# Reinforcement Ratio Calculator

ρ = As/(b·h0), and the reverse calculation for required steel.

- Page: https://jianzhujisuan.com/en/rebar-density-rate
- Category: Rebar (https://jianzhujisuan.com/en/c/rebar)
- Chinese version: https://jianzhujisuan.com/zh/rebar-density-rate.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Direction Known steel area → ratio, Section width b 300 mm, Effective depth h0 560 mm, Steel area As 1256 mm², Target ratio 1.0%, Bar diameter 20 mm): **Reinforcement ratio ρ 0.748 %**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Direction | select | Known steel area → ratio | — |  |
| Section width b | number | 300 | mm |  |
| Effective depth h0 | number | 560 | mm |  |
| Steel area As | number | 1256 | mm² |  |
| Target ratio | number | 1.0 | % |  |
| Bar diameter | number | 20 | mm |  |

## 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

## Worked example

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%).

## Questions

### 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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