Reinforcement Ratio Calculator
ρ = As/(b·h0), and the reverse calculation for required steel.
Inputs
Result
Formula
| ρ = As / (b·h0) × 100% | h0 is effective depth |
| As = ρ × b × h0 | reverse |
| Bars = ceil(As ÷ area per bar) | πd²/4 each |
| Unit weight = 0.00617·d² | kg/m |
Beam 300×600 with h0 = 560 and 4Φ20 (As = 1256 mm²): b·h0 = 168000 mm², so ρ = 0.748%. For a 1.0% target, As must be 1680 mm²; each Φ20 is 314.2 mm², so 6 bars are needed (1885 mm², 1.12%).
FAQ
Why effective depth instead of overall height?
The tension steel sits near the bottom, and the lever arm runs from its centroid to the compression face - that is the depth doing the work. Using full height counts the cover above the steel as effective, understating the ratio: 300×600 gives 0.70% on h = 600 but 0.75% on h0 = 560, a 7% gap that matters near the limit.
Is more reinforcement always safer?
No. Over-reinforced sections crush the concrete before the steel yields - a sudden brittle failure with no warning, more dangerous than under-reinforced. Codes therefore cap the ratio (via the balanced relative compression depth ξb) as well as setting a minimum. Safety means under-reinforced behaviour: steel yields first and the member deforms visibly before failure.
Do slabs, columns and walls use the same formula?
The form is the same but the reference area differs: slabs per metre width; columns on the gross section (b×h, not b×h0) with separate one-face and total ratios; walls separately in each direction, plus boundary element rules for seismic walls. Always ask which basis a quoted ratio uses.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.