Rebar Spacing Calculator
Enter the slab size and choose a bar spacing of 12, 16, 18 or 24 inches. You get the total feet of rebar for a grid in both directions, along with the concrete.
Last updated November 13, 2026
How rebar spacing changes the amount of steel
Rebar in a slab is laid as a grid, with bars running both ways. The spacing is the distance from the center of one bar to the center of the next. Closer spacing means more bars and more steel. The calculator counts the bars in each direction, including the bar at each edge, and multiplies by their length.
Worked example. A 10 × 10 ft slab at 12 inch spacing has 11 bars each way. 11 × 10 ft + 11 × 10 ft = 220 ft of rebar. At 24 inch spacing it has 6 bars each way, or 120 ft.
Feet of rebar by slab size and spacing
| Slab (ft) | 12 in spacing | 16 in spacing | 18 in spacing | 24 in spacing |
|---|---|---|---|---|
| 10 × 10 | 220 ft | 160 ft | 140 ft | 120 ft |
| 20 × 20 | 840 ft | 640 ft | 560 ft | 440 ft |
| 20 × 30 | 1,250 ft | 940 ft | 840 ft | 650 ft |
These are straight lengths with no allowance for laps or cover. Bars are sold in fixed lengths, so where two bars meet they overlap, and that adds steel. Add about 10% for laps and cuts, or more if your plans call for long laps.
Converting feet to bars and weight
Divide the total feet by the bar length you are buying, commonly 20 ft, and round up. For the 10 × 10 slab at 12 inches, 220 ft is 11 bars. #4 rebar weighs 0.668 lb per foot, so 220 ft weighs about 147 lb. Other bar sizes are in the rebar weight chart.
Which spacing should I use?
The calculator does not choose the spacing for you. Bar size and spacing depend on what the slab carries and on your local building code, and for anything structural they are set by an engineer or the approved plans. Use this page to count the steel once you know the spacing. For welded wire mesh instead of bars, use the wire mesh calculator.
Frequently asked questions
How do I calculate rebar for a slab?
Count the bars in each direction: slab width ÷ spacing, plus one, then multiply by the bar length. Do the same the other way and add the two. A 10 × 10 ft slab at 12 inch spacing needs 220 ft.
How much rebar do I need for a 20×20 slab?
840 ft at 12 inch spacing, 640 ft at 16 inches, 560 ft at 18 inches or 440 ft at 24 inches, before laps and waste.
Does rebar spacing include the edge bars?
In this calculator, yes. It counts a bar at each edge. In practice the outer bars sit a few inches inside the slab edge for concrete cover.