Concrete Footing Calculator

Trenches never come out the size you drew them. They are wider at the top, deeper in the soft spots, and always take more concrete than the drawing says. Enter the trench dimensions below, then add a generous wastage figure.

Footing trench cutaway: width, depth and length

Calculating footing concrete

Volume = Trench length × Trench width × Depth

Worked example. A strip footing 12 m long, 600 mm wide and 1 m deep works out at 12 × 0.6 × 1 = 7.2 m³. At 2,400 kg per cubic metre that is over 17 tonnes — firmly ready-mix territory. Pouring several identical pad footings? Calculate one and set the quantity field.

Measure the trench you actually dug. Over-dig of 50 mm on each side of a 12 m trench adds 1.2 m³ before you have poured anything, and that is the volume that catches people out.

Typical footing sizes

Application	Width	Depth	Notes
Garden wall, single skin	300–450 mm	300–500 mm	Below the frost line
Garden wall, double skin	450–600 mm	500 mm+	Roughly 2–3× wall thickness
Single-storey extension	600 mm	1 m typical	Building control confirm on site
Two-storey house	600–750 mm	1 m+	Engineer designed
Deck or porch pad (US)	16–24 in square	Below frost line	Check local code

Depth is driven by soil type, nearby trees and frost. Clay soils near mature trees can need 1.5 m or more because of seasonal shrinkage. In the UK your building control officer signs off the depth after inspecting the open trench; in the US the local frost line sets the minimum.

Trench fill or traditional strip?

Two ways to do the same job. Trench fill pours concrete to within about 150 mm of ground level and builds masonry off that — more concrete, far less bricklaying below ground, and much faster. Traditional strip puts a 225 to 300 mm layer at the bottom and builds blockwork up from it — less concrete, more labour, and more time in a muddy trench.

With concrete at £110 a metre and a bricklayer at £250 a day, trench fill usually wins on cost for anything but the deepest foundations.

What mix to order

Foundations normally take GEN1 or GEN3 (C15 to C20) for mass fill, or C25 where the drawing specifies it. Mixing on site, the classic footing ratio is 1:2:4 cement, sand and coarse aggregate, or 1:6 with all-in ballast. The mix ratio calculator and ballast calculator give exact quantities for either.

Frequently asked questions

How much concrete do I need for footings?

Trench length × width × depth, plus around 10% for over-dig. Ten linear metres at 600 mm wide and 1 m deep comes to 6 m³ before wastage.

What is the mix ratio for footings?

1:2:4 cement, sand and aggregate — roughly C20. With all-in ballast, 1:6. Ready-mix orders are usually placed as GEN3 or C20/C25.

How deep should footings be for a garden wall?

At least 300 mm for a low wall, 500 mm on soft ground or where frost bites. Width should be around two to three times the wall thickness.

Can I pour footings in the rain?

Light rain is survivable once the pour is finished and covered. Never pour into standing water — bail the trench first. Water sitting in the trench dilutes the mix at exactly the point carrying your building.

Do footings need reinforcement?

Plain mass concrete is standard for domestic strip footings. Reinforcement comes in where ground conditions are poor, the footing spans soft spots, or an engineer specifies it after a soil investigation.