How the foundation calculator works
A strip foundation is a continuous trench of concrete under a wall. The calculator works out the concrete from the trench length, width and concrete thickness, then adds an "overbreak" allowance, because trench sides are never perfectly straight. It also calculates the soil you'll dig out and, optionally, the trench blocks needed to build from the top of the concrete up to damp-proof course (DPC) level, about 150mm above ground.
Strip footing vs trench fill
| Strip footing | Trench fill | |
|---|---|---|
| Concrete | 150–300mm thick | Fills trench to ~150mm below ground |
| Below-ground masonry | Trench blocks or blockwork up to DPC | Only a few courses |
| Concrete cost | Lower | Higher |
| Labour & time | More bricklaying in the trench | Faster, less labour |
Before you pour
- Most foundations need building regulations approval. Building control must inspect the open trench before concrete goes in.
- Depth depends on the soil, nearby trees and drains. Clay near trees can need much deeper or engineered foundations.
- Set level pegs in the trench to mark the top of the concrete, and pour on the same day the trench is approved so rain doesn't soften the bottom.
- Order ready-mix with a little extra. Running short mid-pour leaves a weak cold joint.
Worked example: 10m of footing for a small extension
A trench 10m long, 600mm wide and 1m deep is 6m³ of digging, about 7.8m³ of loose spoil weighing nearly 11 tonnes.
- As a strip footing with 225mm of concrete: 10 × 0.6 × 0.225 = 1.35m³, plus 10% overbreak = 1.49m³, so order 1.5m³ of ready-mix. Building back up to DPC (0.925m of masonry) takes about 98 trench blocks, 5 bags of cement and half a tonne of sand.
- As trench fill with 850mm of concrete: about 5.6m³ (order 5.75m³), with only around 32 trench blocks to reach DPC.
Trench fill costs more in concrete but saves a day or more of block-laying in a narrow, deep trench. That's why many builders prefer it for extensions. Choose the "Strip footing" or "Trench fill" preset above to compare.
Ordering ready-mix
- Access: a full-size mixer lorry is heavy and needs to get close. If it can't reach the trench, you'll need barrows and helpers, a concrete pump, or a "mix on site" supplier with a smaller volumetric lorry.
- Small loads: suppliers often charge extra for part loads. Ask for the price of the exact volume and whether there's a short-load charge.
- Timing: book the pour for the day after the building control inspection, and have tools, barrows and the level pegs ready before the lorry arrives.
Garden walls are different
A garden wall doesn't usually need building control, but it still needs a proper footing on firm ground below the topsoil. A typical footing for a low wall is about 450mm wide and 150–300mm thick at 450–600mm deep, deeper on clay. Choose the "Garden wall" preset, and read the garden wall guide for sizes by wall height.
Common mistakes
- Pouring into a wet or soft trench. Pump out water and remove any mud before the concrete goes in. The bottom must be firm, undisturbed ground.
- Running short. Overbreak is real: trenches are always a little wider and deeper than planned.
- Ignoring drains and trees. Foundations near drains or trees may need to be deeper or bridged. Building control will advise.
For the dig and spoil, see the excavation and skip size calculators. For blockwork above ground, see the block calculator. To budget the whole project, try the extension cost calculator.