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 much concrete does a strip foundation need?
Length × width × depth for each run, summed, plus 10% for spillage and over-dig — trench sides are never as neat as the drawing. A 36 m run at 450 mm wide and 300 mm deep is 4.86 m³, so order 5.35 m³. In US units a 120 ft run at 18 in by 12 in is about 6.7 cubic yards before the allowance.
How deep should a footing be?
Deeper than the frost line, and deep enough for the ground you are building on — which is a local question, not a universal number. Frost depth alone ranges from a few hundred millimetres to well over a metre (roughly one to four feet) between regions. Get the depth from your local building authority or a ground investigation before ordering anything.
Save to project
Free to try — no sign-up
Plan the whole job — not just this calculation
Save results to a project: budget and shopping list build themselves, and the whole plan exports as a PDF. Draw your floor plan and room sizes pre-fill the calculators.
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: simplified estimator uses two longitudinal bars along the footing length plus 5% lap/waste; actual bar count and spacing must be designed locally
- 5Lap length: 40 × bar diameter (e.g. 40 × 12 mm = 480 mm laps; a #4 bar at ½ in gives 20 in)
- 6Formwork: 2 × (footing depth × total footing length) for strip footings above ground
Worked example
Strip footing for a 10 m × 8 m house extension (33 × 26 ft): perimeter 36 m, or 118 ft, footing 450 mm wide × 300 mm deep — 18 in by 12 in.
Concrete: 36 × 0.45 × 0.30 = 4.86 m³ + 10% = 5.35 m³, or 7 cubic yards. Rebar (T12 at 200 mm spacing — roughly a #4 bar at 8 in): 3 bars per section × 36 m = 108 lm, 354 linear feet, plus laps.
Frequently asked questions
How deep should domestic foundations be?
Minimum 450 mm (18 in) below finished ground level for frost protection in mild climates, and deeper in colder regions. In shrinkable clay soils: 750 mm–1 m, or 30–40 in, 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 — 18 by 12 in — uses 0.135 m³ per linear metre, about 1.45 cu ft per foot. For 36 m of footing: 4.86 m³. Ready-mix is recommended above 2 m³, roughly 2.6 cubic yards, 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?
Many building codes require foundation width to be at least 3× the wall thickness above. A 215 mm, one-brick wall — 8½ in — needs a minimum 650 mm, or 26 in, wide foundation. Wider footings are needed for poor-bearing soils.