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Two-Way Slab Design Calculator

Estimate the short-span design moment and required reinforcement for a two-way spanning slab panel using the moment-coefficient method, worked per metre width.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

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

  1. Enter the factored uniform design load w in kPa and the short span Lx in metres.
  2. Enter the bending moment coefficient alpha for your panel (default 0.05) taken from a code table for the correct support conditions and aspect ratio.
  3. Enter the effective depth d in millimetres and, optionally, the steel yield fy to read the short-span moment and required steel area.

How it works

For a two-way panel the short-span moment is Mx = alpha * w * Lx^2, where alpha bundles the support conditions and the long/short aspect ratio. Required steel uses the simplified lever arm z = 0.85 * d, so As = Mx*1e6 / (0.85 * fy * z), with Mx in kNm/m and d in mm giving As in mm2/m.

Worked example

Worked example. For w = 8 kPa, Lx = 4 m, alpha = 0.05, d = 140 mm and fy = 500 MPa: Mx = 0.05 * 8 * 4^2 = 6.4 kNm/m and As = 6.4e6 / (0.85 * 500 * 119) = 126.545 mm2/m.

Common mistakes

  • Guessing alpha instead of reading it from the correct code table for the panel's edge conditions and aspect ratio.
  • Designing only the short span and forgetting the long-span direction needs its own coefficient and steel.
  • Using the short span Lx interchangeably with the long span; alpha is defined against the short span.

Frequently asked questions

Where does alpha come from?

From a two-way slab coefficient table (for example in AS 3600), selected for the panel support conditions and the ratio of long to short span.

Does this cover both directions?

No. It solves the short-span (Lx) direction only. Repeat the design for the long span using its own moment coefficient.

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