Concrete · Slabs & post-tensioning
One-way slab calculator
Bending steel, bar spacing and deemed-to-comply span/depth for a one-way slab.
Standard: AS 3600 Cl 9 · AS/NZS 1170.0
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Span | 4.5 | m |
| Thickness D | 200 | mm |
| Cover | 25 | mm |
| Bar diameter | 12 | mm |
| f'c | 32 | MPa |
| fsy | 500 | MPa |
| Superimposed dead | 1 | kPa |
| Live load | 3 | kPa |
| Support | Simply supported | |
| Deflection limit L/ | 250 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Effective depth d | 169 | mm |
| w* = 1.2G + 1.5Q | 11.7 | kPa |
| M* | 29.6 | kNm/m |
| Ast required | 423 | mm²/m |
| Ast,min | 305 | mm²/m |
| Use N12 @ | 260 | mm |
| φMu provided | 30.5 | kNm/m |
| Span/depth L/d | 26.6 | |
| Limit (Cl 9.3.4) | 24.5 |
- ✓ Section can carry M*
- ✓ ku ≤ 0.36 (ductile) — ku = 0.059
- ✗ Deemed-to-comply span/depth
- ✓ Spacing ≤ min(2D, 300 mm)
Method, formulas and assumptions
- Self-weight = 25 × D = 5.00 kPa; G = 6.00 kPa
- w* = 1.2G + 1.5Q = 11.7 kPa (AS/NZS 1170.0)
- M* = w*L²/8 = 29.6 kNm/m
- α2 = 0.85 − 0.0015f'c = 0.802; φ = 0.85
- Ast = (α2 f'c b d / fsy)[1 − √(1 − 2M*/(φ α2 f'c b d²))] = 423 mm²/m
- Ast,min = 0.19(D/d)² f'ct,f/fsy · bd, f'ct,f = 0.6√f'c → 305 mm²/m
- Fd,eff = (1 + kcs)g + (ψs + kcs ψl)q, kcs = 2, ψs = 0.7, ψl = 0.4 → 22.5 kPa
- Lef/d ≤ k3 k4 [(Δ/Lef) 1000 Ec / Fd,eff]^(1/3), Ec = 30,100 MPa
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.