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Measure the hole, not the post. The concrete fills the gap around the post, so a 4x4 post in a 10-inch hole leaves a ring of concrete about 3 inches thick on each side. A hole that is wider or deeper than planned uses more mix than the chart below.
The chart is rounded up to whole bags, because you cannot buy part of a bag. It assumes the hole is a cylinder and the post displaces a small share of the volume; for a 4x4 post the displacement is minor at these sizes.
| Hole diameter | Depth | Volume (cu ft) | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 8 in | 24 in | 0.70 | 2 | 2 |
| 8 in | 30 in | 0.87 | 2 | 2 |
| 8 in | 36 in | 1.05 | 2 | 3 |
| 10 in | 24 in | 1.09 | 2 | 3 |
| 10 in | 30 in | 1.36 | 3 | 4 |
| 10 in | 36 in | 1.64 | 3 | 4 |
| 12 in | 24 in | 1.57 | 3 | 4 |
| 12 in | 30 in | 1.96 | 4 | 5 |
| 12 in | 36 in | 2.36 | 4 | 6 |
Bag yields are the manufacturer's published figures; actual yield depends on how much water is added.
For the matching post count, see how many fence posts you need. If the fence run includes a gate, add two heavier gate posts and their concrete; gate posts are usually set deeper and in wider holes, so do not reuse the line-post number.
The common rule is a hole about three times the post width, with one third of the post buried, and the bottom below the local frost line. A 4x4 post is 3.5 inches square, so a 10-inch hole is typical. In cold climates the frost line can push the depth to 36 inches or more, which moves the bag count up a row in the chart.
Fast-setting concrete is poured dry into the hole, then water is added, and the post stays plumb while it sets. Regular concrete is mixed in a wheelbarrow or tub first, which gives a more consistent mix but takes longer per post and needs bracing. For a handful of posts, fast-set saves time; for a long run, mixed concrete can be cheaper per post.
Whichever you use, follow the bag's water instruction. Too much water weakens the mix; too little leaves dry pockets that crumble later.
Mix only what you can place before it starts to stiffen. On a warm day that window is short, so work one or two posts at a time rather than mixing a full wheelbarrow and letting it sit. Add water gradually and stop at the bag's stated amount; a mix that looks dry often needs another minute of mixing, not another litre of water.
Place the post, check it plumb on two faces, then pour and tamp the concrete around it in layers. Tamp with a piece of timber, not a metal bar, so the mix fills the gap without splitting. Slope the top of the concrete slightly away from the post so water runs off, and re-check plumb before walking away. Bracing the post for the first day is cheap insurance against a lean.
The concrete calculator converts hole size and post count into bags and cost.
For a 10-inch hole 30 inches deep, 3 bags of 80 lb or 4 bags of 60 lb. Smaller 8-inch holes take 2 bags of 80 lb. The chart above covers the common sizes.
Yes, fast-setting mixes are made for posts. Pour the dry mix, add water as the bag directs, and hold the post plumb while it sets. For a long run, mixed concrete gives a more consistent result.
About one third of the post length is the common rule, with the bottom below the frost line. Local code and manufacturer instructions decide the actual depth for your soil and climate.
A little drainage gravel at the bottom is common where the soil holds water, but it reduces the depth of concrete. Follow the manufacturer's instructions, and keep the post above the gravel.
At 10 inches wide and 30 inches deep, 3 bags of 80 lb per post, so 60 bags plus about 5% for spillage, or 63 bags. Check the chart if your holes are a different size.
The mix weakens and can shrink or crack, and the post can loosen. Add water to the bag's stated amount, mix thoroughly, and do not top up "because it looks dry" without checking the consistency.
Enter the hole size and the number of posts to get bag counts.
Estimates, not quotes. Based on our October 2026 price index. See the full index.
Estimates and general guidance, not an engineered design.