How Much Rebar for a 12x12 Slab?
Rebar to buy
16 sticks
144 sq ft, #4 bar at 18 in both ways, 3 in cover, 20 ft sticks, 40 diameter lap splices.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Slab length12 ft
- Slab width12 ft
- Bar spacing, both ways18 in
- Bar size#4, 1/2 in
- Cover from the edge3 in
- Bar stock length20 ft
- Lap splice40 bar diameters
Rebar to buy16 sticks
| Slab area | 144 sq ft |
|---|---|
| Bars running the length | 811.5 ft each, spaced across the width |
| Bars running the width | 811.5 ft each, spaced along the length |
| Linear feet, no laps | 184 ft |
| Lap allowance | 0 ft0 splices at 1.67 ft each |
| Total linear feet | 184 ft |
| Sticks at 20 ft | 16320 ft bought, one run per stick |
| Weight | 123 lb0.06 tons |
| Intersections to tie | 64 |
| Chairs | 36one per 4 sq ft |
The material list this produces
| #4 rebar, 20 ft sticks | 16 |
|---|---|
| Tie wire, lb | 1 |
| Chairs | 36 |
| Poly vapor barrier, sq ft | 159 |
Slab length compared
Length swept with the width held at 12 ft. Watch the first column rather than the two on the right: it climbs gently from 10 to 11 and then jumps to 16, because that is exactly where the run stops fitting two to a stick.
| Slab length | Rebar to buy | Linear feet, no laps | Weight |
|---|---|---|---|
| 8 by 12 ft | 10 sticks | 129 ft | 86 lb |
| 10 by 12 ft | 11 sticks | 156.5 ft | 105 lb |
| 12 by 12 ft | 16 sticks | 184 ft | 123 lb |
| 16 by 12 ft | 19 sticks | 250.5 ft | 167 lb |
| 20 by 12 ft | 22 sticks | 317 ft | 212 lb |
| 24 by 12 ft | 26 sticks | 372 ft | 251 lb |
What else matters here
Sixteen sticks for 184 ft of steel, and the 10 by 12 slab sitting next to it in the table takes 11 sticks for 156.5 ft. A sixth less slab, five fewer sticks. That step is not a rounding artifact and it is worth understanding before the order goes in.
The reason is the stick. Bars on a 12 ft slab come out at 11.5 ft after 3 in of cover at each end, and two 11.5 ft runs do not fit in a 20 ft stick. So every run on this slab takes a stick of its own and leaves an 8.5 ft offcut that is too short to be a second run. On a 10 ft slab the runs are 9.5 ft and two of them do fit, which is why five more sticks buy only 27.5 more feet of steel.
The table below sweeps the length with the width held at 12 ft, and the cliff is visible in it: 8 by 12 is 10 sticks, 10 by 12 is 11, and 12 by 12 jumps to 16. If this is a shed pad and the size is still open, landing on the short side of that step costs nothing. If it has to be 12 ft, buy the 16 sticks and expect a pile of 8.5 ft offcuts at the end of the day.
What this calculator assumes
- Bars are figured as a grid running both ways at the same spacing, which is how a residential slab, a driveway and a footing mat are normally drawn.
- Weights are the published ones: #3 is 0.376 lb per foot, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044 and #8 is 2.670.
- The stick count is nested, not divided. When a run fits inside a stock bar, the count says how many runs are cut from one stick and never splices a bar that did not need splicing. When a run is longer than a stock bar it is built from a chain of sticks with a lap at every joint, and the offcut from one run starts the next. That is why the stick line and the linear feet line never agree, and the gap between them is the offcut you pay for.
- A 40 diameter lap is the common residential rule of thumb. ACI 318 sets the real development length from the concrete strength, the bar position and the cover, and an engineered drawing overrides anything on this page.
- Chairs are one per four square feet, which keeps the mat off the ground on an ordinary pour. Wet setting bar into poured concrete is a different method and this list does not price it.
Questions people ask
How much rebar for a 12x12 slab?
8 bars each way at 18 in with #4 bar, 184 ft of steel, 16 sticks of 20 ft bar and 123 lb.
Why does a 12x12 slab need so many more sticks than a 10x12?
Because an 11.5 ft run cannot be cut two to a 20 ft stick and a 9.5 ft run can. The 12 ft slab takes 16 sticks against 11 for only 27.5 more feet of steel.
How many chairs does a 12x12 slab need?
36, at one per four square feet, plus 64 intersections to tie and about a pound of tie wire.
The rebar calculator takes any dimensions you like and hands back the same list.