Concrete Footing Calculator — Strip & Pad Footings
Calculate concrete volume for strip footings and pad footings, with a simplified rebar estimate and formwork area. Enter footing width, depth and run length to get the volume to order for walls, extensions and garden structures.
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How the footing volume is calculated
- 1Strip footing: width × depth × run length per run; sum all runs
- 2Pad footing: length × width × depth per pad
- 3Add 10% for over-dig and spillage — trench sides are never perfectly straight
- 4Rebar (simplified): two longitudinal bars per strip plus 5% laps; actual reinforcement must be designed locally
- 5Footing depth: below local frost depth — this varies enormously by region
- 6Formwork: 2 × depth × run length where the footing is formed above ground
Worked example
Strip footing for a garage: 24 m perimeter (79 ft), 500 mm wide × 250 mm deep — 20 in by 10 in.
Volume: 24 × 0.5 × 0.25 = 3.0 m³. With 10%: 3.3 m³ of ready-mix, or 4.3 cubic yards. Rebar at 2 bars per run: 48 lm — 157 linear feet — plus laps.
Frequently asked questions
How deep do concrete footings need to be?
Below the local frost line — from 450 mm (18 in) in mild climates to 1.2 m, or 48 in, and more in cold regions — and always into undisturbed load-bearing ground. Your local building authority publishes the required depth.
How wide should a footing be for a wall?
A common minimum is 3× the wall thickness — a 215 mm masonry wall — 8½ in — needs at least a 650 mm (26 in) wide footing. Poor soils need wider footings; a structural engineer sizes anything load-bearing.
Should I use bags or ready-mix for footings?
Ready-mix above about 1 m³, roughly 1.3 cubic yards — footings need to be poured in one continuous operation, and hand-mixing 100+ bags risks cold joints. Small pads and post footings suit bags.
Do footings need rebar?
Mass-concrete strip footings on stable ground often don't. Pad footings, stepped footings and anything on soft or shrinkable soils normally do. This is a structural design question — get it specified.
What concrete mix for footings?
C25/30 (GEN3-type) is the common specification for domestic footings. Sulphate-bearing soils need sulphate-resistant cement — a soil check is worth it before ordering.