Slab Thickness Estimator
Enter slab span and type (one-way/two-way) to estimate recommended slab thickness from common span/depth ratios for preliminary structural sizing.
Input Data
Results
At a glance:Slab thickness is deflection-controlled, using the empirical span/depth ratio: one-way slab about 20–30, two-way about 30–35. Recommended thickness = span(mm) ÷ ratio. This calculator uses 25 (one-way) and 30 (two-way) for quick estimates. Actual must match loads, reinforcement and fire rating (minimum thickness), confirmed by a structural engineer.
Formula
Recommended thickness = span(mm) / span-to-depth ratio
$$t = \frac{L_{mm}}{r}$$How to Use
- Enter the slab span (m).
- Select the slab type (one-way/two-way).
- The tool outputs the suggested thickness.
Case Studies
Residential one-way slab
Span 4 m, one-way.
Suggested thickness 160 mm.
Basement two-way slab
Span 5 m, two-way.
Suggested thickness about 167 mm.
FAQ
How to distinguish one-way and two-way?
Span ratio ≤2 is usually designed as two-way (stressed both directions); >2 as one-way (mainly short-span).
What else affects thickness?
Fire-rating minimum thickness, rebar cover and embedded services all push up the actual slab thickness.
Related Tools
References
Content review: Calculatorism Construction Team. Results are for reference only; please refer to the relevant authorities for the official figures.