How the footing calculator works
A footing is the widened concrete base that spreads the weight of a wall, post or pier over enough soil to stop it sinking. Most small projects combine two shapes: a continuous strip under walls and isolated square pads under posts. The tool treats each as a rectangular block, converts the width and depth from centimetres or inches into the same unit as the length, and sums the two before adding waste.
Concrete = (L × W × D + n × side² × pad depth) × (1 + waste ÷ 100)Trench walls are never perfectly vertical and the bottom is rarely flat, so a dug footing almost always takes more concrete than the drawing suggests. Bag counts use a dry premix density of about 2,130 kg per cubic metre for 25 kg sacks and the printed yield of 0.60 ft³ for an 80 lb sack.
Worked example
A garden room sits on 12 m of strip footing, 45 cm wide and 30 cm deep, plus four 60 cm square pads 30 cm deep for a veranda. The strip holds 12 × 0.45 × 0.30 = 1.62 m³ and each pad 0.6 × 0.6 × 0.3 = 0.108 m³, so the four pads add 0.432 m³. Together that is 2.052 m³, or 2.26 m³ with 10% waste. Mixed from 25 kg bags this would be 193 sacks, close to five tonnes of dry mix, which is why a job this size is normally poured from a ready-mix truck. The imperial equivalent, 40 ft of 18 × 12 in footing plus four 24 × 24 × 12 in pads, comes to 60 + 16 = 76 ft³, or 3.10 yd³ after waste.
Common footing sizes for light structures
| Use | Typical width | Typical depth of concrete |
|---|---|---|
| Garden wall, one brick thick | 30–45 cm (12–18 in) | 15–25 cm (6–10 in) |
| Single-storey house wall (US IRC minimum) | 12 in (30 cm) | 6 in (15 cm) |
| Deck post pad | 40–60 cm (16–24 in) square | 20–30 cm (8–12 in) |
| Garden room or extension wall | 45–60 cm (18–24 in) | 20–30 cm (8–12 in) |
Treat these as common starting points only. The size that is actually required depends on the load, the bearing capacity of your soil and the local frost depth, and it is set by your building code or a structural engineer. Anything that carries a house, an extension or a retaining wall should be designed or checked by a professional and inspected before the pour.
Frost depth and trench depth are different numbers
The depth entered here is the thickness of concrete, not the depth of the hole. In cold regions the underside of a footing must sit below the frost line, which can be 45 cm in mild climates and over 1.2 m (48 in) in northern states or Alpine areas. You can dig to that level and pour only the specified thickness, backfilling above it, or fill the whole trench with concrete, in which case enter the full trench depth.
Mistakes that waste concrete or money
- Counting corners twice. Measure the trench along its centreline so overlapping ends at corners are not added twice.
- Forgetting the step-downs. On a slope, stepped footings add short vertical sections; add their volume by hand.
- Pouring into water or loose soil. Pump out standing water and remove soft spoil so the concrete bears on firm, undisturbed ground.
- Leaving out reinforcement. Many codes call for steel bars in strip footings; the bars barely change the volume but must be ordered and positioned before the pour.
Frequently asked questions
How much concrete do I need for a 10 m footing?
A 10 m strip 45 cm wide and 30 cm deep holds 1.35 m³; with a 10% allowance order about 1.5 m³.
How many bags of concrete for a deck post footing?
A 24 × 24 × 12 in pad holds 4 ft³, so it takes seven 80 lb bags at 0.60 ft³ each. A 60 cm square pad 30 cm deep needs about ten 25 kg bags.
How deep should a footing be?
The bottom must sit on firm ground below the local frost line, often 45 cm to over 1.2 m. The concrete itself is usually 15–30 cm (6–12 in) thick for light structures; your local code sets the minimum.
Is it cheaper to use bags or ready-mix for footings?
Up to roughly half a cubic metre (two-thirds of a yard) bags are practical. Above about 1 m³, ready-mix is usually cheaper and lets you pour each footing in one go.
How wide should a footing be?
A common rule is at least twice the width of the wall it supports, and the US residential code sets 12 in as a minimum for light loads. Soft soil or heavy walls need wider footings designed by an engineer.
Last reviewed: 2026-09-27 · How we calculate