Raft Foundation Thickness Calculator

Work out raft foundation thickness instantly with clear inputs, formula shown and shareable results.

About Raft Foundation Thickness Calculator

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.

Formulas

Stiffness rule

Thickness = column spacing / 10

Punching shear

d such that V / (perimeter x d) is within allowable shear, perimeter = 4(c + 2d)

Worked example

With the default values below, Raft Foundation Thickness Calculator returns Raft thickness to adopt of 550. Change any input above and every figure here is recalculated in your browser.

Inputs used

Column spacingminimum 0.5 m: 5 m
Column ultimate loadminimum 1 kN: 900 kN
Column sizeminimum 100 mm: 400 mm
Allowable punching shear stressminimum 0.05 MPa: 0.36 MPa

Results

Raft thickness to adopt: 550 mm
Thickness from span over ten rule: 500 mm
Effective depth for punching shear: 468 mm
Punching shear perimeter: 5,343 mm

Step by step

1Values used
2Stiffness rule
3Punching shear
4Raft thickness to adopt= 550 mm
5Thickness from span over ten rule= 500 mm
6Effective depth for punching shear= 468 mm
7Punching shear perimeter= 5,343 mm

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Raft design must be carried out by a qualified structural engineer with soil-structure interaction, settlement and full reinforcement design.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Raft design must be carried out by a qualified structural engineer with soil-structure interaction, settlement and full reinforcement design.

Frequently Asked Questions

How is the Raft Foundation Thickness figure worked out?

Thickness = column spacing / 10. Raft thickness is governed by two checks.

Why add 75 mm to the effective depth?

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.

What values do I need to enter?

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.