How to use the concrete footing calculator
- Choose the shape: round for augered holes and tube forms, square for dug or formed square footings.
- Enter the diameter (or width), depth and number of holes.
- Pick your bag size. The table compares every bag size per hole and in total.
- Subtract the post if posts are set in the concrete, and add a gravel layer under Adjust assumptions if you use one.
The footing formulas
Round hole: cubic feet = π × (diameter ÷ 2)² × depth ÷ 1,728 (all in inches)
Square hole: cubic feet = width × width × depth ÷ 1,728
Post: subtract post width² × depth in concrete ÷ 1,728
Bags = holes × cubic feet × (1 + extra) ÷ yield per bag, rounded up
Worked example
8 fence post holes, 10 in wide and 36 in deep
- One hole = π × 5² × 36 ÷ 1,728 = 1.636 cu ft.
- 8 holes = 13.09 cu ft; with 10% extra = 14.40 cu ft (0.533 cu yd).
- 80 lb bags: 14.40 ÷ 0.60 = 24 bags, 3 per hole.
Bags per hole
| Hole | Depth | Volume | 80 lb bags (+10%) | 60 lb bags (+10%) |
|---|---|---|---|---|
| 8 in round | 24 in | 0.70 cu ft | 1.3 | 1.7 |
| 8 in round | 36 in | 1.05 cu ft | 1.9 | 2.6 |
| 10 in round | 36 in | 1.64 cu ft | 3.0 | 4.0 |
| 12 in round | 36 in | 2.36 cu ft | 4.3 | 5.8 |
| 12 in round | 48 in | 3.14 cu ft | 5.8 | 7.7 |
Common mistakes to avoid
- Measuring the auger, not the hole. Dug holes flare at the top; add extra or measure the real width.
- Too shallow. Posts above the frost line can heave in winter.
- Concrete to the surface with no slope. Crown the top so water runs away from the post.
- Buying the exact number of bags. Running out mid-hole is worse than one spare bag.
Frequently asked questions
How many bags of concrete per fence post?
A 10 in round hole 36 in deep holds about 1.64 cu ft. With 10% extra that is 1.8 cu ft, or 3 bags of 80 lb concrete (each yields 0.6 cu ft). Subtract a 4 × 4 post and it drops to about 2.5 bags.
How big should a post hole be?
A common rule of thumb is a diameter about three times the post width, so 10 to 12 in for a 4 × 4. Depth is usually about a third of the post length above ground, and below the frost line. Your local code sets the minimum for decks and structures.
How much concrete for a sono tube?
Use the round option with the tube diameter and the length of tube you will fill. An 8 in tube 4 ft long holds about 1.4 cu ft, or 3 bags of 80 lb mix with a little extra.
Should I subtract the post?
For a precise count, yes. A 4 × 4 post takes up about 0.085 cu ft per foot of depth, which adds up across many holes. For a few holes, the extra concrete is cheap insurance.
Is fast-setting concrete measured the same way?
Yes, but check the yield on the bag. Fast-setting post mixes often yield slightly less per bag than standard mix.