Rebar Calculator
Calculate rebar count and linear footage for a concrete slab from dimensions and bar spacing.
Returns total weight and cost in imperial and metric units.
Rebar (reinforcing bar) is embedded in concrete to handle tensile stress — the pulling forces concrete cannot resist on its own. Correct spacing and sizing prevents cracking, spalling, and structural failure.
Spacing formula:
Number of Bars = (Slab Length − 2 × Cover) / Spacing + 1
Cover = the concrete margin between rebar and the slab edge (typically 1.5–3 inches)
Weight calculation:
Weight (lbs) = Length (ft) × Unit Weight (lbs/ft)
Rebar size and weight reference:
| Bar Designation | Diameter (in) | Weight (lbs/ft) | Common use |
|---|---|---|---|
| #3 | 0.375" | 0.376 | Footings, slabs |
| #4 | 0.500" | 0.668 | Driveways, foundations |
| #5 | 0.625" | 1.043 | Walls, columns |
| #6 | 0.750" | 1.502 | Structural beams |
| #8 | 1.000" | 2.670 | Heavy structural |
Worked example — 20 × 30 ft driveway:
- Rebar: #4 bars at 18" spacing, 3" edge cover
- Length direction: (20 ft − 0.5 ft cover each side) / 1.5 ft + 1 = 13 bars × 30 ft = 390 ft
- Width direction: (30 ft − 0.5 ft) / 1.5 ft + 1 = 20 bars × 20 ft = 400 ft
- Total length: 790 ft | Weight: 790 × 0.668 = 527.7 lbs
Standard spacing guidelines:
- Residential slabs: 18" centers maximum
- Driveways: 12–18" centers (#4 bar minimum)
- Foundation walls: 12" vertical, 18" horizontal
- Always maintain minimum 3" concrete cover from edges and 1.5" from top/bottom surfaces
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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