A 10 × 10 ft slab with #4 bar every 12 in, kept 3 in from the edges, takes 10 sticks of 20 ft rebar, about 127 lb of steel. Change the size, spacing, bar or lap length (the overlap where two bars join) for your own slab, or switch to wire mesh to count sheets.
Quick answers
Rebar weighs 0.376 lb per ft for #3, 0.668 lb for #4 and 1.043 lb for #5. The number is the diameter in eighths of an inch: #4 is 1/2 in (Engineering ToolBox).
Spacing, bar size and lap length for structural slabs come from your plans or local code. This calculator counts material; it doesn't size reinforcement.
How the rebar count works
Take the edge clearance off both ends: a 10 ft side with 3 in clearance leaves a 9.5 ft run.
Bars across a run = run ÷ spacing, rounded down, plus one. At 12 in spacing, 9.5 ft gives 10 bars.
Each bar is as long as the run the other way. If that's longer than one stick, bars overlap by the lap length.
Sticks to buy = bars × sticks per run. Weight = linear feet × weight per foot for the bar size.
Example: a 10 × 10 ft slab, #4 bar at 12 in, 3 in clearance. 10 bars each way, each 9.5 ft long, is 190 linear feet. Each 20 ft stick gives two 9.5 ft bars, so you need 10 sticks and about 127 lb of steel.
Rebar for common slab sizes
#4 bar, 20 ft sticks, 3 in edge clearance, 20 in laps where needed. When two bars fit in one stick, the count assumes you cut both from it.
Slab
Sticks at 12 in
Linear ft at 12 in
Sticks at 18 in
Linear ft at 18 in
10 × 10 ft
10
190
8
133
10 × 12 ft
16
229
11
157
12 × 12 ft
24
276
16
184
12 × 20 ft
32
464
22
317
12 × 24 ft
48
578
32
385
20 × 20 ft
40
780
28
546
24 × 24 ft
96
1,208
64
805
Planning the whole slab? The concrete slab planner puts concrete, gravel base, rebar or mesh and forms on one shopping list.
Common questions
How do I calculate how much rebar I need for a slab?
Subtract the edge clearance from both ends of each side. Divide the remaining width by the spacing and add one to get the bars that run the long way; do the same with the length for the bars that run the short way. Multiply each set by its bar length and add any lap splices. Divide by the stick length to get sticks.
How far should rebar be from the edge of the slab?
Bars need concrete cover on every side so they don't rust. The cover your project needs is set by your plans or local code; the calculator uses 3 inches from the edges by default, which you can change. Use chairs or dobies to hold the grid up in the slab rather than letting it rest on the ground.
What is a lap splice?
When a run is longer than one stick, two bars overlap and are tied together. The overlap is the lap length. Your plans or local code set it; the calculator lets you enter it and adds the extra length.
Rebar or wire mesh?
Both are used in home slabs, and the calculator counts either one. Which one your slab needs comes from your plans, or from your local building department if the slab is structural.
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