Rebar Calculator
A 20x20 ft concrete slab with #4 rebar on 12-inch centers requires 39 bars each way (78 total), or about 1,560 linear feet of rebar weighing approximately 1,043 lbs. At $0.60-$0.85/ft, materials cost $936-$1,326.
Rebar Calculator
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How to Calculate Rebar
Rebar is laid in a grid pattern inside concrete. Calculate bars in each direction separately: divide the slab dimension by the spacing, add 1, and multiply by the perpendicular dimension for bar length.
| Rebar Size | Diameter | Weight per Foot | Common Use |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lbs | Light slabs, patios |
| #4 | 1/2 in | 0.668 lbs | Driveways, standard slabs |
| #5 | 5/8 in | 1.043 lbs | Footings, walls |
| #6 | 3/4 in | 1.502 lbs | Foundation walls |
| #7 | 7/8 in | 2.044 lbs | Structural beams |
| #8 | 1 in | 2.670 lbs | Heavy commercial |
| Application | Rebar Size | Spacing | Slab Thickness |
|---|---|---|---|
| Patio/walkway | #3 | 18 in | 4 in |
| Driveway | #4 | 12 in | 4-5 in |
| Garage floor | #4 | 12 in | 4-6 in |
| Foundation footing | #4-#5 | 12 in | 8-12 in |
| Retaining wall | #4-#5 | 12-16 in | 6-8 in |
Tips
- Rebar should sit at 1/3 from the bottom of the slab; use chairs or supports to position it
- Standard rebar comes in 20-foot lengths; plan cuts to minimize waste
- Overlap splices by 40 bar diameters (#4 = 20 inches minimum overlap)
- Add 2-3 inches of cover from the rebar to the concrete surface edge
- Tie intersections with 16-gauge wire every other crossing minimum
Frequently Asked Questions
How much rebar do I need for a concrete slab?
For a standard 4-inch slab with #4 rebar on 12-inch centers: divide each dimension by 12 inches, add 1 for the edge bar, then multiply bars by the slab dimension for total linear feet. A 20x20 ft slab needs about 1,560 linear feet of rebar.
What size rebar should I use for a driveway?
#4 rebar (1/2 inch diameter) on 12-inch centers is the standard for residential driveways. For heavy vehicle loads or poor soil conditions, upgrade to #5 rebar or reduce spacing to 8-10 inches.
How much does rebar cost per foot?
Standard #4 rebar costs $0.60-$0.85 per linear foot at home improvement stores. Buying in bulk (full 20-ft bars) is cheaper than cut pieces. #3 costs $0.40-$0.55/ft and #5 costs $0.85-$1.20/ft.
Do I need rebar in a 4-inch slab?
It depends on use. A 4-inch patio slab on stable soil can use wire mesh instead. But driveways, garage floors, and any slab with vehicle loads should have #4 rebar on 12-inch centers for crack resistance.
How do I calculate rebar overlap?
Rebar splices should overlap by at least 40 bar diameters. For #4 rebar (0.5 in diameter): 40 x 0.5 = 20 inches minimum overlap. For #5 (0.625 in): 40 x 0.625 = 25 inches. Always tie overlaps securely with wire.
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