How Much Rebar Do I Need for a Slab? Spacing, Bars and Weight Chart
Part of the project planner: Planning a concrete patio, slab or driveway.
A 20 × 12 ft slab with #3 rebar on 16 inch centers takes 20 bars of 20 ft (383 linear feet and 144 pounds of steel), with 10% extra for laps and waste, and 135 intersections to tie. The rebar calculator works out the bars and the weight for your own slab, spacing and bar size.
Rebar for common slab sizes
#3 bars, with 3 inches of cover at the edges and 10% extra. The bars, in 20 ft pieces, and the weight, by spacing:
| Slab | 16 in spacing | 12 in spacing | 24 in spacing | Ties at 16 in |
|---|---|---|---|---|
| 10 × 10 ft | 9 (63 lb) | 11 (79 lb) | 6 (39 lb) | 64 |
| 12 × 12 ft | 12 (86 lb) | 16 (114 lb) | 8 (57 lb) | 81 |
| 12 × 16 ft | 16 (115 lb) | 21 (153 lb) | 11 (77 lb) | 108 |
| 20 × 12 ft | 20 (144 lb) | 26 (192 lb) | 13 (96 lb) | 135 |
| 20 × 20 ft | 33 (242 lb) | 43 (323 lb) | 22 (161 lb) | 225 |
| 24 × 24 ft | 47 (350 lb) | 63 (467 lb) | 32 (233 lb) | 324 |
Bar sizes and weights
For the 20 × 12 ft slab at 16 inch spacing:
| Bar | Weight per foot | Total weight | Bars to buy |
|---|---|---|---|
| #3 (3/8 in) | 0.376 lb per ft | 144 lb | 20 bars of 20 ft |
| #4 (1/2 in) | 0.668 lb per ft | 256 lb | 20 bars of 20 ft |
| #5 (5/8 in) | 1.043 lb per ft | 399 lb | 20 bars of 20 ft |
How to place rebar in a slab
- Lay the grid with the bars at the spacing, and tie every crossing with tie wire.
- Lift it off the ground on chairs or bricks, so that it sits in the middle or the lower third of the slab, about 3 inches off the bottom.
- Keep it clear of the forms by about 2 inches, so that the steel is covered by concrete and does not rust.
- Lap the bars where they meet by 30 to 40 bar diameters (about 12 to 20 inches for #3 and #4 bars) and tie the laps.
- Bend and cut rebar with a bender and a cutter, or an angle grinder. Wear eye protection.
Rebar or wire mesh?
Wire mesh is a lighter alternative for patios and sidewalks: welded wire sheets are quick to lay but tend to sit on the ground unless they are lifted. Rebar is stronger and is usual for driveways, garage slabs and footings. Neither stops a slab from cracking: they hold it together when it does. Control joints decide where the cracks go. See the slab thickness guide.
For the concrete see the concrete calculator, and for the whole slab the concrete slab materials calculator. Follow your plans and local code for any structural work.
Frequently asked questions
How much rebar do I need for a slab?
The bars run in a grid at the spacing you choose: the length of all the bars divided by the stock length, plus 10% for laps and waste. A 20 × 12 ft slab at 16 inch centers with #3 bars needs 20 bars of 20 ft, 383 linear feet in all.
What spacing should rebar be for a slab?
Typically 12 to 24 inches on center in each direction: 16 to 18 inches for a patio or sidewalk, and 12 inches for a driveway or a garage slab. Your plans or the local code may specify more.
How much does rebar weigh?
#3 rebar weighs 0.376 lb per foot, #4 0.668 and #5 1.043. The 20 × 12 ft slab takes 144 lb of #3 bar.
What size rebar for a concrete slab?
#3 (3/8 inch) is common for patios and walkways, and #4 (1/2 inch) for driveways and garage floors. Footings and walls use #4 and larger.
How far should rebar be from the edge and the bottom?
About 3 inches from the bottom (on chairs or bricks) so that the concrete covers the steel, and at least 2 inches from the sides. Lap the bars by 30 to 40 bar diameters.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.