Footing & Column Calculator
Concrete for a strip footing under a wall, or for a row of round piers under a deck. Gives the answer by the cubic yard and by the bag, so you can see which way is cheaper.
Concrete needed
2.17cubic yards
- Cubic yards
- 2.17
- Cubic feet
- 58.7
- Weight
- 4.39 short tons
- 40lb bags
- 196
- 50lb bags
- 157
- 60lb bags
- 131
- 80lb bags
- 98
- Includes a 10% waste allowance. The bare figure is 1.98 cubic yards.
- Past half a cubic yard, ready-mix is usually cheaper than bags and always less work. A footing also wants pouring in one go, which is hard to do by hand.
- Footing size and depth are set by your local code, not by this. Frost depth is the usual controlling number and it varies from nothing to more than four feet.
- Ten per cent waste is the normal allowance on a footing because the trench walls are never as neat as the drawing, and an over-dug section still has to be filled with concrete.
About the footing calculator
Two shapes cover nearly every footing a house needs. A strip footing is a trench filled with concrete, running under a wall and spreading its load over a wider area of soil. A round pier is a hole, usually formed with a cardboard tube, carrying a post — deck posts, porch columns, a pergola.
The dimensions are not yours to choose. Footing width, depth and how far down it starts are set by your local building code and by what the soil will carry. Frost depth is usually the controlling number, and it varies from nothing in a warm climate to more than four feet in a cold one. A footing above the frost line moves every winter and takes whatever is on it with it.
Ten per cent is the standard waste allowance on a footing, and it is higher than for a slab for a good reason. A trench dug in soil is never the neat rectangle on the drawing — it slumps, it over-digs at the corners, and every extra inch of width along its whole length is concrete you have to buy. A slab poured into formwork has no such problem.
One practical point the arithmetic will not tell you: a footing wants pouring in one go. A cold joint halfway along is a weak line. Once the answer is past about half a cubic yard, mixing it by hand fast enough to avoid that is genuinely difficult, and ready-mix stops being the expensive option.
What it works out
- Strip footings and trenches
- Round piers and sonotube columns, any number of them
- Cubic yards, cubic feet and bag counts in all four bag sizes
- A waste allowance for over-dug trenches
The formula
Strip: Volume = Length × Width × Depth Pier: Volume = π × (Diameter ÷ 2)² × Height
A strip footing is a box: length times width times depth. The only care needed is with units, since the length is in feet and the section is in inches. A sixty-foot footing sixteen inches wide and eight deep is 60 times 1.333 times 0.667, which is 53.3 cubic feet, or just under two cubic yards before waste.
A pier is a cylinder: pi times the radius squared, times the height. A twelve-inch pier four feet deep holds 3.14 cubic feet, so six of them come to 18.8 cubic feet — about 0.7 of a yard.
Ten per cent goes on top, and on a footing that is not padding. A trench dug into soil slumps at the sides and over-digs at the corners, and an extra inch of width along sixty feet of trench is another three and a third cubic feet of concrete you have to pay for and place.
Whether to order ready-mix or mix bags turns on the same number. Ninety eight 80lb bags is what this footing takes, and mixing that many fast enough to avoid a cold joint partway along is not realistic.
- Length
- The run of a strip footing, in feet.
- Width
- Footing width in inches, set by your code rather than by preference.
- Depth
- Footing thickness in inches — how deep the concrete is, not how far down it sits.
- Diameter
- Pier diameter in inches, usually the size of the forming tube.
- Height
- The full depth of the pier hole, in feet.
- Waste
- Extra for an over-dug trench. Ten per cent is standard on a footing.
A worked example
A sixty-foot strip footing, sixteen inches wide and eight deep, with ten per cent for an over-dug trench.
That works out to 2.17 cubic yards.
- Cubic yards
- 2.17
- Cubic feet
- 58.7
- Weight
- 4.39 short tons
- 40lb bags
- 196
- 50lb bags
- 157
- 60lb bags
- 131
- 80lb bags
- 98
Questions
How much concrete do I need for a 60 foot footing?
2.17 cubic yards for a footing sixteen inches wide and eight deep, including a ten per cent allowance. That is 98 80lb bags, which is a ready-mix job rather than a bag job.
How much concrete does a deck pier take?
A twelve-inch pier four feet deep takes 3.14 cubic feet, which is six 80lb bags. Six of those piers come to 0.7 of a cubic yard.
How deep does a footing have to be?
Below the frost line, and that is a local figure — nothing in a warm climate, over four feet in a cold one. Your building department publishes it. A footing that sits above the frost line lifts every winter as the ground freezes and takes the structure with it.
How wide should a strip footing be?
Set by the load and the soil, and specified by your code rather than chosen. A common starting point is twice the width of the wall above it, but a footing on weak soil needs to be wider to spread the same load, and that is a question for whoever is approving the work.
Should I use bags or order ready-mix?
Past about half a cubic yard, ready-mix is usually cheaper and always less work. On a footing there is a second reason: it wants pouring in one go, and mixing bags fast enough to avoid a cold joint partway along gets very hard at that size.
Why is the waste allowance higher than for a slab?
Because a trench is dug rather than formed. Soil slumps, corners over-dig, and every extra inch of width runs the whole length of the footing. A slab poured into timber formwork is the size you built the forms.
Do I need rebar in a footing?
Usually, and how much is set by your code — often two or three continuous bars in a strip footing, with the laps and cover specified. Rebar in a pier is common too. It is not in this calculation because it is a structural requirement rather than a quantity you choose.
Does this include the wall or pad above the footing?
No — this is the footing itself. A stem wall or a pad above it is a separate pour and usually a separate day. Work it out as its own strip or slab and add the two together.