This rebar calculator estimates the bars, total length and weight needed for a flat grid of reinforcing steel across a concrete slab from its size, bar spacing and bar size.
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At 12 inch spacing with #4 bars you need 11 bars each way, 220 ft in total. With a 15% allowance that is 253 ft, or 13 bars of 20 ft, weighing about 169 lb.
Rebar to buy, including laps and waste
30 bars
Inputs
Results are estimates for planning only. Supplier specifications, product yields, site conditions and local codes vary. Confirm quantities with your supplier or a qualified professional before you buy or build.
slab length 20 ft, slab width 12 ft, bar spacing (centre to centre) 12 in, #4 (13 mm), bar length as sold 20 ft.
Rebar to buy, including laps and waste
30 bars
slab length 6 m, slab width 4 m, bar spacing (centre to centre) 30 cm, #4 (13 mm), bar length as sold 6 m.
Rebar to buy, including laps and waste
33 bars
slab length 10 ft, slab width 10 ft, bar spacing (centre to centre) 16 in, #3 (10 mm), bar length as sold 20 ft.
Rebar to buy, including laps and waste
10 bars
slab length 24 ft, slab width 24 ft, bar spacing (centre to centre) 16 in, #4 (13 mm), bar length as sold 20 ft.
Rebar to buy, including laps and waste
53 bars
| Size | Diameter | Weight per foot | Weight per metre |
|---|---|---|---|
| #3 | 3/8 in (10 mm) | 0.376 lb | 0.56 kg |
| #4 | 1/2 in (13 mm) | 0.668 lb | 0.99 kg |
| #5 | 5/8 in (16 mm) | 1.043 lb | 1.55 kg |
| #6 | 3/4 in (19 mm) | 1.502 lb | 2.24 kg |
#4 bars, 20 ft stock length, 15% laps and waste.
| Slab size | Total bar length | Bars (20 ft) | Weight |
|---|---|---|---|
| 10 × 10 ft | 253 ft | 13 | 169 lb |
| 12 × 12 ft | 359 ft | 18 | 240 lb |
| 16 × 16 ft | 626 ft | 32 | 418 lb |
| 20 × 20 ft | 966 ft | 49 | 645 lb |
| 24 × 24 ft | 1,380 ft | 69 | 922 lb |
A slab grid has bars running both ways. Bars running along the length are spaced across the width, and bars running across the width are spaced along the length, so a slab needs bars in two directions. Each direction needs one more bar than the number of gaps, because there is a bar at both edges.
The spacing, bar size and number of layers come from your plans, an engineer or local code. This tool turns those numbers into a shopping list. Pair it with the concrete calculator for the slab volume and the gravel calculator for the base.
Rebar only works if it sits in the right place in the slab. It is held off the ground on plastic or wire chairs so concrete flows beneath it, and the bars are tied together where they cross. Your plans specify the cover, which is the concrete between the bar and the surface. Buy chairs and tie wire with the bars, and ask your supplier to size them.
At 12 inch spacing with #4 bars, that is 11 bars each way, 220 feet in total. With a 15% allowance for laps it is about 253 feet, or 13 twenty-foot bars, weighing roughly 169 pounds.
It depends on the slab's design, thickness and load. Spacing of 12 to 24 inches is common in residential work. Follow your plans or ask an engineer or your local building department.
A #3 bar weighs 0.376 lb per foot, #4 weighs 0.668, #5 weighs 1.043 and #6 weighs 1.502. In metric, 0.56, 0.99, 1.55 and 2.24 kg per metre.
Not always. Some residential slabs use welded wire mesh or fibre, and some light-duty slabs use neither. Requirements depend on the soil, load and local code.
At 16 inch spacing with #4 bars there are 19 bars each way, 912 feet in total. With a 15% allowance that is about 1,049 feet, or 53 twenty-foot bars, weighing roughly 700 pounds.
Often, for light-duty slabs. Mesh is sold in sheets or rolls by area, so you can use the slab area from the concrete calculator. Follow your plans for which to use.
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Get the embed codeResults are estimates for planning only. Supplier specifications, product yields, site conditions and local codes vary. Confirm quantities with your supplier or a qualified professional before you buy or build.