Laying out a rebar grid
A slab reinforced with rebar uses two sets of parallel bars, one in each direction, tied where they cross. The number of bars depends on the slab size, the spacing and the edge cover. Bars come in sticks of fixed length, so long runs need lap splices. This calculator counts the bars, the sticks to buy, their weight and the chairs that hold the grid up.
The method
- Bar run = slab dimension − 2 × edge cover.
- Bars in each direction = the other bar run ÷ spacing + 1.
- Sticks per bar: one if the run fits in one stick; otherwise the run is split with a lap of 40 bar diameters at each splice.
- Weight = sticks × stick length × weight per foot: 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5.
Worked example: a 20 × 20 ft slab
The slab uses #4 bars at 18 in on center, with 3 in of edge cover and 20 ft sticks.
- Bar run: 20 − 0.5 = 19.5 ft each way, which fits in one 20 ft stick.
- Bars each way: 19.5 × 12 ÷ 18 + 1 = 14 bars.
- Sticks: 14 + 14 = 28 sticks.
- Rebar in place: 28 × 19.5 = 546 ft.
- Weight: 28 × 20 × 0.668 = 374 lb.
- Chairs at about one per 9 sq ft: 45 chairs.
Rebar by spacing for the same slab
| Spacing | Sticks | Linear feet | Weight |
|---|---|---|---|
| 12 in | 40 | 780 ft | 534 lb |
| 16 in | 30 | 585 ft | 401 lb |
| 18 in | 28 | 546 ft | 374 lb |
| 24 in | 20 | 390 ft | 267 lb |
Bar sizes
Rebar sizes in the US are numbered in eighths of an inch: #3 is 3/8 in, #4 is 1/2 in and #5 is 5/8 in. Residential slabs and footings mostly use #3 to #5. Your plans or local code set the size and spacing; this calculator only counts the material.
Lap splices
When a bar run is longer than a stick, two bars overlap and are tied together. A common rule is 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5. Engineers may specify more. Stagger splices so they do not all fall in the same line.
Placing the grid
- Height: the grid sits in the slab, usually around mid-depth or slightly above, never on the ground. Use chairs or small concrete blocks every 3 to 4 ft.
- Cover: keep about 3 in between bars and soil at the edges, so moisture does not reach the steel.
- Ties: tie bars at every other intersection with wire ties.
Rebar or wire mesh?
Thin patios and sidewalks often use welded wire mesh or synthetic fibers instead of rebar. Rebar gives more strength for driveways, garage slabs and slabs on poor soil. Whatever you choose, it only works if it ends up inside the concrete, not pressed to the bottom.