How Much Concrete Do I Need for Fence Posts?

Hole minus post — forget the subtraction and you over-order by a seventh. And widening the hole two inches costs far more than it sounds.

The post takes up room too

Eight posts, 12-inch holes, 30 inches deep, 4×4 posts: 23 bags.

Forget to subtract the posts and the same job says 27. The post occupies about 14% of the hole, so ignoring it over-orders by roughly a seventh — four bags to carry, pay for and store.

Your details

Hole diameter(in)

Volume rises with the SQUARE of this — a 12in hole is 44% more volume than a 10in one, and needs 55% more concrete once the post is subtracted.

Hole depth(in)

Often about a third of the post's total length, and below the local frost line.

Post width (square)(in)

The post displaces concrete. Enter 0 to ignore it and see what that costs. A square post only fits a round hole if its DIAGONAL does — width x 1.41 must be less than the hole diameter, so a 4 in post needs at least 5.7 in.

How many posts(posts)

Yield per bag(cu ft)

Printed on the bag. An 80 lb bag of mix is commonly around 0.6 cu ft.

Result

Bags to buy

Rounded up. Buy a spare — a hole that hits a rock gets wider.

Concrete neededCubic feet, hole minus post.: 13.49
Per holeCubic feet — useful for checking one post before committing to all of them.: 1.686
In cubic yardsFor ready-mix. Below about half a yard, bags are usually the sensible choice.: 0.499
Concrete saved by counting the postsCubic feet you would have over-ordered by ignoring the post.: 2.22

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The arithmetic

Hole — a cylinder, π/4 × diameter² × depth:

0.7854 × 12² × 30 = 3,393 cu in

Post — width² × depth:

4² × 30 = 480 cu in

Concrete per hole:

3,393 − 480 = 2,913 cu in ÷ 1,728 = 1.686 cu ft

× 8 posts = 13.49 cu ft, ÷ 0.6 per bag = 22.5 → 23 bags

Two inches of diameter is not a small change

This is the one worth internalising, because it's a decision you make when you pick up an auger bit.

Volume rises with the square of the diameter. Going from a 10-inch hole to a 12-inch one is:

20% more width
44% more hole volume
55% more concrete

That last figure is higher than the second because the post displaces the same fixed amount either way — so the subtraction shrinks the smaller hole proportionally more.

In bags: 15 versus 23. The bit size is a bigger cost lever than it looks.

Yield per bag is an input, not an assumption

It varies with bag size and mix, it's printed on the front of the bag, and it's the number the whole count turns on. A calculator that guessed at it would be confidently wrong for a lot of people.

An 80 lb bag of mix is commonly around 0.6 cu ft — but check yours.

Bags or ready-mix?

Under about half a cubic yard, bags. The default job here is 0.499 cu yd — right on the line.

Ready-mix carries minimum orders and delivery charges that dominate at small volumes. For a long run of posts the arithmetic flips and hiring a mixer starts to make sense.

How deep, and how wide?

The common rules of thumb: a third of the post's total length below ground, and a hole about three times the post's width across.

Those are starting points, not requirements. Fence height, soil type and wind exposure all change what's appropriate.

And in many places the hole must reach below the frost line — which is a genuine local requirement rather than a guideline, and not something any calculator can supply. Look yours up.

Buy a spare bag

The reason is specific to this job: holes get bigger, never smaller.

One that hits a rock or a root ends up wider than planned. And a hole you've already dug isn't something you can leave open while you drive back to the yard — especially not with a post standing in it waiting to be braced.

A spare bag costs a few pounds and removes the worst version of the afternoon.

Two things the calculator doesn't model

Gravel at the bottom for drainage displaces some concrete, so you need slightly less. A bell-shaped or flared hole holds more. They pull in opposite directions, and if you're doing both they partly cancel — but neither is a measurement, so neither is in the arithmetic.

Before any of this

Locate the utilities. Everything else on this page is arithmetic; this isn't.

Buried services don't announce themselves, an auger finds them very efficiently, and the consequences run from an expensive afternoon to a fatal one. Free location services exist in most places and take a phone call.

Print the post setting sheet — it has a row per post, because depths genuinely vary along a run.