Concrete Footing and Post Hole Calculator
Enter the hole size and number of posts, or the size of a continuous footing, to get the concrete you need in cubic yards and bags.
On this page
Enter sizes greater than zero.
Prices are US national ranges from published sources, checked September 23, 2026. Where the numbers come from.
Concrete for post holes
Per hole, before waste. The post takes up some of the hole, so these figures are slightly high, which is the safe side.
| Hole (diameter × depth) | Volume | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 8 × 24 in | 0.70 cu ft | 2 | 2 | 3 |
| 10 × 30 in | 1.36 cu ft | 3 | 4 | 5 |
| 10 × 36 in | 1.64 cu ft | 3 | 4 | 6 |
| 12 × 36 in | 2.36 cu ft | 4 | 6 | 8 |
| 12 × 42 in | 2.75 cu ft | 5 | 7 | 10 |
| 12 × 48 in | 3.14 cu ft | 6 | 7 | 11 |
| 16 × 48 in | 5.59 cu ft | 10 | 13 | 19 |
Bag counts use yields of 0.6, 0.45 and 0.3 cu ft for 80, 60 and 40 lb bags. Across several holes, add up the volume first and then divide: it often saves a bag.
The formulas
Round hole: cubic feet = 3.1416 × (diameter in ÷ 24)² × (depth in ÷ 12)
Continuous footing: cubic feet = length ft × (width in ÷ 12) × (depth in ÷ 12)
Divide cubic feet by 27 for cubic yards. Footing size, depth and reinforcement are set by your local building code, not by this calculator.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 10 in wide, 36 in deep hole: about 1.64 cubic feet, or 3 bags of 80 lb. For a 12 in × 48 in deck footing: 6 bags of 80 lb.
How do I calculate concrete for a round footing?
Volume = π × radius² × depth. With the diameter in inches: 3.1416 × (diameter ÷ 24)² × (depth ÷ 12) gives cubic feet.
How deep should post holes be?
It depends on the post height, the load and your frost line. Local building codes set the minimum depth for deck and structural footings; check with your building department.
How do I calculate a continuous footing?
Length (ft) × width (ft) × depth (ft). A 40 ft footing, 16 in wide and 8 in deep, is 35.6 cubic feet, or 1.32 cubic yards.
Sources
- Quikrete, Concrete Mix No. 1101 data sheet, updated 2022-10. Checked September 23, 2026.
- The Home Depot, Quikrete 80 lb Concrete Mix. Figure as shown in the page's search listing; checked September 23, 2026.
- The Home Depot, Sakrete 80 lb Gray Concrete Mix. Figure as shown in the page's search listing; checked September 23, 2026.
- Concrete Network, Concrete prices, updated 2026-07-14. Checked September 23, 2026.