requirements: 4. 4" is plenty, be sure the base is good - compacted, free draining, etc use a relatively tight mix, ie low water to cement ration, slump in the 3 to 3. The spacing of the rebar should increase to between 18 and 24 inches apart for slabs between four and six inches thick. 13. (i) The maximum spacing between two parallel main reinforcing bars shall be 3dor 300mm or whichever is less, and (ii) The maximum spacing between two secondary parallel. For concrete not presented to climate or in contact with the ground: chunks, dividers, and joists – #14 and #18 bars – ½ inch. The compacted aggregate base is first, followed by a 6 mil polyethylene vapor barrier, then insulation, then rebar or wire mesh, or both. 6. For beams and walls, the spacing should be closer and is usually 3-4 inches apart. Set the forms at the proper height before bringing in the last lift. Specify the spacings: rebar-rebar spacing = 40 cm, edge-grid spacing = 8 cm. I'd go #4 rebar at 18-inch max spacing. Do you need rebar for 4 inch slab? No, you do not need rebar for a 4 – inch slab of concrete on grade. For flat slabs, grid spacing of 18 inches for driveways and 24 inches. Add one more length to allow for that extra, giving you a total of 18. This doesn't really eliminate cracks, but results in very. The Rebar calculator for slab is best to find the Rebar size. For slabs of septic tanks, you usually use #3 and #4 bars. 525 mm. Per ACI 332. The 6mm rebar should be arranged in a grid pattern, spacing out the rebar at 6 to 12 inches. 34 inch (8. 33 ft). The spacing of the rebar is also determined by the weight and size of the slab. Similarly, rebar with a size 15 will be 15/8. To use our rebar calculator, simply input the dimensions and specifications of your project. My driveway is 4 inch thick concrete. Designed to be extremely strong yet able to straddle a lower rebar mat, these chairs have a wide, stable base with a footprint that. 561 kg/m. Concrete doesn't dry out; it undergoes a chemical curing process that creates a dry, solid mass. 2. 6, the designer can exceed the CJ spacing in Table 10. A 4-inch-thick slab cast on the ground and in permanent contact with it will float and rebar is not required. the calculator will give you the default spacing values (for rebar – rebar it’s 40cm, and for edge – grid it’s 8cm). For example, the percentage of reinforcement of a 30-inch slab with two layers of No. Rebar Chair is a patented polypropylene support system ideal for concrete applications requiring a 6 in. 87 (round up to 13) Add one rebar to your result: 13 + 1 = 14. In addition, the spacing of the bars will also vary by the project. 4 x Modulus of Rupture fs = allowable working stress of the reinforcement (psi) T =range of temperature the slab is expected to be subjected to (F o) Stretch a string between two screws installed at the edge of both ends of the form boards. Provide minimum horizontal joint reinforcement of two No. When you build a 4-inch slab, you’ll need a higher grade of rebar. Slabs, walls/dividers, and joists – 11 bars and littler – ¾ inch. According to 26. User note: About this chapter: Chapter 4 provides requirements for constructing footings and walls for foundations of wood, masonry, concrete and precast concrete. Add one more length to allow for that extra, giving you a total of 18. Therefore, you will need approximately 2. Leave a few inches of space between the mesh and the edges of the forms to prevent rust. It’s crucial to ensure that welded wire mesh be placed on chairs of the proper height. For a 4-inch slab, you’ll need a higher grade of rebar. The appropriate rebar size depends on the project, with #3 and #4 being common choices. 3. 2)(12 in. 5 inches up to 12 inches. To keep the rebar at least 2 inches from all sides you need at least 4 3/4 inches of concrete. 5 inches of spacing (18" + 1/2" diameter of the rebar). Placement of Rebars. Use a metal-cutting blade or disc in a reciprocating saw, circular saw or grinder to cut the rebar. Generally speaking, a 4-inch slab of normal weight concrete (145-165 pounds per cubic foot) with a 28-day compressive strength of at least. It will be different from the concrete right next to it. 16000 lbs divided by 144 is 111 pounds per square inch. and type of reinforcement bars are crucial to the success of any project. October 9, 2021. A 4-inch-thick slab cast on the ground and in permanent contact with it will float and rebar is not required . According to this method, you will need two rebars for a footing width of 16 inches, four rebars for a footing width of 24 inches, and so on. Legs on other types of. When spacing the rebar, it is important to ensure it is evenly distributed and in accordance with the local building codes. lengths of rebar to just below. I was thinking of going with the same spacing. 1 Type A - The dowel structure shall comprise a single metallic material meeting the requirements of AASHTO M 255M/M 255, AASHTO M 334, ASTM A276, ASTM A312, or ASTM A1035 (CS, CM and CL). Make multiple passes if needed to create a flat, evenly filled area. 1. 3 of the Eurocode 2. With our rebar calculator, you can eliminate. Step-4: Rebar Calculation Process Of The Slab Panel-A 4. 440 3. 4 inch of p r otection. The rebar needs to be correctly. 4. 1 in2. For example, if a 6-inch-thick slab is used with #3 rebar at 18 inches on center, the percentage of steel for a 12-inch width would be: For enough reinforcing to accomplish. diameter. And a cross-sectional area of 0. 5 bars or littler – 1½ inches. Commonly called “rebar,” these 1/2-inch-diameter steel rods are embedded in the center of the slab to create a grid that extends lengthwise and crosswise to help the concrete better resist pulling and bending forces. 2. 0 inches use 4 inch Schedule 40 PVC duct and internal joining coupling for 4 inch I. If the slab is more than 24 inches thick, #5 (#5/8 inch diameter) rebar may be required. Great for use when pouring concrete slabs. For example, a slab that is 6 inches thick might have rebar marked as size 6 or 3/4-inch. Additionally, any slab poured over 4 inches should also be bonded with mesh fabric to the 2 preceding layers of clear of construction joint material to ensure the slab is adequately supported. I believe the rebar should be 1/4 to 1/3 of the slab thickness from the bottom. slabs are actually the 3-1/2-in. Advertise Here. spacing reinforcing bars is through. Cracking is really due to a number of causes. 25" 3,000 PSI / 28 Day Aging #6 Rebar: 12 in: D2-10 Models: 4. Step-4: Rebar Calculation Process Of The Slab Panel-A 4. For example, a 4 inch slab of 3000 psi concrete, 4 feet by 8 feet, with reinforcing mesh will hold up to 34,800 lbs. 7: Garage footers must be 24 inches deep, size of framing for footer is 6 inches deep X 16 inches wide, with 2 runs of rebar size 1/2 inch or 5/8 inch. Instead, to comply with ACI minimum reinforcing ratio and bar spacing requirements, slabs from 4 to 7 inches thick should have at least #4 bars at 12 inches on centers; closer spacing or larger bar sizes are needed for thicker slabs. Joint spacing in the topping should be coordinated with joint spacing in the base slab. Here: Multiply the measurements together to calculate the square footage of the area. For a flat slab, such as a driveway, consider placing the rebar in an 18 inch grid spacing, keeping the edge rebar at the same distance from each side. Raise the form sides 1/2-inch to create a slab that is a full 4 inches thick. 65*6) = 1. What size rebar do I need for a 4 inch slab? You will need a #3 rebar for a 4-inch slab. No, you do not need rebar for a 4-inch slab of concrete on grade. The general rule of thumb is that control joints should be place no more than 3X the slab thickness (in inches). 014/4 = 0. An 8-inch slab or thicker is required for CJ’s spaced up to 20 feet. To form consistent, parallel sides for a curved sidewalk, build one side first. Ask Your Own Structural Engineering Question. Unique fastening system ensures accurate positioning of rebar, to code, during concrete placement. Having installed thousands of slab on grade foundations over some 26 years as an ironworker, I can say that if it was my driveway, I would probably do 5 inch thick concrete with #4 rebar at 18 to 24 inches each way at center of slab in other words 2" chairs or dobies at every intersection and deepened perimeter footings. Do you need rebar for 6 inch slab? Rebar is recommended for. Using the rebar tying tool, twist wires around the intersections of the bars. Concrete has great compression strength, but it can crack or br eak under. 9 3 in. Workmanship and jobsite conditions. 6 rebar spaced 4 inches on center is P = 8. Excavate the perimeter 1 foot deep and 1 foot wide to form a deeper trench around the excavated area. of Main bars = (Width of Slab/Spacing) + 1. For footings that are poured before the slab, two #4 horizontal pieces of rebar are required – one 3” from the top and the other 3” from the bottom. Minimum sub-base (stone) under slab shall be 4 inches for both Table 1 and Table 2. 5-inch cover to the beam stirrups, dagg = 3⁄4 inch, #3 stirrups used with #4, #5, and #6 longitudinal bars, and #4 stirrups used for #7 and larger longitudinal bars. Horizontal rebar starting at 12 inches and at 18 inch intervals after to the top. 25" 3,000 PSI / 28 Day Aging #6 Rebar: 12 in: D2-10 Models: 4. 5% of the slab cross-sectional area. anchor bolts and the 4 inches (102 mm) φ pipes. For the short side, 240 inches divided by 14 inches gives you 17 lengths of 20-foot rebar plus a bit left over. Minimum sub-base (stone) under slab shall be 4 inches for both Table 1 and Table 2. To find the Bar Bending Schedule for Slab we use the following formula, No. However, if the slab is subject to heavy load or if it’s used as a structural support, you may need to go larger, such as #6 (3/4-inch diameter) or even #7 (7/8-inch diameter). Every concrete slab should have a minimum. 3: Minimum 8mmdia rebar is required for slab construction. Standard spacing for a re-bar grill typically ranges from 4” to 6” for most residential projects. 59 per square foot. Similarly, a 4 inch slab of 4,000 psi concrete, 4 feet by 8 feet, with reinforcing mesh can hold up to 45,000 lbs. If the slab is going to support extra weight, such as the weight of a vehicle or the weight of the soil for a retaining wall, then stronger rebar such as #5 (5/8-inch diameter) or #6 (3/4-inch diameter) might be. 34 inch (8. ) = 0. Rebar Spacing. This type of rebar is typically 6mm in diameter and can span up to 24. Tying should include bars near the ends of the bridge and bars near the curbs. 26 inches (18" controls) #3 bar = 15. I poured a full 4. We would like to show you a description here but the site won’t allow us. Say your forklift has a contact area of 1 square foot. Grade of steel;Size and spacing of the steel reinforcing; and. The grade shall be as specified by the. All thick slabs can be categorized into:. MAX and #5 top placed at center of slab" I installed the #3 dowels 12" apart and 6" into the existing surrounding floor. For example, if the engineer calls for rebar spaced every 4 inches, you need to place three bars for every 12 inches of the form. A six inch slab can handle some thin spots - a 4" slab can't. 4 wire mesh in a 4-inch slab, P = 8. Gravel or stone provides a layer for the installation of underslab conduit and pipes, allows for water and radon to escape, and helps keep the slab thickness uniform,. Generally, rebar spacing in slab footings ranges from 12 to 24 inches, both for horizontal and vertical bars. For example in a slab of 6-inch depth, a rebar of 6/8 inch will be used. 1%. View More. For example, if both slabs have rebar mats that are spaced at 18 inches and use No. Anchor bolts are located a minimum of 15 anchor diameters from the edge of the concrete perpendicular to the length of the wood sill plate. A confused student is a good student. Add 5%-10% additional material for some waste or spills. Does a 4-inch slab need rebar? By: Olin Wade (Remodel or Move Stuff) Watch This Video To Learn More About Using or Not Using Rebar For Concrete Driveway Watch on It. Tie two bands of 1/2-inch steel reinforcing rods (rebar) to stakes set about four inches from the perimeter of the forms. What should be the size rebar for a 4-inch slab? For a 4-inch slab, the thickness of the rebar should be ⅛ of the slab. For example: A #5 [#16] bar is approximately 5/8 inch [16 millimeters] in diameter: a #9 [#29] bar is approximately 9/8 inches [29. If you are in a climate subjecting the floor to freezing, get air added to the concrete. For example, if the area is 10 feet long and 6 feet wide, the square footage is 60 feet. #5 bars are 5/8”. A single rebar mat is only going to hold the slab together when it cracks. Calculates approximate Rebar Length from total slab area and perimeter. Here:The size of rebar needed for a 4-inch slab depends on several factors, such as the type of material used for the slab, the type of project, and the size and weight of the load it will be required to carry. Special surface finishes, including coatings. 4/1.