Work out raft foundation thickness instantly with clear inputs, formula shown and shareable results.
Raft thickness is governed by two checks. A span over ten rule gives a starting depth for flexural stiffness, and punching shear at each column requires enough effective depth that the load divided by the critical perimeter times the depth stays within the allowable shear stress. The critical perimeter itself grows with depth, so the check is solved iteratively.
Stiffness rule
Thickness = column spacing / 10
Punching shear
d such that V / (perimeter x d) is within allowable shear, perimeter = 4(c + 2d)
Preliminary estimate only. Raft design must be carried out by a qualified structural engineer with soil-structure interaction, settlement and full reinforcement design.
Thickness = column spacing / 10. Raft thickness is governed by two checks.
Effective depth is measured to the reinforcement centroid. Cover plus half a bar diameter, roughly 75 mm on a raft in contact with soil, converts it to overall thickness.
This calculator takes 4 inputs: Column spacing, Column ultimate load, Column size, Allowable punching shear stress. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.