How Much Concrete Do You Need for Fence Posts?

Sizing the hole

A post stays put because the concrete around it spreads the load into the soil and resists the leverage of wind on the fence. Two rules of thumb set the hole. Depth should be about one third of the post's above-ground height, so a 6-foot fence post is buried about 2 feet. Width should be about three times the post: a 4-inch post gets a 12-inch hole. Deeper and wider is stronger, especially in soft ground or for a gate post that takes extra strain.

Once the hole size is set, its volume is straightforward. For a round hole it is pi times the radius squared times the depth; for a square hole it is width times width times depth. This gross volume assumes the hole is completely filled with concrete, which it is not — the post occupies part of it, and that part must be removed from the total.

Subtracting the post, converting to bags

Compute the volume of the part of the post that sits in the hole — its cross-section times the buried depth — and subtract it from the hole volume. What is left is the concrete you actually pour. For a 4-inch post in a 12-inch hole, the post takes up only about a ninth of the hole's cross-section, so the subtraction is modest but worth doing across many posts.

Turn that net volume into bags using the yield printed on the sack. A 60-pound bag of ready-mix makes about 0.45 cubic feet of concrete, and an 80-pound bag about 0.6. Divide the concrete you need per post by the bag yield, round up to a whole bag per hole, then multiply by the post count. It is normal to need one to two bags per fence post, and more for a chunky gate post.

Setting posts that last

Add a few inches of gravel at the bottom of each hole for drainage so water does not pool against the post and rot or rust it. Set the post, brace it plumb, then pour the concrete and slope the top surface away from the post so rain runs off. Let it cure before hanging panels or a gate — a day for the concrete to hold the post, and longer for full strength.

Buy a spare bag or two beyond the calculation. Holes are rarely perfect cylinders, soft spots eat extra concrete, and running out mid-pour with one post half-set is worse than a leftover bag. Keep dry bags sealed off the ground, and they store for months for the next project.

Frequently asked questions

How deep should a fence post go?

About one third of the height above ground. A 6-foot fence post sits roughly 2 feet deep. Go deeper in sandy or wet soil, for tall or gate posts, or below the frost line where the ground freezes, so heaving does not lift the post.

Do I really subtract the post volume?

Yes, for an accurate figure. The post displaces concrete, so the hole never takes its full gross volume. For a slim post in a wide hole the difference is small, but across a long fence line it can change the bag count, and it stops you overbuying.

Can I pour dry concrete straight into the hole?

Some fast-setting mixes are designed for it: set the post, pour the dry mix around it, then add water on top. For standard ready-mix, mix with water first. Either way the volume math is the same — you are filling the same net space around the post.

How long before I can attach the fence?

Give the concrete a day to hold the post firmly, and several days to a week for it to reach most of its strength before you load it with panels, wind or a gate. Fast-setting mixes cut the initial wait to under an hour, but full cure still takes time.