Concrete Footing Calculator

Concrete volume for strip and pad footings: length, width and depth in feet and inches or metres and millimetres, with bag counts and weight.

A footing spreads the load of a wall, post or pier into the ground. Strip (continuous) footings run under walls; pad footings sit under single posts and columns. Either way the concrete is length × width × depth, so a strip footing 20 ft long, 16 in wide and 8 in deep is 20 × 1.333 × 0.667 = 17.8 cubic feet, about 0.66 cubic yards. Enter the length in feet or metres and the width and depth in inches or millimetres, set the number of identical footings, and the calculator gives the volume, the premixed bags and the weight. Footing sizes come from your building code, the ground and the load, and in cold climates from the frost depth, so take the dimensions from your plans.

Units
Bag sizes
Pour
ft
in
in
%
Concrete
0.72 yd³

19.56 ft³ · 0.554 m³

Bags
33

80 lb bag (0.60 ft³)

Weight when mixed
2,930 lb

1.46 tons

Premixed bags needed by size
80 lb bag (0.60 ft³)33
60 lb bag (0.45 ft³)44
50 lb bag (0.375 ft³)53
40 lb bag (0.30 ft³)66
20 kg bag60
25 kg bag48
30 kg bag40
How it was worked out
  1. 20 ft long × 16 in wide × 8 in deep × 1 = 17.78 ft³ (0.66 yd³)
  2. Plus 10% extra = 19.56 ft³ (0.72 yd³)
  3. 80 lb bag (0.60 ft³): 19.56 ft³ ÷ 0.6 ft³ made per bag = 32.59, rounded up to 33 bags

Bag yields are typical figures: 0.30, 0.375, 0.45 and 0.60 ft³ for 40, 50, 60 and 80 lb bags, and about 108 × 20 kg bags per m³. Check the yield printed on the bag you buy. Mixed concrete is taken as 2,400 kg/m³ (about 4,050 lb per cubic yard).

Common use cases

  • Concrete for the strip footing under a garden or retaining wall
  • Pad footings for a deck, pergola or carport
  • Foundations for a shed or extension (with the sizes from your plans)
  • Compare bagged concrete with a ready-mix delivery for a long trench

How to use

  1. Choose Footing (it's selected already) and your units.
  2. Enter the length in feet or metres, and the width and depth in inches or millimetres.
  3. For several identical pad footings, enter one footing's size and the number of footings.
  4. Read the volume, bags and weight. Switch to Round column for tube-formed piers.

FAQ

How deep and wide should a footing be?

It depends on the load, the soil and the climate. Codes generally require footings to sit on firm, undisturbed ground and, where the ground freezes, below the local frost depth. Take the sizes from your plans or your building department rather than a rule of thumb.

Can I use this for a trench filled with concrete?

Yes. A trench fill is a strip footing as deep as the trench: enter the trench's length, width and the depth of concrete. Trenches with sloping or crumbling sides take more, so allow extra.

What about a thickened edge on a slab?

Work out the slab with the slab calculator, then add the extra depth around the edge here as a footing: the perimeter as the length, the thickened width, and only the depth below the slab's underside.

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