One-Way Slab Design Calculator
Estimate the design bending moment and required tension reinforcement for a simply-supported one-way concrete slab, worked per metre width of slab.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the factored uniform design load w in kPa (this equals kN/m2 acting on a 1 m wide strip).
- Enter the clear span L in metres and the effective depth d in millimetres (depth to the centre of the tension steel).
- Optionally change the steel yield strength fy (default 500 MPa) to read the design moment and required steel area per metre.
How it works
The slab is treated as a 1 m wide strip spanning simply between two supports. The maximum sagging moment is M = w * L^2 / 8. Required steel is found from a simplified internal lever arm z = 0.85 * d, giving As = M*1e6 / (0.85 * fy * z), with M in kNm/m and d in mm so As comes out in mm2/m.
Worked example
Worked example. For w = 8 kPa, L = 4 m, d = 140 mm and fy = 500 MPa: M = 8 * 4^2 / 8 = 16 kNm/m, z = 0.85 * 140 = 119 mm, and As = 16e6 / (0.85 * 500 * 119) = 316.362 mm2/m.
Common mistakes
- Using this simply-supported formula for a continuous slab, which needs smaller coefficient moments over supports and in spans.
- Entering the overall slab thickness instead of the effective depth d to the steel centroid.
- Forgetting to factor the load first, or mixing service and ultimate loads.
Frequently asked questions
Is the required steel the final answer?
No. It is a first estimate. You must still check minimum reinforcement, deflection, crack control and detailing to the governing code.
What load should I enter?
Enter the factored (ultimate) uniformly distributed load in kPa, which is numerically the load in kN per square metre on the 1 m wide design strip.
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Tip: Enter any known values to calculate the remaining results.
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