Rebar Calculator
Calculate how many linear feet and pieces of rebar you need to reinforce a concrete slab.
How to use this tool
- Enter the slab dimensions in feet.
- Set the rebar spacing (12 inches is standard for residential slabs).
- Enter the standard bar length you'll purchase (typically 20 ft).
- The result shows total linear footage and number of bars to buy.
Calculate rebar requirements for a concrete slab reinforcement grid.
Formula
Rows along length = ⌈width ÷ spacing_ft⌉ + 1
Rows along width = ⌈length ÷ spacing_ft⌉ + 1
Total linear ft = (rows_along_length × length) + (rows_along_width × width)
Bars needed = ⌈total_linear_ft ÷ bar_length⌉
How it works
The calculator models a standard two-direction (grid) rebar layout: one set of parallel bars running the full length of the slab, and a perpendicular set running the full width. The number of bars in each direction is determined by dividing the span by the on-center spacing and adding one for the starting bar, a standard construction rule ensuring bars cover both edges.
Total linear footage is the sum of both directions, which is then divided by the purchased bar length and rounded up to whole bars. The formula assumes a flat rectangular slab and a uniform grid; curved slabs or variable spacing will require adjustments.
Worked example
Worked example
- Spacing in feet = 12 in ÷ 12 = 1 ft.
- Rows running lengthwise (across width) = ⌈10 ÷ 1⌉ + 1 = 10 + 1 = 11 rows, each 20 ft long → 220 ft.
- Rows running widthwise (across length) = ⌈20 ÷ 1⌉ + 1 = 20 + 1 = 21 rows, each 10 ft long → 210 ft.
- Total linear feet = 220 + 210 = 430 ft.
- Bars needed = ⌈430 ÷ 20⌉ = ⌈21.5⌉ = 22 bars.
Total linear feet: 430 ft; Rebar bars needed: 22
Key terms
- Rebar
- Short for reinforcing bar — a steel rod embedded in concrete to add tensile strength, preventing cracking under load.
- On-center spacing
- The distance measured from the center of one rebar to the center of the next. Common spacings for residential slabs are 12 in and 16 in.
- Grid layout
- Placing rebar in two perpendicular directions to create a mesh that resists cracking in all horizontal directions.
- Cover
- The minimum thickness of concrete between the rebar and the slab surface, protecting steel from moisture and corrosion. Typically 1.5–2 in for slabs on grade.
- Lap splice
- The overlap length required where two rebar pieces meet end-to-end to transfer load. Longer lap splices mean more total footage is needed than a simple grid calculation suggests.
Frequently asked questions
- How much rebar do I need for a slab?
- For a standard grid pattern with 12-inch spacing, calculate both directions of bars, multiply by the slab dimension in each direction, then add them together.
- What rebar size for a concrete slab?
- #3 rebar (3/8-inch diameter) is standard for residential slabs 4 inches thick. Use #4 (1/2-inch) for 5–6 inch driveways or higher-load applications.