Post Hole Concrete Calculator

How this is calculated

The concrete you need is the volume of the HOLE minus the volume of the POST sitting in it. The hole is a cylinder: pi divided by 4, times the diameter squared, times the depth. A square post is simply its width squared times the depth. That subtraction matters more than people expect. A 4 inch post in a 12 inch hole occupies about 14 percent of it, so ignoring the post over-orders by roughly a seventh - on the default eight posts that is more than two cubic feet, or about four extra bags. Everything is worked in inches and converted at the end: 1728 cubic inches to the cubic foot, and 27 cubic feet to the cubic yard. The yield per bag is an input rather than a built-in figure because it genuinely varies with bag size and mix. Read it off the bag you intend to buy - it is usually printed on the front, and it is the number the whole bag count turns on. ⚠️ The hole DIAMETER is the number people get wrong, and getting it wrong is expensive because the volume goes up with the SQUARE of it. A 12 inch hole is 44 percent more VOLUME than a 10 inch one at the same depth, not the 20 percent the width suggests - and because the post displaces a fixed amount whatever the hole size, the CONCRETE needed rises faster still: with a 4 inch post it is 55 percent more. If you are hiring an auger, the bit size you choose is a bigger cost decision than it looks. A common rule of thumb is that a third of the post's total length should be below ground, and that the hole should be about three times the post's width across. Those are starting points rather than requirements - fence height, soil type, wind exposure and frost depth all change what is appropriate, and in many places the hole must extend below the frost line, which is a local figure no calculator can supply. This assumes a straight-sided hole. A bell-shaped or flared hole holds more, and gravel at the bottom for drainage displaces some concrete - neither is modelled, and they pull in opposite directions. ⚠️ A SQUARE POST FITS A ROUND HOLE ONLY IF ITS DIAGONAL DOES. A 4 inch post measures 5.66 inches corner to corner, so it will not go into anything under about 5.7 inches however the areas compare. If the calculator shows a dash instead of a number, that is why - the post you entered cannot physically enter the hole you entered, and the honest answer is that the combination does not exist rather than some quantity of concrete. In practice you want the hole about THREE TIMES the post width for a footing that holds, so a 4x4 wants a 12 inch hole - which is the default here.

Common questions

Do I really need to subtract the post?

It is worth doing. A 4 inch post in a 12 inch hole takes up about 14 percent of the volume, so ignoring it over-orders by roughly a seventh - on eight posts that is over two cubic feet, or about four bags. The calculator shows exactly what that comes to, and you can set the post width to 0 to see the difference. Over-ordering is not a disaster, but four unnecessary bags is four bags to carry, pay for and store.

Why does hole diameter matter so much?

Because volume rises with the SQUARE of the diameter. Going from a 10 inch hole to a 12 inch one is a 20 percent increase in width and a 44 percent increase in VOLUME. The concrete rises faster than that again, because the post displaces the same amount either way - with a 4 inch post the default job needs 55 percent more. That makes the auger bit you hire a bigger cost decision than it appears, and it is why a couple of holes that ended up oversized because they hit a root can throw a whole estimate out.

How deep should the hole be?

The common rule of thumb is about a third of the post's total length below ground, with the hole roughly three times the post's width across. Those are starting points, not requirements. Fence height, soil type and wind exposure all change what is appropriate, and in many regions the hole must reach below the frost line - a local figure that is a genuine requirement rather than a guideline, and one no calculator can supply.

Bags or ready-mix?

Below roughly half a cubic yard, bags are almost always the sensible choice - ready-mix has minimum orders and delivery charges that dominate at small volumes. The calculator gives cubic yards as well so you can see which side of that line you are on. For a large run of posts the arithmetic can flip, and a mixer becomes worth hiring.

What about gravel in the bottom of the hole?

It displaces concrete, so you need slightly less - the calculator does not model it because the depth of gravel is a judgement rather than a measurement. Gravel is common for drainage under a post, and it pulls in the opposite direction from a flared or bell-shaped hole, which needs MORE than a straight-sided one. If you are doing both, they partly cancel.

Should I buy extra?

Yes, and 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 have already dug is not something you can leave half-filled while you drive back to the yard. A spare bag or two costs little and removes the worst version of the afternoon.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Post Hole Concrete Calculator at https://www.bfcbrilliance.com/tools/post-hole-concrete-calculator.

What I entered:
- Hole diameter (in): ___
- Hole depth (in): ___
- Post width (square) (in): ___
- How many posts (posts): ___
- Yield per bag (cu ft): ___

What it calculated:
- Bags to buy: ___
- Concrete needed: ___
- Per hole: ___
- In cubic yards: ___
- Concrete saved by counting the posts: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

More concrete tools