Foundation Calculator — Concrete Volume, Rebar & Formwork
Calculate concrete volume for strip footings and pad foundations, rebar quantity and formwork area. Enter footing dimensions and get a complete material list for domestic and light commercial foundations.
How the foundation calculator works
- 1Strip footing volume: length × width × depth (m³) per run; sum all runs
- 2Pad foundation volume: length × width × depth (m³) per pad
- 3Add 10% for spillage and over-pour
- 4Rebar: number of bars = footing width ÷ bar spacing; total lm = bars × footing length
- 5Lap length: 40 × bar diameter (e.g. 40 × 12 mm = 480 mm laps)
- 6Formwork: 2 × (footing depth × total footing length) for strip footings above ground
Worked example
Strip footing for a 10 m × 8 m house extension: perimeter 36 m, footing 450 mm wide × 300 mm deep.
Concrete: 36 × 0.45 × 0.30 = 4.86 m³ + 10% = 5.35 m³. Rebar (T12 at 200 mm spacing): 3 bars per section × 36 m = 108 lm + laps.
Frequently asked questions
How deep should domestic foundations be?
Minimum 450 mm below finished ground level for frost protection in the UK (deeper in colder regions). In shrinkable clay soils: 750 mm–1 m minimum. Always consult a structural engineer and local building control.
How much concrete do I need for a strip foundation?
A standard 450 mm wide × 300 mm deep strip footing uses 0.135 m³ per linear metre. For 36 m of footing: 4.86 m³. Ready-mix is recommended for volumes above 2 m³ for consistent quality.
What concrete mix should I use for foundations?
C25/30 (GEN3 / ST4) is the standard mix for house foundations in the UK. In sulphate-bearing soils, use sulphate-resistant cement (SRPC). Always follow a structural engineer's specification.
Do I need rebar in house foundations?
Strip foundations for domestic extensions often do not require rebar if the ground is stable and the concrete is unreinforced mass concrete. However, pad foundations and any foundation on poor ground should be designed and specified by a structural engineer.
What is the minimum foundation width for a brick wall?
UK Building Regs (Part A) require foundation width to be at least 3× the wall thickness above. A 215 mm (one-brick) wall needs a minimum 650 mm wide foundation. Wider footings are needed for poor-bearing soils.