How Many Bags of Concrete Per Fence Post?

Updated 2026-07-19

Ask ten fence contractors how many bags of concrete a post needs and you will get the same answer in ten different accents: it depends on the hole. But the range is tighter than most homeowners expect, and once you understand the simple cylinder math behind it, you can budget an entire fence line to within a bag or two.

This guide gives you the quick answer for the most common setup — a 4x4 post in an 8-inch hole — then shows the math so you can adjust for 6x6 posts, deeper frost lines, and oversized gate-post holes.

The quick answer for a standard 4x4 post

A standard 4x4 fence post (which actually measures 3.5 inches square) set in an 8-inch diameter hole 2 feet deep needs about 0.53 cubic feet of concrete. A 50 lb bag of fast-setting mix yields roughly 0.375 cubic feet, so the math lands at about 1.4 bags — call it 1.5 to 2 bags per post. Buy 2 and you will never come up short, because holes are never perfectly cylindrical and the leftover third of a bag disappears fast.

If you are loading up on standard 80 lb bags instead (about 0.60 cubic feet each), one bag per post is technically enough for this hole size — but most pros still round up, because a post hole that comes out 2 inches wider than planned suddenly needs half a bag more.

The math: hole volume minus post displacement

A post hole is a cylinder: pi times the radius squared times the depth. An 8-inch hole has a 4-inch radius, so 2 feet of depth holds about 1,206 cubic inches — 0.70 cubic feet. The post itself takes up space: 3.5 × 3.5 × 24 inches is 294 cubic inches, about 0.17 cubic feet. Subtract the post from the hole and you get 0.53 cubic feet, the number the bag count is built on.

Setting posts on metal post bases above the footing, the way deck builders do it? Skip the subtraction and use the full hole volume.

Bag counts for common post and hole sizes

1. 4x4 post, 8-inch hole, 2 feet deep: about 0.53 cu ft — 2 bags of 50 lb fast-setting, or 1 bag of 80 lb standard.
2. 4x4 post, 10-inch hole, 2 feet deep: about 0.92 cu ft — 3 bags of 50 lb, or 2 bags of 80 lb.
3. 4x4 post, 8-inch hole, 3 feet deep: about 0.79 cu ft — 2 to 3 bags of 50 lb.
4. 6x6 post, 12-inch hole, 3 feet deep: about 1.73 cu ft — 5 bags of 50 lb, or 3 bags of 80 lb.
5. Gate and corner posts: bump the hole up one size and add a bag — they carry the load of the whole run.

How deep should the hole be?

The classic rule is one-third of the above-ground post height in the ground, plus 6 inches of gravel at the bottom for drainage. For a 6-foot fence, that means 2 feet of post embedment in concrete over the gravel layer.

In cold climates the frost line overrides the rule of thumb. If local frost depth is 36 or 42 inches, the bottom of the footing has to sit below it or freeze-thaw cycles will heave the post out of plumb within a few winters. Your building department publishes the number, and deeper is always better — just budget roughly one extra 50 lb bag for every additional foot of depth on an 8-inch hole.

Fast-setting vs. standard mix

Fast-setting mix — the 50 lb bag made specifically for setting posts — is the right call for most fences. Dump it in dry around a braced, plumb post, add about a gallon of water, and it firms up in 20 to 40 minutes. No mixer, no wheelbarrow, and you can hang panels the same day.

Standard 80 lb mix makes sense when a mixer is already on site for a bigger pour, or when you want working time to brace and align a long run of posts before anything sets. For gate posts, many contractors prefer standard mix poured around a well-braced post and left to cure overnight, because a gate punishes a sloppy set for years.

Budget the whole fence line

Full-run example: a 100-foot fence with posts every 8 feet has 13 line posts. At 2 bags each that is 26 bags of 50 lb fast-setting mix; add 2 or 3 extra for the two gate posts and one spare for the hole that caves in, and you are loading about 30 bags — roughly 1,500 pounds, so bring the trailer, not the sedan.

Once you know your post size, hole diameter, and depth, drop them into the concrete calculator and it will convert the hole volume to an exact bag count — load the truck once and dig in peace.