How much rebar do I need for a 20×20 slab?
A 20 × 20 ft slab with #4 rebar on a 16 in grid takes 32 stock bars of 20 ft, which is 585 linear feet weighing about 391 lb. Tying the grid takes 225 ties and 1 roll of tie wire, and holding it at mid-slab takes 134 chairs. At 12 in spacing for a driveway it is 42 bars; at 24 in for a light patio, 21.
Rebar for a 20×20 slab by grid spacing
| Grid spacing | 20 ft stock bars (5% extra) | Linear feet | Weight | Ties | Chairs |
|---|---|---|---|---|---|
| 12 in | 42 | 780 ft | 521 lb | 400 | 134 |
| 16 in | 32 | 585 ft | 391 lb | 225 | 134 |
| 18 in | 30 | 546 ft | 365 lb | 196 | 134 |
| 24 in | 21 | 390 ft | 261 lb | 100 | 134 |
#4 (1/2 in) rebar with 3 in edge clearance in a 20 × 20 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 20×20 slab?
For a 4 in patio or shed slab, 6 × 6 W1.4 welded wire mesh is usually enough: 9 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 32 bars weighing 610 lb.
Rebar cost and installation for 400 square feet
#4 rebar sells for about $0.70 to $1.00 per foot, or $14 to $20 per 20 ft stick, so the 32 bars for a 16 in grid cost $448 to $640. Chairs are about 25 cents each ($34) 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 225 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 20×20 slab.
Common questions
How many pieces of rebar for a 20x20 slab?
15 bars running the 20 ft direction and 15 running the 20 ft direction at 16 in on center, for 585 linear feet. That cuts from 32 stock bars of 20 ft with 5 percent extra.
How much does rebar for a 20x20 slab weigh?
About 391 lb for #4 bar on a 16 in grid, or 521 lb at 12 in. #4 rebar weighs 0.668 lb per foot.
What spacing should rebar be in a 20x20 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.