Fence Post Concrete Calculator
Work out how many bags of concrete it takes to set fence or deck posts.
Last reviewed September 19, 2026
Result
Buy 22 bags (80 lb)Estimate
Augered holes come out wider than the bit, especially in loose soil, so this includes 10% extra.
- Concrete per hole
- 1.1509 ft³
- Concrete for all posts, waste included
- 12.6595 ft³ (0.4689 yd³)
- Minimum if everything goes perfectly
- 11.5086 ft³
https://calcgallery.com/concrete/fence-post-concrete-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Measure or decide the hole diameter. A common rule of thumb is about three times the post width.
- Set the depth. Roughly a third of the post length is typical, but frost depth and local codes decide this.
- Enter the post size, using the real measurement. A nominal 4×4 is 3.5 in.
- Read the bag count. It already allows 10% for holes that come out wider than the auger.
How it is calculated
A round hole is a cylinder, so its volume is the area of the circle multiplied by the depth:
Hole = π × (diameter ÷ 2)² × depthThe post occupies part of that, so it comes out:
Concrete per hole = hole − post
Total = that × number of posts × (1 + waste)The post matters more than people expect. A 3.5 in square post in a 10 in hole displaces about 15% of the volume. On ten posts that is a bag and a half you would otherwise have bought and not used.
Diameter matters even more, because it is squared. Going from a 10 in hole to a 12 in hole is a 44% increase in concrete, not 20%.
Practical tips
Mix stiffer than you think. Concrete for post holes wants to be firm enough to hold the post while you brace it. A soupy mix takes hours to grip and the post drifts while you are not looking.
Brace every post before you walk away. Two lengths of scrap timber at right angles will hold a post plumb while the concrete sets. Checking it an hour later and finding it leaning is a hole you have to dig out again.
Crown the top. Finish the concrete slightly higher than the surrounding ground and slope it away from the post. Water that sits against the post is what rots it, and a flat or dished top collects it.
Do the whole run before setting any of them. Dig and dry-fit the posts along a string line first. Discovering that hole seven is in the neighbour’s drain after you have concreted six is an expensive way to find out.
Check depth against your frost line. Where the ground freezes, a post set above the frost line will be lifted by it, a little each winter. Your local building department publishes the depth, and it is not a number worth guessing.
With your numbers
- Hole = π × (10 in ÷ 2)² × 30 in
- Less the post, each hole takes 1.151 ft³
- 10 posts = 11.51 ft³
- With 10% waste = 12.66 ft³
- Rounded up to 22 bags
Quick reference
Augered holes come out wider than the bit, especially in loose soil, so this includes 10% extra.
| Job | Bags to buy |
|---|---|
| 4 posts, 8 in hole × 24 in deep, 4×4 post | 4 |
| 10 posts, 10 in hole × 30 in deep, 4×4 post | 22 |
| 14 posts, 10 in hole × 30 in deep, 4×4 post | 30 |
| 10 posts, 12 in hole × 36 in deep, 6×6 post | 32 |
| 6 posts, 10 in hole × 30 in deep, round 4 in post | 13 |
| 10 posts, 250 mm hole × 760 mm deep, 90 mm post | 37 |
| 14 posts, 300 mm hole × 900 mm deep, 100 mm post | 90 |
Common questions
- How many bags of concrete per fence post?
- For a 10 in hole 30 in deep with a 4×4 post, about two 80 lb bags per post. That is the worked example above. Wider or deeper holes climb quickly, because volume grows with the square of the diameter.
- How deep should a fence post go?
- A common guideline is a third of the post's above-ground height, and below the frost line where the ground freezes. Both vary by jurisdiction and soil, so treat those as starting points and check what applies locally.
- Should the hole be wider at the bottom?
- A bell-shaped hole resists frost heave better, and it uses more concrete than this calculates. If you are belling the holes, add 10 to 20% on top of the figure here.
- Do I need to subtract the post?
- Yes, and this does. A 4×4 in a 10 in hole takes up about 15% of the volume. Ignoring it means buying bags you will not open.
- Can I pour the mix in dry?
- Some products are made for it and say so on the bag. Most are not. Dry-setting a bag that is not designed for it gives you a post held by powder until enough rain arrives, which may be never at the depth that matters.
- What about gravel at the bottom?
- A few inches of compacted gravel under the post helps water drain away from the end grain. It also reduces the concrete slightly, by whatever depth of gravel you use.