Rebar Calculator
Enter the slab size, bar size and grid spacing from your plans. You'll get the bars each way, how many sticks to buy, the total length and weight, and the crossings to tie.
- Tested formulas
- Feet or metric
- Printable shopping list
2 What to buy
Updates as you type| Bars running the length | 8 bars × 19.5 ft |
|---|---|
| Bars running the width | 14 bars × 11.5 ft |
| Spacing | 18" each way, held 3" back from the edges |
| Crossings to tie | 112 |
The residential code requires slab reinforcement to be supported so it stays between the center and the upper third of the slab while the concrete is placed. Chairs or other supports under the grid hold it there.
Shopping list
Quick answer
A 20 × 12 ft slab with #4 rebar at 18 inches each way needs 22 bars: 8 running the length and 14 across, 317 ft in all. That's 22 sticks of 20 ft rebar (every bar here needs its own stick), weighing about 212 lb.
Rebar for common slab sizes
| Slab | 12" grid | 16" grid | 18" grid | 24" grid |
|---|---|---|---|---|
| 10 × 10 ft | 10 (190 ft) | 8 (152 ft) | 8 (133 ft) | 6 (95 ft) |
| 12 × 12 ft | 24 (276 ft) | 18 (207 ft) | 16 (184 ft) | 12 (138 ft) |
| 10 × 20 ft | 20 (385 ft) | 16 (299 ft) | 14 (270 ft) | 10 (193 ft) |
| 12 × 20 ft | 32 (464 ft) | 24 (348 ft) | 22 (317 ft) | 16 (232 ft) |
| 20 × 20 ft | 40 (780 ft) | 30 (585 ft) | 28 (546 ft) | 20 (390 ft) |
| 24 × 24 ft | 58 (1,128 ft) | 44 (846 ft) | 38 (752 ft) | 30 (564 ft) |
#4 bar, 3" from the edges, 20 ft sticks. Sticks to buy, with total feet in brackets. Bars in the 24 × 24 ft slab need splices, and the lap overlap isn't included. Use the calculator above with your lap length for exact numbers.
How the rebar grid is worked out
A slab grid has bars running both ways at the same spacing. For each direction:
- Take off the edge distance at both ends. With bars held 3 inches back, a 20 ft slab leaves 19.5 ft of bar.
- Count the bars that fit across the other way: divide the distance across (edges taken off) by the spacing, round down, then add one for the first bar. The last bar can land a little farther in than the edge distance, because the spacing rarely divides evenly.
- Count the sticks (the 20 ft lengths rebar is sold in). If two bars fit in one stick, you cut two from it; otherwise each bar uses its own stick. A bar longer than a stick is made from overlapping pieces tied together, called a lap splice, and the overlap adds length.
For a 20 × 12 ft slab at 18 inches: 11.5 ft across ÷ 1.5 ft = 7.67. Round down to 7 full spaces and add 1 for the first bar: 8 bars of 19.5 ft run the length. Along the 19.5 ft length, 19.5 ÷ 1.5 = 13 spaces, so 14 bars of 11.5 ft run the width. That's 317 ft of #4 bar, about 212 lb at 0.668 lb per foot. A 19.5 ft bar takes a whole 20 ft stick. Each 11.5 ft bar leaves an 8.5 ft offcut, too short for another one, so every bar here needs its own stick: 22 sticks, 440 ft bought to place 317 ft.
Rebar sizes and weights
A bar's number is its diameter in eighths of an inch. Weights are the nominal values in ASTM A615, the standard for US rebar.
| Bar | Diameter | Weight per foot | 20 ft stick |
|---|---|---|---|
| #3 | ⅜" | 0.376 lb | 7.5 lb |
| #4 | ½" | 0.668 lb | 13.4 lb |
| #5 | ⅝" | 1.043 lb | 20.9 lb |
| #6 | ¾" | 1.502 lb | 30 lb |
Where the rebar sits in the slab
The International Residential Code requires reinforcement in a slab on ground to be supported so it stays between the center and the upper third of the slab during the pour. Rebar left on the ground ends up at the bottom of the slab, below where the code wants it. Set the grid on chairs and tie the crossings so it doesn't shift while the concrete goes in.
Spacing, bar size and edge distance come from your plans or local code. This calculator counts what you've chosen; it doesn't design the slab. For the concrete itself, use the concrete slab calculator.
Frequently asked questions
How much does a 20 ft stick of #4 rebar weigh?
About 13.4 lb. #4 bar weighs 0.668 lb per foot (ASTM A615), so 20 ft is 13.36 lb.
How do I calculate rebar for a slab?
For each direction, take the edge distance off both sides, divide by the spacing, round down, then add one to get the number of bars. Each bar is the slab length minus the edge distance at both ends. Add both directions for the total length.
How high should rebar be in a slab?
The International Residential Code requires slab reinforcement to be supported so it stays between the center and the upper third of the slab during placement. Use chairs or other supports.
What size rebar is #4?
Half an inch. Bar numbers are the diameter in eighths of an inch, so #3 is ⅜ inch, #4 is ½ inch and #5 is ⅝ inch.
Last reviewed 2026-10-26. See how we calculate for our sources and assumptions.