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Rebar Calculator: Bars, Weight and Cost for a Slab
Lay out a rebar grid for a concrete slab, patio or driveway. Get the number of bars each way, stock lengths to buy with lap splices, total weight, tie wire, chairs and cost.
How to calculate rebar for a slab
Rebar in a slab forms a grid of bars running both ways. Keep the bars back from the edges (usually 3 in) so concrete covers them. For each direction:
bars = (slab width − 2 × edge clearance) ÷ spacing, rounded down, + 1
Each of those bars is as long as the other side of the slab minus the clearance at both ends. When a bar is longer than the stock you can buy, two bars overlap in a lap splice. A common rule is 40 bar diameters: 20 in for #4 bar.
Worked example
A 20 × 20 ft slab with #4 bar at 12 in on center and 3 in clearance has 19.5 ft of usable width, so 20 bars each way, each 19.5 ft long. That is 40 bars from 20 ft stock, 780 linear ft, and at 0.668 lb per foot about 520 lb of steel. You also need about 400 ties where the bars cross and roughly 45 chairs to hold the grid up in the middle of the slab.
Rebar sizes and weights
| Bar | Diameter | Weight per ft | Common use |
|---|---|---|---|
| #3 | ⅜ in | 0.376 lb | Patios, sidewalks, light slabs |
| #4 | ½ in | 0.668 lb | Driveways, garage slabs, footings |
| #5 | ⅝ in | 1.043 lb | Footings, walls, heavy slabs |
| #6 | ¾ in | 1.502 lb | Structural foundations |
Spacing and placement tips
- 12 to 18 in on center is typical for driveways and garage floors; 18 to 24 in for patios and walks. Follow plans or local code where they apply.
- Rebar must sit in the middle or upper third of the slab to do its job. Bars lying on the ground do almost nothing, so use chairs or concrete dobies.
- Keep at least 3 in of concrete between the bars and the soil, and 1.5 to 2 in from exposed surfaces.
- Wire mesh is an alternative for thin slabs, but it is hard to keep lifted. Rebar is easier to place correctly.
Need the concrete too? Use the concrete calculator.
Frequently asked questions
How much rebar do I need for a 20x20 slab?
With #4 bar at 12 in on center and 3 in edge clearance, a 20 × 20 ft slab needs 20 bars each way, 40 bars of 20 ft stock, about 780 linear ft and 520 lb.
How far apart should rebar be in a slab?
12 to 18 in on center each way is typical for driveways and garage slabs, and 18 to 24 in for patios and sidewalks, unless plans or code say otherwise.
How much should rebar overlap?
A common rule is 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5 bar. Engineers may specify more.