How to calculate rebar for a slab
Take off the edge clearance on both sides, divide each direction by the spacing, round down and add one: that is the number of bars in each direction.
- Bar length: slab length − 2 × clearance. A 20 ft slab with 3 in clearance takes bars 19.5 ft long.
- Bars across: 19.5 ft ÷ 1 ft spacing = 19.5, round down to 19, add 1 = 20 bars each way.
- Total length: 20 × 19.5 + 20 × 19.5 = 780 ft.
- Weight: 780 ft × 0.668 lb/ft for #4 = about 521 lb.[1]
bars each way = floor((span − 2 × clearance) ÷ spacing) + 1
When a run is longer than a stock bar, the bars overlap at a lap splice. Each splice adds one lap length of steel, so enter the lap your plans or code require; the calculator adds it to every long run.
Rebar sizes and weights
US bar sizes are numbered in eighths of an inch: a #4 bar is 4/8 = ½ inch across and weighs 0.668 lb per foot.[1]
| Bar | Diameter (in) | Diameter (mm) | lb per ft | kg per m | lb per 20 ft |
|---|---|---|---|---|---|
| #3 | 0.375 | 9.5 | 0.376 | 0.56 | 7.5 |
| #4 | 0.5 | 12.7 | 0.668 | 0.994 | 13.4 |
| #5 | 0.625 | 15.9 | 1.043 | 1.552 | 20.9 |
| #6 | 0.75 | 19 | 1.502 | 2.235 | 30 |
| #7 | 0.875 | 22.2 | 2.044 | 3.042 | 40.9 |
| #8 | 1 | 25.4 | 2.67 | 3.973 | 53.4 |
Where rebar goes in a slab
Reinforcement goes in the center to upper third of the slab: the residential code (California's R506.3.4, based on the International Residential Code) requires it to be supported there for the whole pour.[2] Chairs or blocks hold the grid at that height.
Rebar for common slabs
A 12 × 12 ft patio at 18 inch spacing takes 8 + 8 bars of 11.5 ft; a 24 × 24 ft garage at 16 inches takes 18 + 18 runs of 23.5 ft, each needing a splice if your stock bars are 20 ft.
| Slab (ft) | Spacing | Runs each way | Total length | #3 weight | #4 weight |
|---|---|---|---|---|---|
| 12 × 12 | 18 in | 8 × 11.5 ft, 8 × 11.5 ft | 184 ft | 69 lb | 123 lb |
| 12 × 12 | 12 in | 12 × 11.5 ft, 12 × 11.5 ft | 276 ft | 104 lb | 184 lb |
| 10 × 20 | 12 in | 20 × 9.5 ft, 10 × 19.5 ft | 385 ft | 145 lb | 257 lb |
| 20 × 20 | 12 in | 20 × 19.5 ft, 20 × 19.5 ft | 780 ft | 293 lb | 521 lb |
| 24 × 24 | 16 in | 18 × 23.5 ft, 18 × 23.5 ft | 846 ft | 318 lb | 565 lb |
| 12 × 40 | 18 in | 27 × 11.5 ft, 8 × 39.5 ft | 627 ft | 236 lb | 419 lb |
Common rebar estimating mistakes
The most common mistake is dividing the slab size by the spacing and forgetting the extra bar at the end of each row.
- No lap allowance: runs longer than a stock bar need overlapping bars.
- Short offcuts: a 19.5 ft run cut from a 20 ft bar leaves 6 inches of scrap. The calculator counts whole stock bars, cutting short pieces several to a bar where they fit.
- Footings and walls: count those runs separately; the concrete for them is in the concrete calculator.
Concrete for the same slabUse the concrete calculator for cubic yards and bags, and the concrete slab cost calculator to price it.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
At 12 inch spacing with 3 inches of edge clearance: 20 bars each way, each 19.5 ft long, 780 ft in all. With 20 ft stock bars that is 40 bars, about 521 lb of #4.
How much does a 20 ft piece of #4 rebar weigh?
About 13.4 lb: 20 ft × 0.668 lb/ft.[1]
How do I calculate how many rebar pieces I need?
For each direction: (slab dimension − 2 × edge clearance) ÷ spacing, round down, add one. Add the two directions together.
What size is #4 rebar?
½ inch nominal diameter (4 eighths of an inch), about 12.7 mm, weighing 0.668 lb per foot.[1]