This span-to-depth calculator estimates a slab depth that should satisfy deflection criteria based on suitable L/d rules from the design code. You can get to it by clicking the "Concrete Design/Check" button in the top toolbar, via the "Span to Depth Calculator" button in the "Concrete manager" tool or via the "Concrete Span-to-Depth Calculators" option in the Design menu.
The calculations are done in accordance with AS3600:2018 clause 9.4.4 or IS456:2000 clause 23.2.
Refer also to the discussion about deflections in "Concrete slab design".
