Calculate rebar quantities, spacing, weight, and coverage for slabs, footings, and walls.
Rebar is designated by a number that represents its diameter in eighths of an inch. The most common sizes for residential and light commercial work are #3 through #8. Here is what each size means: #3 rebar is 3/8 inch in diameter and weighs 0.376 pounds per foot. #4 rebar is 1/2 inch in diameter and weighs 0.668 pounds per foot. This is the most commonly used size for residential slabs and footings. #5 rebar is 5/8 inch in diameter and weighs 1.043 pounds per foot, often used in thicker slabs, walls, and structural footings. #6 rebar is 3/4 inch in diameter at 1.502 pounds per foot, typically found in commercial foundations. #7 rebar is 7/8 inch in diameter at 2.044 pounds per foot, and #8 rebar is 1 inch in diameter at 2.670 pounds per foot, used in heavy structural applications. Standard rebar lengths are 20 feet, though 40-foot and 60-foot lengths are available for commercial projects. For most residential work, you will be ordering #4 or #5 rebar in 20-foot sticks.
The American Concrete Institute standard ACI 318 governs rebar spacing in structural concrete. The maximum spacing for slab reinforcement is the lesser of 3 times the slab thickness or 18 inches. For a standard 4-inch residential slab, maximum spacing is 12 inches on center (3 x 4 = 12). For a 6-inch slab, the maximum is 18 inches (3 x 6 = 18, equal to the 18-inch cap). Most contractors default to 12 inches on center in both directions for residential slabs because it provides a good balance of strength and material cost. Closer spacing like 8 or 10 inches on center is used when slabs carry heavier loads, span longer distances, or sit on weak or expansive soils. For walls, vertical rebar is typically spaced at 12 to 16 inches on center, and horizontal bars at 12 to 18 inches on center, depending on the wall height and loading conditions.
There are three common types of concrete reinforcement, each suited to different applications. Rebar is the strongest option, providing structural reinforcement that holds cracked concrete together and resists tensile forces. It is required for any structural application, driveway slabs, garage floors, and slabs over 5 inches thick. Welded wire mesh (typically 6x6 W1.4/W1.4) is lighter-duty reinforcement made from thin steel wire welded into a grid. It is acceptable for sidewalks, patios, and thin slabs carrying only foot traffic. The major disadvantage of wire mesh is that it tends to sink to the bottom of the pour during placement, where it provides little benefit. Unless it is properly supported on chairs throughout the pour, it often ends up in the wrong position. Fiber reinforcement consists of synthetic or steel fibers mixed directly into the concrete. Fiber reduces plastic shrinkage cracking during curing but does not provide structural reinforcement. It is a complement to rebar, not a replacement. Many contractors use fiber-reinforced concrete with rebar for the best results.
Rebar must be placed in the tension zone of the concrete, which is the bottom third of a slab. For a 4-inch slab, the rebar should sit approximately 1 to 1.5 inches from the bottom surface, supported on rebar chairs or concrete dobies spaced every 3 to 4 feet. ACI 318 requires a minimum concrete cover of 3 inches for concrete cast directly against earth, 2 inches for formed surfaces exposed to weather, and 1.5 inches for slabs on grade with a vapor barrier. Getting the placement right is critical. Rebar that sits on the ground provides almost no reinforcement. Rebar that is too high in the slab can cause surface cracking. Use the correct height chairs for your slab thickness and check placement before the pour begins.
When rebar must be extended beyond a single bar length, the bars are overlapped and tied together at a lap splice. The standard lap splice length is 40 times the bar diameter. For #4 rebar (1/2 inch diameter), the minimum lap splice is 20 inches. For #5 rebar (5/8 inch), it is 25 inches. For #6 rebar (3/4 inch), the lap splice must be at least 30 inches. Lap splices should be staggered so that not all splices occur at the same location in the slab, which creates a weak point. A good rule of thumb is to offset adjacent splices by at least 24 inches. At intersections and corners, bars should extend past the intersection by at least 12 inches.
Not every concrete project requires rebar reinforcement. Small non-structural slabs like garden paths, stepping pads, and small patios under 100 square feet that carry only foot traffic can be poured without rebar. Concrete countertops typically use wire mesh or fiber rather than rebar due to their thin profile. Fence post holes filled with concrete generally do not need rebar unless required by local code for gate posts. However, any concrete that serves a structural purpose, supports vehicles, or is thicker than 4 inches should include rebar reinforcement. When in doubt, check with your local building department or a structural engineer, because adding rebar to a pour is inexpensive insurance compared to replacing a cracked slab later.
Example: 20 ft x 24 ft garage slab, #4 rebar at 12" on center, both directions
Long direction bars: (20 x 12 / 12) + 1 = 21 bars x 24 ft = 504 linear ft
Short direction bars: (24 x 12 / 12) + 1 = 25 bars x 20 ft = 500 linear ft
Total linear feet: 504 + 500 = 1,004 linear feet
20-ft sticks needed: 1,004 / 20 = 51 sticks of #4 rebar
Total weight: 1,004 x 0.668 = 671 pounds
For most residential slabs 4 inches thick, #3 rebar (3/8 inch diameter) is sufficient. For driveways and garage floors at 5 to 6 inches thick, #4 rebar (1/2 inch diameter) is the standard choice. Slabs thicker than 6 inches or those supporting heavy equipment typically require #5 rebar (5/8 inch diameter). Always check your local building code or consult a structural engineer for load-bearing applications.
The most common spacing is 12 inches on center in both directions. ACI 318 limits maximum spacing to 3 times the slab thickness or 18 inches, whichever is smaller. For a 4-inch slab, maximum spacing is 12 inches. For a 6-inch slab, the maximum is 18 inches. Closer spacing like 8 or 10 inches on center is used for heavier loads or problematic soil conditions.
Weight depends on bar size. #3 rebar weighs 0.376 pounds per foot, #4 weighs 0.668 pounds per foot, #5 weighs 1.043 pounds per foot, #6 weighs 1.502 pounds per foot, #7 weighs 2.044 pounds per foot, and #8 weighs 2.670 pounds per foot. A single 20-foot stick of #4 rebar weighs about 13.4 pounds, while a stick of #5 weighs about 20.9 pounds.
Rebar is stronger and more reliable for most applications. Wire mesh (6x6 W1.4/W1.4) is acceptable for thin sidewalks and patios carrying only foot traffic. Rebar is required for driveways, garage floors, structural slabs, and anything over 5 inches thick. The main problem with wire mesh is that it tends to sink to the bottom of the pour if not properly supported, which makes it ineffective as reinforcement.
Rebar belongs in the bottom third of the slab, supported on rebar chairs or concrete dobies. For a 4-inch slab, the rebar should sit about 1 to 1.5 inches from the bottom. This positions the steel in the tension zone where concrete is most likely to crack. Never lay rebar directly on the ground. Use properly sized chairs spaced every 3 to 4 feet to maintain consistent height throughout the pour.