Rebar Calculator
Bars each way = (slab width − 2 × cover) ÷ spacing, rounded down, + 1. A 20 × 20 ft slab with #4 bars at 12 in and 3 in cover takes 780 ft (521 lb): 41 bars of 20 ft with 5 % extra. Size, spacing, cover and laps come from the plans: this tool counts the steel, it does not design the slab.
Rebar to order, #4 bars
41 bars of 20 ft
780 ft of bar + 5 % waste, cut from 20 ft bars
Estimate only. Check dimensions, loads and local code requirements with a qualified professional before you build.
- Bars one way
- 20 × 19.5 ft
- Bars the other way
- 20 × 19.5 ft
- Total length of bar
- 780 ft
- Steel weight
- 521 lb
Quantities only: bar size and spacing come from the plans or a professional.
Lap 0: each run is counted as one length, with no overlap.
Checked by the Yardstick Math team · How we check
Worked examples
A 20 × 20 ft slab, #4 at 12 in
- 20 ft − 2 × 3 in = 19.5 ft = 234 in
- 234 ÷ 12 = 19.5, rounded down 19, + 1 = 20 bars of 19.5 ft each way
- 40 × 19.5 = 780 ft
- 780 × 0.668 = 521 lb
- 780 × 1.05 ÷ 20 = 40.95, so 41 bars of 20 ft
Result: 780 linear ft of #4 bar, 521 lb: 41 bars of 20 ft with 5 % extra.
Checked by the automated test “rebar — 20 × 20 ft slab, #4 at 12 in, 3 in cover → 20 + 20 bars of 19.5 ft = 780 ft, 521 lb”.
A 12 × 20 ft slab at 16 in
- Across the 12 ft: 11.5 ft = 138 in; 138 ÷ 16 = 8.6, so 8 + 1 = 9 bars of 19.5 ft
- Across the 20 ft: 234 ÷ 16 = 14.6, so 14 + 1 = 15 bars of 11.5 ft
- 15 × 11.5 + 9 × 19.5 = 172.5 + 175.5 = 348 ft
- 348 × 0.668 = 232 lb
Result: 348 linear ft of #4 bar, 232 lb.
Checked by the automated test “rebar — 12 × 20 ft slab at 16 in → 15 bars of 11.5 ft + 9 of 19.5 ft = 348 ft, 232 lb”.
How it works
Bars one way = (slab width − 2 × cover) ÷ spacing, rounded down, + 1; each bar is (slab length − 2 × cover) long
Bars the other way: same with length and width swapped
Linear feet = Σ bars × bar length (+ one lap per splice when a lap is typed)
Weight = linear feet × unit weight of the bar size (SDDOT, NCDOT)
Bars to buy = linear feet × (1 + extra %) ÷ stock bar length, rounded up
Assumptions
- Unit weights are the SDDOT and NCDOT tables: #3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft.
- One layer of bars in a square grid at the same spacing both ways, starting one cover from each edge.
- Lap 0 (default): each run is counted as one length, with no overlap. A run longer than the stock bar needs the lap from the plans (SDDOT: “check plans for overlap”).
- Pieces are cut from stock bars and offcuts are not reused: the extra % covers the cut waste.
- Quantities only: no bar size, spacing, cover or slab thickness is chosen here.
Reference table
| Bar size (metric) | Weight, lb/ft (kg/m) | Diameter, in (mm) | Area, in² (mm²) |
|---|---|---|---|
| #3 (#10) | 0.376 (0.560) | 0.375 (9.5) | 0.11 (71) |
| #4 (#13) | 0.668 (0.994) | 0.500 (12.7) | 0.20 (129) |
| #5 (#16) | 1.043 (1.552) | 0.625 (15.9) | 0.31 (199) |
| #6 (#19) | 1.502 (2.235) | 0.750 (19.1) | 0.44 (284) |
In this table the bar number is the diameter in eighths of an inch: #4 = 4/8 in = 0.500 in. NCDOT Fig. 1 gives the same weights.
Before you buy
- Read the plans. Bar size, spacing, cover and lap length are on the drawings or come from a professional.
- Long slabs. A run longer than the bar you buy needs a lap: type it in bar diameters to count the overlaps.
- Supports and ties. Chairs, tie wire and extra bars at openings are not counted here.
Common mistakes
- Forgetting the cover at both edges The bars stop one cover short of each edge: a 20 ft slab with 3 in cover takes 19.5 ft bars, not 20 ft.
- Leaving out the starting bar 234 in ÷ 12 in gives 19 spaces but 20 bars: the +1 is the first bar at the edge.
- Mixing lb/ft and kg/m A #4 bar weighs 0.668 lb/ft, which is 0.994 kg/m: multiply feet by lb/ft, or meters by kg/m, never the other pair.
- Buying linear feet instead of bars Rebar is sold in stock lengths: 780 ft is 39 bars of 20 ft with no cut waste at all, 41 with 5 % extra.
- Forgetting the laps on long runs A 24 × 24 ft slab at 18 in has 23.5 ft runs, longer than a 20 ft bar. If the plans call for a 40-diameter lap, #4 takes 805.3 ft, not 752. SDDOT: “check plans for overlap”.
Frequently asked questions
How much rebar do I need?
Count the bars each way from the plan's spacing, then add their lengths: a 20 × 20 ft slab with #4 bars at 12 in and 3 in cover takes 40 bars of 19.5 ft, 780 ft in all.
How much does rebar weigh?
Per foot, #3 weighs 0.376 lb, #4 0.668 lb, #5 1.043 lb and #6 1.502 lb (SDDOT, NCDOT). A 20 ft #4 bar weighs 13.36 lb.
How much rebar do I need for a slab?
A 10 × 10 ft slab with #4 bars at 12 in takes 10 bars of 9.5 ft each way: 190 ft, 127 lb. Bar size and spacing come from the plans or a professional.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- Structures Construction Manual, Chapter 7: Reinforcing Steel — South Dakota Department of Transportation, accessed
- Structure Design Manual, Fig. 1: Areas, Weights, and Spacing of Bars — North Carolina Department of Transportation, accessed