Concrete Footing Calculator

What the concrete footing calculator does

A footing is the wide concrete base that spreads a wall or post's load onto the ground. Before you pour one, you need to know how much concrete it will take. This works that out. You enter the length, width, and depth, say whether there is a stem wall on top, and how many footings you are pouring, and it returns the volume in cubic yards and cubic metres, plus pre-mixed bag counts.

The stem wall option is there because many footings carry a short concrete wall above them, and that wall is part of the same pour. Below is how it adds it all up, with a worked example.

How to use it

1. Enter length, width, and depth. The three dimensions of the footing itself, each with its own unit. You can mix units freely.
2. Set the stem wall. Leave it on No for a plain footing. Switch it to Yes and the calculator asks for the wall's thickness and height, and adds that concrete on top.
3. Set the quantity. The number of identical footings you are pouring.
4. Press Calculate for the breakdown, or Reset to clear it.

How the volume is worked out

A footing is a long box, so its concrete is just the three sides multiplied together:

Volume = length × width × depth

If there is a stem wall, the calculator works out that wall the same way, running it along the length of the footing, and adds the two volumes. Then it multiplies by your quantity. Behind the scenes every measurement is converted to one common unit first so the multiplication lines up, then the total is given to you in both cubic yards and cubic metres.

Units, rounding, and how precise this is

You can enter measurements in inches, feet, yards, centimetres, or metres, in any mix. The answer comes back in cubic yards, which is how ready-mix is ordered in the US, and cubic metres for everywhere metric. One cubic yard is 0.7646 cubic metres, and the calculator just multiplies by that.

Volume is rounded to two decimal places, which is plenty for ordering. The bag counts are always rounded up to a whole bag, since you cannot buy part of one. The figure you get is the exact volume of the footing you described, so on the day you will want a little extra for the reasons in the ordering section below.

A worked example: a 20 ft footing

Say you are pouring a continuous footing 20 feet long, 1.5 feet (18 inches) wide, and 1 foot deep, with no stem wall.

The volume is 20 × 1.5 × 1 = 30 cubic feet, which is 30 ÷ 27 = 1.11 cubic yards, or about 0.85 cubic metres.

That is right around the point where bagged concrete stops making sense, since 1.11 yards is roughly 50 bags of 80 lb mix to carry and mix by hand. For a footing this size, most people order ready-mix, which the last section gets into.

How deep and how wide a footing should be

The calculator takes whatever dimensions you give it, but two rules of thumb help you pick them. A footing is usually about twice as wide as the wall it carries, so an 8 inch block wall often sits on a 16 inch footing. And the bottom of the footing needs to sit below the frost line, the depth to which the ground freezes in winter, so the cold cannot heave it.

That frost depth changes a lot by region, from almost nothing in warm climates to several feet in cold ones, and your local building code sets the number you have to meet. So for anything the house relies on, check the code or have the footing sized by an engineer before you commit to a depth.

Bags or a ready-mix truck

Small footings, like a few post pads, are fine from bags. Once a pour gets near a full cubic yard, a ready-mix truck is usually faster and often cheaper per yard, and it lets you place a continuous footing in one go rather than mixing bag after bag while the first part starts to set.

Whichever way you go, order a bit more than the math. The ground under a footing is rarely a perfect trench, and any over-dig quietly adds to the volume. A margin of 5 to 10 percent covers it, and coming up short on a footing is far more trouble than a little left over.

Questions people ask

How much concrete do I need for a footing?

Multiply the footing's length, width, and depth for the volume, then add the stem wall if there is one. A 20 ft footing at 18 inches wide and 1 foot deep comes to about 1.11 cubic yards.

What is a stem wall?

It is the short concrete wall that sits on top of the footing and carries the structure up to ground level or above. Since it is poured with the footing, the calculator adds its concrete to the total.

How deep should a footing be?

Deep enough to sit below your local frost line, which varies by region and is set by building code. The width is usually about twice the wall it supports.

How many bags of concrete for a footing?

It depends on the volume. As a rough guide a cubic yard is about 45 bags of 80 lb mix, so once a footing passes a yard, ready-mix is usually the easier call.

References

A quick note on where these figures come from. The unit conversions, including that one cubic yard is 0.7646 cubic metres, follow the US National Institute of Standards and Technology guide. The weight used to estimate pre-mixed bags is the standard density of normal-weight concrete, about 2,400 kg per cubic metre, as tabulated in the Portland Cement Association's Design and Control of Concrete Mixtures. Footing depth and width rules come from local building codes, commonly based on the International Residential Code.

1. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
2. Portland Cement Association (PCA), Design and Control of Concrete Mixtures. https://www.cement.org
3. International Code Council, International Residential Code (footing depth and width provisions). https://codes.iccsafe.org