Rebar Calculator
Estimate rebar quantities for a rectangular slab — bar counts, total length, and steel mass.
Estimates for planning only — add ~10% for laps and wastage. Verify bar sizes and spacing with a structural engineer before ordering.
How it works
- 1Enter the slab length and width, then choose bar spacing and diameter.
- 2The calculator divides each slab dimension by the spacing to count bars running each way.
- 3Total length is bars-along-length × length + bars-along-width × width; mass uses steel density 7850 kg/m³.
Use cases
- Estimating rebar quantities for a ground-floor concrete slab before ordering steel.
- Comparing mesh density options (spacing or diameter) to balance cost against reinforcement.
- Producing a quick material take-off for small residential or commercial slabs.
Frequently asked questions
How is rebar mass calculated?
Each bar size has a mass per metre from steel density 7850 kg/m³ and the bar cross-section. A 12 mm bar (≈113 mm²) is ≈ 0.888 kg/m. Total mass = total length × mass per metre.
Why add about 10% for laps and wastage?
Bars overlap (lap) at joints for continuity — typically 40–50 bar diameters — and cutting produces offcuts. A 10% allowance is common; your structural engineer may specify more.
Can you show a worked example?
A 5 m × 4 m slab with 12 mm bars at 200 mm spacing: 21 bars one way, 26 the other. Total length = 21 × 5 + 26 × 4 = 209 m → about 185.6 kg of steel.
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