Rebar calculator
Rebar is laid in a grid, so the count comes from how many bars fit across each direction at your spacing, plus the overlap where bars splice. Enter the slab size, spacing, and bar size and the calculator returns bars to buy, weight, and the chairs and ties to install them.
How the rebar calculator works
The calculator subtracts edge clearance from each side of the slab, then counts how many bars fit across the width at your spacing to get the bars that run lengthwise, and the reverse for the bars that run across. Where a run is longer than a stock bar, it adds a lap splice of 40 bar diameters, the common rule for tension laps in slabs, and rounds up to whole stock bars.
Chairs hold the grid at mid-depth so the steel actually does its job. One per 3 square feet is a workable spacing for #4 bar; use more for #3, which sags. Tie wire is counted at one tie per intersection, with a 3.5 lb roll making about 350 ties.
Formula
bars one way = floor((slab width − 2 × cover) ÷ spacing) + 1
lap length = 40 × bar diameter (minimum 12 in)
Example: a 20 × 12 ft slab with #4 bar at 16 in on center and 3 in clearance has 9 bars running 19.5 ft and 15 bars running 11.5 ft. Nothing is longer than a 20 ft stock bar, so no splices are needed: 348 linear feet, 232 lb of steel, 26 stock bars with extra, 135 ties, and 80 chairs.
Rebar sizes and when to use them
| Size | Diameter | Weight | Typical use |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | Thin slabs, patios, stair treads |
| #4 | 1/2 in | 0.668 lb/ft | Driveways, garage slabs, footings, walls |
| #5 | 5/8 in | 1.043 lb/ft | Heavy footings, retaining walls, structural slabs |
| #6 | 3/4 in | 1.502 lb/ft | Engineered foundations, piers |
For a 4 in patio slab, 6 × 6 welded wire mesh is often enough. Go to #4 rebar at 16 in for anything a vehicle will park on. Size the pour itself with the concrete calculator.
Common sizes, already worked out
Common questions
How much rebar do I need for a 10x10 slab?
With #4 bar at 16 in on center and 3 in clearance, 8 bars each way at 9.5 ft, about 152 linear feet, or 9 stock 20 ft bars with 5 percent extra, each cut in half. That is roughly 100 lb of steel.
How far apart should rebar be in a concrete slab?
12 to 18 in on center in both directions for most residential slabs, with 16 in the usual choice. Driveways and slabs carrying vehicles use 12 in; light-duty patios can go to 24 in or use wire mesh instead.
How much should rebar overlap?
40 times the bar diameter is the standard lap for slabs: 15 in for #3, 20 in for #4, 25 in for #5. Tie the lap in at least two places. Stagger splices so they do not all land in the same line.
Does rebar need to be in the middle of the slab?
Yes. Steel on the ground does nothing. Set it on chairs or dobies so it sits in the middle third of the slab thickness, at least 1.5 in from the bottom and 3 in from any edge exposed to soil.