Our Rebar Calculator works out exactly how many rebar pieces, total linear feet and total weight you need for a concrete slab grid — enter your slab size and spacing to avoid over-ordering or running short mid-pour.
Rebar Calculator
Rebar Formula
Total Linear Feet = (Bars across width × Slab length) + (Bars across length × Slab width)
Where the number of bars in each direction = (Slab dimension ÷ spacing in feet) + 1, rounded down. Add 5-10% waste allowance for overlaps, cuts and damaged pieces, then divide total linear feet by your standard bar length (usually 20 ft) to get the number of pieces to order.
Standard Rebar Sizes and Weight
| Rebar Size | Diameter | Weight (lb/ft) |
|---|---|---|
| #3 | 3/8 in | 0.376 |
| #4 | 1/2 in | 0.668 |
| #5 | 5/8 in | 1.043 |
| #6 | 3/4 in | 1.502 |
Common Rebar Spacing by Application
- Residential driveways/patios: 18 inch grid, #3 or #4 rebar.
- Garage floor slabs: 12-16 inch grid, #4 rebar.
- Footings and foundations: 12 inch grid or per engineer spec, #4 or #5 rebar.
- Sidewalks: Often mesh instead of rebar grid, but if used, 18-24 inch grid with #3 rebar.
FAQs
How much rebar do I need for a 10×10 slab?
At 18-inch spacing, a 10×10 ft slab needs roughly 70-80 linear feet of rebar in a basic grid — use the calculator above with your exact spacing for a precise number.
What size rebar should I use for a driveway?
#3 or #4 rebar on an 18-inch grid is standard for residential driveways. Heavier traffic areas may use #4 on a tighter 12-16 inch grid.
How long does rebar usually come in?
Standard rebar stock length is 20 feet, though some suppliers also offer 10-foot and 12-foot lengths for smaller projects.
Should I add extra rebar for waste?
Yes — a 5-10% waste allowance covers overlaps at splices, cutting waste, and bent or damaged pieces.