How to use the rebar calculator
For a slab, enter its length and width, the spacing between bars and how far the grid stops short of the edges. The calculator counts the bars in each direction, works out how many stock bars that takes once long runs are spliced, and gives the weight. For a footing or wall, switch to Straight runs and enter the length and how many bars run side by side.
The plan drawing shows the grid at your spacing, so you can check it against your drawings before ordering.
The formulas
bars one way = ⌊(width − 2 × cover) ÷ spacing⌋ + 1
each bar = length − 2 × cover
lap = lap diameters × bar diameter
stock bars per run = ⌈(run − lap) ÷ (stock length − lap)⌉
weight = feet in place × unit weight
Worked example: a 30 × 20 ft slab, #4 at 24 in, 3 in cover
- Long bars: (20 − 0.5) ÷ 2 = 9.75, so 9 spaces and 10 bars, each 29.5 ft long.
- Short bars: (30 − 0.5) ÷ 2 = 14.75, so 15 bars, each 19.5 ft long.
- Laps: 40 × 0.5 in = 20 in. A 29.5 ft run needs two 20 ft bars with one lap; a 19.5 ft run fits in one bar.
- Stock bars: 10 × 2 + 15 × 1 = 35 bars.
- Steel in place: 10 × (29.5 + 1.67) + 15 × 19.5 = 604 ft, which at 0.668 lb/ft is about 404 lb.
Bar sizes and weights
| Bar | Diameter | Weight per ft | 20 ft bar | 40d lap |
|---|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.5 lb | 15 in |
| #4 | 0.500 in | 0.668 lb | 13.4 lb | 20 in |
| #5 | 0.625 in | 1.043 lb | 20.9 lb | 25 in |
| #6 | 0.750 in | 1.502 lb | 30.0 lb | 30 in |
The bar number is its diameter in eighths of an inch: #4 is 4/8 = ½ in.
Placing rebar properly
- Hold it up. Rebar lying on the ground does nothing. Set it on chairs or concrete dobies so it sits in the middle of a slab, or about 3 in above the soil in a footing.
- Tie the crossings. Tie every second or third intersection with wire so the grid doesn't shift when concrete is placed.
- Stagger the laps. Don't put all the splices in one line across the slab.
- Bend at corners. In footings, use bent corner bars so the steel is continuous around the corner.
Common mistakes
- Forgetting laps. On a long slab, laps add 5 to 10% to the steel.
- Too little cover. Bars near the surface rust, expand and crack the concrete.
- Buying exact counts. Bars get bent, cut wrong and borrowed. A spare bar or two is cheap insurance.
Questions people ask
How much rebar do I need for a slab?
Count the bars in each direction: (slab width − 2 × edge cover) ÷ spacing, plus one, for the bars running the long way, and the same for the other way. A 30 × 20 ft slab at 24 in spacing with 3 in cover takes 10 long bars and 15 short ones, about 604 ft of #4 including laps.
What size rebar for a concrete slab?
#3 (⅜ in) or #4 (½ in) bars at 12 to 24 inches are common in residential slabs and driveways; footings commonly use #4 or #5. Use what your plans or local code specify.
How much does rebar weigh per foot?
Per ASTM A615: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft and #6 is 1.502 lb/ft. A 20 ft #4 bar weighs about 13.4 lb.
How much should rebar overlap?
A common estimating rule is 40 bar diameters (20 in for #4), and some contractors use 48. The real lap length comes from ACI 318 and depends on concrete strength, bar size and cover, so follow your engineer's drawings.
How far from the edge should rebar be?
Concrete cover protects the steel from rust. Slabs on grade usually keep bars about 2 to 3 inches in from the edges and supported in the middle of the slab's thickness; footings cast against earth need 3 inches.
Sources and assumptions
- CRSI: reinforcing bar identification, sizes and unit weights (ASTM A615)
- ACI 318 governs lap splice and development lengths; 40 bar diameters is an estimating rule of thumb only.
- Offcuts are not reused between runs, so the stock-bar count is slightly on the safe side.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.