How to calculate a concrete foundation
A slab on grade foundation is a flat slab with thickened edges, and the edges can add a lot of concrete.
- Calculate the flat slab area and thickness.
- Calculate the thickened edge beams as trenches and add them.
- Add 5 to 10 percent for waste.
- Confirm the final numbers against your engineer plan.
Worked example
For a typical concrete foundation with length 40 ft, width 30 ft, depth 6 in, the volume is 600.00 cubic feet. Divide by 27 for 22.22 cubic yards. With a 10 percent allowance, plan for 24.44 cubic yards.
In concrete, 22.22 cubic yards weighs about 45.00 US tons at a planning value of 4,050 pounds per cubic yard.
Concrete Foundation 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 |
|---|---|---|---|---|
| 24 by 24 ft, 4 in | 192.00 | 7.11 | 7.82 | 14.40 |
| 30 by 40 ft, 6 in | 600.00 | 22.22 | 24.44 | 45.00 |
| 40 by 50 ft, 6 in | 1,000.00 | 37.04 | 40.74 | 75.00 |
| 50 by 60 ft, 8 in | 2,000.00 | 74.07 | 81.48 | 150.00 |
Tips
- Your plans override any generic figure.
- A concrete pump can reduce labor and waste on large pours.
- Plan the pour so that it can be finished in one day.
Common mistakes to avoid
- Ignoring the footing and edge beam volume.
- Using the slab thickness for the whole structure.
- Not checking the plan for step downs.
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 foundation?
Multiply the dimensions in feet and divide by 27. The example above, length 40 ft, width 30 ft, depth 6 in, needs 22.22 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 foundation material weigh?
22.22 cubic yards of concrete weighs about 45.00 US tons as a planning value.