How Foundation Concrete Calculator Works

Enter the total length of your foundation trenches, the trench width, and the depth of concrete to be poured. The calculator multiplies the three, adds a 10% allowance for uneven trench bottoms and overbreak, and rounds to the ordering increments ready-mix trucks work in.

The calculation formula is:

A 10-metre strip footing at 600mm wide with 300mm of concrete needs 10 × 0.6 × 0.3 = 1.8 m³, called up as 2 m³ once waste is added. The waste allowance matters more for foundations than any other pour, because trench bottoms are never perfectly flat and soft spots dug out on inspection day all swallow extra concrete. For non-structural slab projects, you can estimate requirements using our Slab Calculator or standard volume mixes with our Concrete Mix Calculator.

Strip or Trench Fill: The Decision That Sets the Volume

UK domestic foundations come in two main patterns, and the choice multiplies concrete volumes. A traditional strip foundation pours a relatively thin band of concrete (at least 225mm thick under Approved Document A, and commonly 300mm) and then builds blockwork up to the damp proof course. Trench fill pours concrete nearly to the surface, stopping around 150mm below ground level, using three to four times the concrete but eliminating below-ground blockwork and bricklayer time.

Trench fill has become the default for much UK housebuilding, especially on clay, because deep concrete stabilises the trench and the pour takes hours rather than days of below-ground bricklaying. On a 40-metre run at 600mm wide, the difference is roughly 5.4 m³ of concrete for strip against 20 m³ for trench fill, so run both numbers before assuming one is cheaper.

Depths, Widths and the Clay Problem

Foundation dimensions come from the ground, not from preference. The working minimums are a 450mm depth in firm soils to clear frost action, a 600mm width under a standard cavity wall, and concrete at least 225mm thick. Shrinkable clay changes everything, pushing depths to 750mm on low-shrinkage clay and a metre or more on high-shrinkage ground, because clay swells and shrinks with seasons and shallow foundations ride that movement.

Trees near clay are a multiplier: NHBC guidance can require depths of 1.5 to 2.5 metres depending on species, water demand, and distance, which is when trench fill or engineered alternatives take over. None of this is guesswork territory. Building Control inspects the open trench and confirms the depth against the actual ground before any concrete is poured. Pouring before that inspection risks being ordered to break out fresh concrete at your own cost.

The Right Mix and How to Order It

For standard domestic foundations on normal ground, the specification is GEN3 (equivalent to C20), ordered from any ready-mix supplier as a foundation mix. Sulphate-bearing ground, identified by soil testing, steps the order up to FND series mixes, and reinforced designs use RC grades to the engineer's specification. Site mixing at 1:2:4 works for small pads, but ready-mix wins for consistency on anything structural.

On quantities, the crossover is quick: a cubic metre needs around 84 bags of 25kg concrete, so anything beyond half a cubic metre belongs with ready-mix at roughly £90 to £115 per m³ for GEN3. Order to arrive after the inspection is passed, tell the supplier the pour is a foundation so the slump suits placement, and expect a part-load charge on small volumes.

Worked Example

A single-storey extension with 14 metres of trench, 600mm wide, on clay requiring 900mm deep trenches with trench fill to 150mm below ground:

• Concrete depth: 900 - 150 = 750mm
• Volume: 14 × 0.6 × 0.75 = 6.3 m³
• Plus 10 percent: 6.9 m³, ordered as 7 m³
• Mix: GEN3 foundation mix, roughly £630 to £800 delivered
• Sequence: Trench dug, Building Control inspection passed, then the pour is booked. That is more than a standard 6 m³ truck carries, so the order goes as one full load plus a part load, which is worth knowing before the first truck is booked.