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Foundation calculator: how much concrete for a trench foundation?
Work out the excavation, concrete and spoil removal for strip or trench-fill foundations. Enter the trench length, width and depth, the ground type and how you plan to dig and mix.
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How the calculation works
Concrete volume is the trench length × width × the depth of concrete, which is the dug depth less the gap you leave below ground level. A 10 m trench, 600 mm wide, dug 1 m deep and filled to 250 mm below ground takes 10 × 0.6 × 0.75 = 4.5 m³.
The calculator adds your wastage allowance, then converts the volume into cement and ballast if you mix on site, or into a ready-mix order. It also estimates the spoil to remove and the plant and labour for digging.
Before you dig
Foundation width and depth depend on the ground and the load, and are normally set by building control or an engineer. Clay ground usually needs deeper foundations than sand or gravel because it moves as it wets and dries.
For anything larger than a few cubic metres, ready-mixed concrete is usually quicker and more consistent than mixing by hand.
Worked example
A 10 m trench, 600 mm wide and 1 m deep, filled with site-mixed concrete to 250 mm below ground (about 4.5 m³):
| Material | Quantity | Price (ex VAT) |
|---|---|---|
| Blue Circle Mastercrete Grey Cement in Plastic Bag 25kg | 57 | £437.19 |
| 20mm Ballast Bulk Bag (Min Weight 800kg) | 10 | £614.10 |
| Materials total | £1,051.29 | |
Today's prices, excluding VAT and delivery. Labour, plant hire and some items are priced when you request a quote. Use the calculator above for your own measurements.
Frequently asked questions
How do I work out concrete for a trench foundation?
Multiply the trench length by its width by the depth of concrete, all in metres, then add a little for wastage. The depth of concrete is the dug depth less any gap left below ground level.
How deep should house foundations be?
Commonly at least about 1 m, and deeper in clay or near trees, but the depth for your job should be agreed with building control or a structural engineer.
Should I mix on site or order ready-mix?
Small pours can be mixed on site. For trench fill of several cubic metres, ready-mixed concrete delivered to site is usually faster and gives a more consistent result.