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 Estimate
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