How to calculate a concrete footing
Footings spread the weight of a wall or building over the soil, and their size comes from your plans or the local code.
- Add up the total length of all footing runs.
- Enter the footing width and the depth from your plans.
- Add 10 percent for uneven excavation.
- Order ready mix and ask about a pump if the trench is hard to reach.
Worked example
For a typical concrete footing with length 100 ft, width 16 in, depth 8 in, the volume is 88.89 cubic feet. Divide by 27 for 3.29 cubic yards. With a 10 percent allowance, plan for 3.62 cubic yards.
In concrete, 3.29 cubic yards weighs about 6.67 US tons at a planning value of 4,050 pounds per cubic yard.
Concrete Footing sizes and quantities
These common sizes are calculated with the same formula as the tool above, so you can find your number quickly.
| Size | Cubic feet | Cubic yards | With 10 percent extra | US tons |
|---|---|---|---|---|
| 50 ft, 16 in wide, 8 in deep | 44.44 | 1.65 | 1.81 | 3.33 |
| 100 ft, 16 in wide, 8 in deep | 88.89 | 3.29 | 3.62 | 6.67 |
| 150 ft, 20 in wide, 10 in deep | 208.33 | 7.72 | 8.49 | 15.62 |
| 200 ft, 24 in wide, 12 in deep | 400.00 | 14.81 | 16.30 | 30.00 |
Tips
- Follow the engineer or code on width and depth, since soil and frost depth vary.
- Trench walls that collapse add extra concrete, so dig carefully.
- Check the trench bottom for loose soil before the pour.
Common mistakes to avoid
- Using a width or depth that does not match the plans.
- Forgetting the corners and the thickened sections.
- Not allowing for over excavation.
These figures are planning estimates. Real material varies in moisture, size, and compaction, so confirm weights and order quantities with your supplier.
Frequently asked questions
How many cubic yards do I need for a concrete footing?
Multiply the dimensions in feet and divide by 27. The example above, length 100 ft, width 16 in, depth 8 in, needs 3.29 cubic yards before waste.
How much extra should I order?
Add 5 to 10 percent for waste. Add more if the ground is uneven or the material is loose.
Can I enter inches and feet together?
Yes. Each box has its own unit selector, so you can mix feet and inches.
How much does concrete footing material weigh?
3.29 cubic yards of concrete weighs about 6.67 US tons as a planning value.