Concrete calculator for post holes

Enter the hole and the post, and this gives you bags per post and for the whole run. It subtracts the volume the post occupies, which is the step almost every other post-hole calculator skips.

Hole diameter Three times the post width, per Quikrete’s post-setting guide. A 4x4 wants 10–12 in; a 6x6 wants 12–18 in. Hole depth One-third to one-half of the above-ground post height, plus 6 in for the gravel base — and never shallower than your local frost depth. Post Wood is listed at its dressed size, not the nominal name. A 4x4 is 3.5 in on a side. Gravel in the bottom Quikrete’s guide calls for 6 in of gravel or crushed stone under the post so water drains away from the end grain. Concrete does not fill this. Set to 0 if you are skipping it. Concrete stops below grade The same guide stops the concrete 3–4 in down so you can backfill with soil. Set to 0 if you are instead crowning the concrete at grade to shed water. How many posts Extra for waste % An augered hole is never a clean cylinder. 10% covers ragged sides; use 15% in loose sand or gravel where the hole bells out. Bag Fast-setting mix yields the same 0.0075 cu ft per pound as general mix — but it is sold in a 50 lb bag, so you need more bags for the same hole.

Bags of mix, whole job 2 bags Rounded up. Buy one spare per ten posts, not per post.

Concrete per post 0.89 cu ft

Concrete, whole job 0.89 cu ft

Same volume in cubic yards 0.03 cu yd

Volume the posts displace 0.15 cu ft Already subtracted above. This is what other calculators leave in.

Gravel for the drainage bases 0.27 cu ft

The post is not made of concrete

A post hole is a cylinder with a post standing in the middle of it. The concrete goes in the ring around the post, not the whole cylinder:

concrete = (hole area − post area) × depth of the concrete

A nominal 4x4 is dressed to 3.5 × 3.5 in, which is 12.25 sq in, or 0.085 sq ft (dressed sizes come from Voluntary Product Standard PS 20, the American Softwood Lumber Standard). A 10 in hole has 78.5 sq in of area. So the post occupies 15.6% of the hole. A 6x6 is 5.5 × 5.5 in and takes up 27% of a 12 in hole. Almost every post-hole calculator online ignores this, and over-orders by that much on every post. On a 30-post fence that is four or five wasted bags.

Two more deductions nobody makes

Quikrete's Setting Posts in Concrete guide is specific about what the hole looks like: the diameter is three times the post width, the depth is one-third to one-half of the above-ground post length plus 6 in, you put 6 in of gravel in the bottom for drainage, and you fill with concrete only up to 3–4 in below ground level so you can backfill with soil.

That means a 30 in hole gets about 21 in of concrete, not 30. Combined with the post itself, the real number is roughly 40% below the raw cylinder volume.

Here is the check that proves it. The Fast-Setting Concrete Mix data sheet states that two 50 lb bags set a 3 in square post in a 9 in hole 30 in deep — that is 0.75 cu ft. The raw cylinder is 1.10 cu ft, which would be three bags. Take out the post, the gravel base and the top setback and you get 0.66 cu ft: two bags, exactly as the manufacturer says.

Fast-setting mix is not a different yield — it is a different bag

The Fast-Setting Concrete Mix (No. 1004) data sheet gives 0.375 cu ft for a 50 lb bag and 0.45 cu ft for a 60 lb bag. General-purpose mix gives 0.60 cu ft for 80 lb. Per pound those are identical — 0.0075 cu ft. What changes is the bag: because fast-setting is normally sold in 50 lb sacks, the same hole takes about 60% more bags, and that is where people get caught at the till.

Fast-setting is also placed dry: pour the powder in, then add water. You cannot tip the leftovers back into the bag, so round to whole bags per post and accept a little waste rather than trying to split one.

Depth is a code question, not a strength question

The one-third rule sets how much post you need in the ground to resist wind. Your local frost depth sets how deep the concrete has to go so the ground does not lift it in winter. Take the deeper of the two. In cold climates the frost line usually wins, and no amount of concrete fixes a hole that is too shallow.

Common cases

Questions people ask

How many bags of concrete do I need for a 4x4 fence post?

Two 80 lb bags for a typical 10 in hole 30 in deep. The concrete volume is about 0.9 cu ft once you subtract the post, the 6 in gravel base and the 3–4 in of soil backfill at the top — and an 80 lb bag yields 0.6 cu ft.

Do you really subtract the post from the hole?

Yes, and it matters. A 4x4 fills 15.6% of a 10 in hole and a 6x6 fills 27% of a 12 in hole. Quikrete's own figure — two 50 lb bags for a 3 in post in a 9 in hole 30 in deep — only works out if you subtract the post.

How deep should a fence post be?

One-third to one-half of the height above ground, plus 6 in for the gravel base, per Quikrete's post-setting guide. A 6 ft fence post therefore wants 24–36 in in the ground. If your local frost depth is deeper than that, the frost depth wins.

How wide should the hole be?

Three times the post width. A 4x4 gets a 10–12 in hole, a 6x6 gets 12–18 in. Wider is not stronger past that point, it just costs more concrete — and the concrete volume goes up with the square of the diameter, so a 12 in hole holds 44% more than a 10 in one.

Is fast-setting mix worth it for fence posts?

For a few posts, yes — you can plumb the post and let go within an hour instead of bracing it overnight. For a long run the bag count hurts: fast-setting comes in 50 lb bags at 0.375 cu ft, so you carry about 60% more bags than the 80 lb general-purpose equivalent.

Should I use gravel under the post or concrete all the way down?

Gravel. Six inches of gravel or crushed stone lets water drain out of the bottom of the hole instead of sitting against the end grain of the post. It also reduces the concrete you buy, which is why it is worth entering rather than ignoring.

Last reviewed 2026-07-30