Foundation Calculator

Foundation Concrete Calculator

Estimate the concrete for house and extension foundations: traditional strip foundations, deep trench fill foundations and raft (mat) foundations. Get the order volume, the number of ready-mix truck loads and the cement needed for a site mix.

Strip, trench fill & raftTruck loadsMetric & imperialLast updated: November 2, 2026

Calculate foundation concrete

m
Strip / trench: total length of all trenches. Raft: slab length.
m
m
Strip ≈ 0.225 m · trench fill ≈ 1 m · raft ≈ 0.25–0.3 m
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Strip, trench fill and raft foundations

TypeWhat it isTypical size
StripA wide, shallow band of concrete under load-bearing walls; masonry is built up from it to ground level600 mm wide × 225 mm deep (24 × 9 in)
Trench fillThe whole trench is filled with concrete almost to ground level, saving below-ground brickwork450–600 mm wide × 1 m+ deep
Raft (mat)A single thickened slab under the whole building, used on weak or variable soil150–300 mm thick, with thickened edges

Foundation concrete formula

Volume

V = L × W × D × (1 + waste%)

For strip and trench fill foundations L is the total trench length; for a raft it is the slab length and W the slab width.

Truck loads

Loads = ⌈V ÷ truck capacity⌉

The calculator assumes 6 m³ per truck in metric and 8 yd³ in imperial. Check your supplier’s actual capacity; many trucks carry 6–9 m³ (8–10 yd³), and part loads may carry a surcharge.

Worked examples

Strip foundation, 40 m of trench, 0.6 m × 0.225 m
  1. V = 40 × 0.6 × 0.225 = 5.40 m³
  2. +10% waste = 5.94 m³ → 1 × 6 m³ truck
Trench fill, 40 m of trench, 0.6 m wide, 1 m deep
  1. V = 40 × 0.6 × 1.0 = 24.0 m³
  2. +10% waste = 26.4 m³ → 5 × 6 m³ trucks
Raft 10 m × 8 m × 0.25 m
  1. V = 10 × 8 × 0.25 = 20.0 m³; +10% = 22.0 m³ → 4 trucks

Measuring total trench length

Add the length of every trench along its centreline. For a simple rectangular building with no internal walls this is the perimeter: 2 × (length + width). If you measure the outside edges of every trench, you count the corners twice, so subtract one trench width for each corner (four for a rectangle). Add internal load-bearing wall trenches separately.

Example: a 10 m × 8 m house with 0.6 m trenches measured on the outside: 2 × (10 + 8) = 36 m, minus 4 × 0.6 m = 33.6 m centreline length.

How to use the calculator

  1. Select the foundation type and units. All dimensions use metres or feet.
  2. Enter the total length and width from your foundation plan.
  3. Enter the concrete depth — the poured thickness, not the trench depth for a strip foundation.
  4. Calculate and book the truck loads shown, or use the cement figures for a site mix.

Frequently asked questions

How much concrete do I need for a house foundation?

Multiply total trench length × width × depth. A typical 40 m strip foundation 0.6 m wide and 0.225 m deep needs 5.4 m³, or about 6 m³ with waste. The same trench filled 1 m deep (trench fill) needs about 26 m³.

Is trench fill better than strip foundation?

Trench fill uses more concrete but saves below-ground blockwork, labour and time, and is safer because no one works in a deep trench. Strip foundations use less concrete and suit stable, shallow ground.

What concrete grade is used for foundations?

Plain strip and trench fill foundations commonly use GEN1–GEN3 or C20 (about M20, 2,900 psi). Reinforced rafts usually need C25–C30 or as specified by the engineer.

Excavation CalculatorSpoil to remove Concrete Footing CalculatorStrip and pad footings Concrete Block CalculatorBlocks to build up the walls Reinforcement CalculatorSteel for rafts and slabs

Results are estimates. Foundation type, depth and concrete grade must follow your engineer’s design, a soil investigation where needed, and local building regulations.

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