Footing Size Calculator
Enter the load on one post or column, the type of soil and how many footings you need. The calculator gives the pad size, thickness, the pressure on the soil and the concrete to order.
- Load and soil
- Pad size and depth
- Concrete volume
1 kN is about 100 kg. Include the weight of the structure plus people, snow and stored items.
A footing spreads a heavy point load over enough ground to carry it. Soil can only carry so much pressure before it squeezes out or settles. Divide the load by what the soil can take and you have the area of footing needed. Firm clay takes about 100 kPa, soft clay only half that.
Pad footing chart
Square pad side in mm. Search for a load.
| Load (kN) | Soft clay | Firm clay | Stiff clay | Dense sand |
|---|---|---|---|---|
| 20 | 700 | 500 | 450 | 450 |
| 50 | 1050 | 750 | 650 | 550 |
| 100 | 1500 | 1050 | 900 | 750 |
| 200 | 2100 | 1500 | 1250 | 1050 |
| 400 | 3000 | 2100 | 1750 | 1500 |
Includes 10% for the footing's own weight. 450 mm is used as a practical minimum.
Load and pad size
Four soil types. Slide the load.
The arrows under the pad show soil pressure. A bigger pad spreads the same load more thinly.
How to size a pad footing
Load over soil. Step through it.
Work out the load
On one post, in kN.
Footings must sit below frost depth and on undisturbed ground, never on loose fill or topsoil.
Pad, strip or pile
Foundation types. Compare them.
One block under each post or column.
A continuous strip under a wall.
Deep columns down to firm ground.
On very soft ground or near big trees, piles or screw piles may be cheaper than huge pads.
Before pouring
Site checks. Tick each item.
Tick each item as you go.
Clean out loose soil and water from the base of the hole before the concrete goes in.
What you get
Footing design. Each tile explains one result.
Square side.
Concrete depth.
Against the limit.
Per pad and total.
Capacity used.
Soil pressure under the final pad is shown so you can see how much margin is left.
Frequently Asked Questions
No question matches that search.
How big should a footing be?
Load ÷ soil capacity gives the area. 50 kN on firm clay needs about 750 mm square.
How deep should a footing be?
Below frost and topsoil, usually at least 450 mm, deeper on clay.
How thick should a pad footing be?
At least 200 mm, and about a third of the pad width.
What is the bearing capacity of clay?
Soft clay about 50 kPa, firm about 100, stiff about 150.
How much concrete for a 600 mm footing?
A 600 × 600 × 200 mm pad needs 0.072 m³.
What size footing for a deck post?
For most domestic decks, 450 to 600 mm square.
Do I need an engineer?
For buildings and large loads, yes.
Footings that stay put
Find the load, know your soil and size the pad to spread it. Dig below frost onto firm ground, and your posts and columns will stay level for years.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.