Foundation Size Calculator
Choose how many storeys the walls carry, the type of soil, the total length of loadbearing walls and the concrete thickness. The calculator suggests the strip width, depth and concrete to order.
- Strip width
- Depth by soil
- Concrete and spoil
Typical loads for masonry houses. Heavy roofs, long spans or point loads need an engineer.
Strip foundations carry walls by spreading their weight along a trench of concrete. A two-storey brick house puts around 50 kN on every metre of wall. Divide that by what the soil can carry and you have the width. On firm ground the 450 mm minimum is usually enough; on soft ground it grows.
Strip foundation chart
Width in mm. Search for a soil.
| Soil | Capacity (kPa) | 1 storey | 2 storeys | 3 storeys | Min depth |
|---|---|---|---|---|---|
| Soft clay | 50 | 600 | 1000 | 1400 | 1,000 mm |
| Firm clay | 100 | 450 | 500 | 700 | 1,000 mm |
| Stiff clay | 150 | 450 | 450 | 500 | 1,000 mm |
| Dense sand | 200 | 450 | 450 | 450 | 600 mm |
| Rock | 600 | 450 | 450 | 450 | 450 mm |
450 mm is used as a practical minimum, as a trench narrower than that is hard to dig and inspect.
Wall load and width
Four soils. Slide the wall load.
Soft clay is the problem soil. It needs wide strips and deep trenches, and moves with the seasons.
How to size a strip foundation
Load per metre. Step through it.
Find the wall load
kN per metre.
The concrete should project at least its own thickness either side of the wall for loads to spread properly.
Strip, trench fill or raft
House foundations. Compare them.
Concrete strip with brickwork built up from it.
Trench filled with concrete almost to ground level.
A reinforced slab under the whole building.
Trench fill uses more concrete but saves brickwork below ground and time on site.
Before the pour
Inspections. Tick each item.
Tick each item as you go.
Building control must see the open trench before concrete goes in. Book the visit in advance.
What you get
Foundation sizes. Each tile explains one result.
Strip width.
Minimum by soil.
Volume to order.
Soil to remove.
Capacity used.
Spoil volume is the soil dug out. It bulks up by about 25% once loose, so plan skips accordingly.
Frequently Asked Questions
No question matches that search.
How wide should a strip foundation be?
At least 450 mm. Wider on soft ground: about 1 m for two storeys on soft clay.
How deep should house foundations be?
About 1 m on clay, 450 to 600 mm on sand, gravel or rock.
How thick is a strip foundation?
Usually 150 to 300 mm, commonly 225 mm.
How much concrete for a 10 m strip?
A 600 × 225 mm strip 10 m long needs 1.35 m³.
What load does a house wall put on the ground?
About 30 kN/m for one storey and 50 kN/m for two.
Why are clay foundations deeper?
Clay shrinks and swells with moisture near the surface.
Do I need building regulations approval?
Yes for new foundations in the UK and most countries.
Foundations sized for your ground
Match the wall load to your soil, go deep enough for the ground conditions and order the right concrete. Get the trench inspected, and the walls above will stand straight.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.