Concrete Footing & Post Hole Calculator
Count the bags of concrete needed to set a fence post, deck post, mailbox or sign in a round hole.
Calculator
| 60-lb bags | 6 bags |
|---|---|
| 50-lb bags | 7 bags |
| Cubic feet (exact) | 2.36 ft³ |
| Cubic yards (+10%) | 0.096 yd³ |
How it is calculated
Volume of a cylinder: π × (12 ÷ 2 ÷ 12)² × (36 ÷ 12) = 2.36 cubic feet. Add 10% waste, then divide by 0.6 cubic feet per 80-lb bag: 5 bags.
About this calculator
A post-hole footing is a cylinder of concrete, so its volume is π × radius² × depth. Fence posts, deck posts, mailbox posts and signs usually sit in a hole several times wider than the post.
Volume grows with the square of the diameter: doubling the hole width quadruples the concrete, while doubling the depth only doubles it. That is why a wider hole costs far more concrete than a deeper one.
Worked examples
| Project | 80-lb bags of concrete mix | Rough cost |
|---|---|---|
| a 10-inch diameter, 24-inch deep hole | 2 bags | $10.00 – $18.00 |
| a 12-inch diameter, 36-inch deep hole | 5 bags | $25.00 – $45.00 |
| a 12-inch diameter, 36-inch deep hole | 44 bags | $220 – $396 |
| a 18-inch diameter, 48-inch deep hole | 52 bags | $260 – $468 |
Buying notes
- Use the actual hole diameter, which is normally wider than the post.
- The post itself takes up volume; this calculator ignores that, so it errs on the safe side.
- Deck footings and anything holding up a structure may have code requirements for depth and size; check locally.
Quick answers by size
Click a size for the exact quantity and cost.
How to measure and use this calculator
- Measure the hole's diameter (the auger size) and its depth.
- Enter the number of holes if you are setting several posts.
- Read the bag count for your bag size (80, 60 or 50 lb).
- Optionally subtract the post's volume if you want a tighter estimate.
Common mistakes
- Using the post width instead of the hole diameter.
- Digging shallower than the frost line in cold climates.
- Not leaving room for a gravel base at the bottom for drainage.
Tips
- Use the hole's actual diameter, which is usually wider than the post.
- Subtract the volume of the post itself if you want a tighter number; it is ignored here to stay on the safe side.
- A common rule is a hole depth of about one third of the post's total length, but local frost depth and code may require more.
Frequently asked questions
How many bags of concrete for a fence post?
A 10–12 inch wide, 24–36 inch deep hole usually takes between two and five 80-lb bags; the table above lists exact counts by size.
How deep should a fence post hole be?
A common rule is about one third of the post's length below ground, and below the frost line where the ground freezes. Check local requirements.
How many bags of concrete for 10 fence posts?
Multiply the bags for one hole by 10, or enter 10 in the number-of-holes field to get the total.