Post Hole Concrete Calculator
A post hole concrete calculator finds 80 lb bags per hole: a 10 in wide, 2 ft deep hole holds 1.09 ft³, so 2 bags, $14 at $7 a bag. Multiply by your posts.
Post hole concrete calculator: bags per hole
A post hole concrete calculator treats the hole as a cylinder. Volume in cubic feet is π × (diameter in inches ÷ 24)² × depth in feet, and bags of 80 lb mix are that volume ÷ 0.6. A 10 inch hole 2 feet deep holds 1.09 cubic feet, which is 1.82 bags, so the calculator shows 2.
The result is for a single hole. Multiply it by the number of posts to get the full shopping list.
The formula in plain words
Half of the diameter is the radius. In feet, that is the diameter in inches divided by 24. Square the radius, multiply by π, then multiply by the depth. That gives cubic feet of concrete. One 80 lb bag fills about 0.6 cubic feet once mixed, so you divide by 0.6 and round up.
Worked example, a fence post
A fence post hole is 10 inches across and 2 feet deep. Bags are $7.
- Radius: 10 ÷ 24 = 0.4167 ft
- Volume: π × 0.4167² × 2 = 1.0908 ft³
- Bags: 1.0908 ÷ 0.6 = 1.82, rounded up to 2
- Cost per hole: 2 × $7 = $14
For a 40 ft fence with posts every 8 feet, you have 6 posts, which is 12 bags and $84.
Worked example, a deck post
A deck footing hole is 12 inches across and 3 feet deep.
- Volume: π × 0.5² × 3 = 2.3562 ft³
- Bags: 2.3562 ÷ 0.6 = 3.93, rounded up to 4
- Cost per hole: 4 × $7 = $28
Doubling the diameter would quadruple the volume. Width matters more than depth for cost.
Bags per hole
| Hole (diameter × depth) | Volume | Bags (rounded up) | Cost at $7 |
|---|---|---|---|
| 6 in × 3 ft | 0.59 ft³ | 1 | $7 |
| 8 in × 3 ft | 1.05 ft³ | 2 | $14 |
| 10 in × 2 ft | 1.09 ft³ | 2 | $14 |
| 10 in × 3 ft | 1.64 ft³ | 3 | $21 |
| 12 in × 3 ft | 2.36 ft³ | 4 | $28 |
| 12 in × 4 ft | 3.14 ft³ | 6 | $42 |
How to measure
- Dig the hole first, then measure it. Rocky or loose soil often makes a hole wider than you planned.
- Measure the diameter at the top and the middle. If they differ, use the average.
- Measure depth from the bottom of the hole to the top of the concrete. Concrete is usually stopped a few inches below ground, or slopes away from the post.
- Subtract any gravel you pour in the bottom for drainage.
Common mistakes
- Digging the hole too wide. A wider hole costs a lot more concrete, since volume rises with the square of the diameter.
- Forgetting the gravel layer at the bottom.
- Buying by the hole rather than the total. For many holes, add the partial bags together, and you may save a bag or two if you mix your own batches.
- Pouring concrete level with the ground. Slope the top so water runs away from the post.
- Ignoring local rules on depth and frost. Footing depth for a fence or deck can be set by code.
Bags for a whole fence
Work out one hole, then multiply by the number of posts. A 96 ft fence with posts every 8 feet has 13 posts. With the 10 inch × 3 ft hole from the table:
- Bags per hole: 1.6362 ÷ 0.6 = 2.73, rounded up to 3
- Total: 13 × 3 = 39 bags
- Cost: 39 × $7 = $273
Corner and gate posts often get bigger holes, so work those out separately. If you pour all the holes the same day, you may find that the leftover from one hole fills the next, and you can buy a bag or two fewer. Buying a couple of spare bags is still cheaper than a second trip.
Related pages
For a fence layout, use the fence post calculator. Tube forms for round footings are in the sonotube calculator, and post depth rules of thumb are in the fence post depth calculator.
Frequently asked questions
How many bags of concrete does one post hole need?
It depends on the hole. Volume = π × (diameter ÷ 24)² × depth, in feet, and one 80 lb bag yields 0.6 cubic feet. A 10 inch hole 2 feet deep holds 1.09 cubic feet, or 1.82 bags, so you buy 2 per hole.
Does the post take up concrete volume?
Yes. A 4×4 post is about 3.5 inches square and displaces roughly 0.085 cubic feet per foot of depth. The calculator ignores it, which leaves you a little extra. For round holes with round posts the saving is bigger.
How deep and wide should the hole be?
Check local code and the fence or deck plan. A common rule of thumb is a hole about three times the width of the post, deep enough to bury about a third of the post's length, and below the frost line where the ground freezes. Local rules vary, so confirm yours.
Should I round up the bag count?
Round up per hole if you pour the holes separately, and round the total up for a whole fence. This calculator already rounds each hole up to whole bags, so multiply by the number of holes for the budget.
Is dry-mix concrete poured into the hole the same volume?
The volume of a hole filled with dry mix and soaked with water is about the same, but you need to follow the bag's instructions for water and set time. Some fast-setting bags yield a bit differently, so check your product's label.
Can I use a different bag size?
Yes. The calculator uses 0.6 cu ft per 80 lb bag. A 60 lb bag yields about 0.45 cu ft, so you would need roughly one third more bags by count. Check your bag's printed yield.
Estimates only. Verify quantities with your supplier before purchasing.