How much rebar do I need for a 12×12 slab?
A 12 × 12 ft slab with #4 rebar on a 16 in grid takes 19 stock bars of 20 ft, which is 207 linear feet weighing about 138 lb. Tying the grid takes 81 ties and 1 roll of tie wire, and holding it at mid-slab takes 48 chairs. At 12 in spacing for a driveway it is 26 bars; at 24 in for a light patio, 13.
Rebar for a 12×12 slab by grid spacing
| Grid spacing | 20 ft stock bars (5% extra) | Linear feet | Weight | Ties | Chairs |
|---|---|---|---|---|---|
| 12 in | 26 | 276 ft | 184 lb | 144 | 48 |
| 16 in | 19 | 207 ft | 138 lb | 81 | 48 |
| 18 in | 17 | 184 ft | 123 lb | 64 | 48 |
| 24 in | 13 | 138 ft | 92 lb | 36 | 48 |
#4 (1/2 in) rebar with 3 in edge clearance in a 12 × 12 ft slab. Runs longer than 20 ft include a 20 in lap at each splice.
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Rebar or wire mesh for a 12×12 slab?
For a 4 in patio or shed slab, 6 × 6 W1.4 welded wire mesh is usually enough: 4 sheets of 5 × 10 ft, or 1 roll of 5 × 150 ft. Mesh costs less and goes down faster but is hard to keep at mid-depth; it tends to end up on the ground where it does nothing. Rebar on chairs stays where you put it and is the right call for anything a vehicle drives on, a slab over 4 in thick, or a slab on clay or fill. For a garage or driveway, #4 at 12 in is common; for a workshop floor with heavy equipment, step up to #5 at 12 in, which for this slab is 19 bars weighing 216 lb.
Rebar cost and installation for 144 square feet
#4 rebar sells for about $0.70 to $1.00 per foot, or $14 to $20 per 20 ft stick, so the 19 bars for a 16 in grid cost $266 to $380. Chairs are about 25 cents each ($12) and tie wire a few dollars a roll. A rebar cutter or an angle grinder with a metal wheel handles the cuts; a pigtail tie tool makes the 81 ties go quickly. Keep the grid 3 in from any edge, splice with 20 in laps tied in two places, and stagger the splices. Size the pour itself with the concrete calculator for a 12×12 slab.
Common questions
How many pieces of rebar for a 12x12 slab?
9 bars running the 12 ft direction and 9 running the 12 ft direction at 16 in on center, for 207 linear feet. That cuts from 19 stock bars of 20 ft with 5 percent extra.
How much does rebar for a 12x12 slab weigh?
About 138 lb for #4 bar on a 16 in grid, or 184 lb at 12 in. #4 rebar weighs 0.668 lb per foot.
What spacing should rebar be in a 12x12 slab?
16 in on center for most residential slabs, 12 in for driveways and anything carrying vehicles, and 18 to 24 in for light-duty patios where wire mesh would otherwise be used. Keep it at mid-depth on chairs.