Concrete volumes
Trench fill foundation volume
- where
- depth of concrete (mm)
- trench depth from ground level (mm)
- ground level to top of concrete (mm)
- concrete volume (m³)
- total centreline length of trench (m)
- trench width (mm)
Explain this equation
Works out the concrete to fill a trench foundation up to just below ground level.
The concrete stops a little below ground (g) so the first courses of brick or block can start below the surface. The concrete depth is the trench depth less that gap. Width and depth are converted from millimetres to metres and multiplied by the centreline length.
Volume of a long rectangular strip.
Sense check: 24 m of 600 mm wide trench, 1 m deep, filled to 150 mm below ground takes about 12.2 m³.
Watch out: trench fill uses a lot of concrete. Small errors in width or depth add up fast, so measure the dug trench.
Works out the concrete for a trench fill foundation, where the trench is filled almost to ground level and brickwork starts just below the surface.
Enter your values to see the result.
Good to know
The gap below ground level is usually set so that whole brick courses reach the damp-proof course; agree it with the bricklayer before ordering. Trenches are rarely dug to an exact width: in loose or wet ground, measure the actual width in several places and use the average, as over-dig is concrete you still pay for.
Common mistake
Using the design trench width when the digger bucket has cut it wider, which can leave you several tenths of a cubic metre short on the last load.
Limits
Constant width and level bottom. Add steps, soft spots and over-dig separately. Foundation depth depends on soil, trees and loads: follow the design and Building Control.
Sources
- Geometry: prism volume = plan area × depth; cylinder volume = π d²/4 × h
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.