Post Hole Concrete Calculator

First calculate the volume of the cylindrical hole, then subtract the volume taken up by the embedded part of the post. Concrete only fills what is left: Concrete = Hole Volume − Embedded Post Volume. For several identical holes, Total = Concrete Per Hole × Number of Holes, and your waste allowance is applied to that total.

1. 1Choose a square/rectangular or round post.
2. 2Enter the hole diameter.
3. 3Enter the hole depth.
4. 4Enter the post dimensions.
5. 5Enter the number of identical holes.
6. 6Optionally enter a gravel-base depth.
7. 7Choose a waste allowance.
8. 8Select Calculate.
9. 9Review the concrete volume and bag quantities.

Measurements are normalized before multiplying, so you can mix inches, feet, centimeters and meters freely. The hole uses the full-precision value of π, and only displayed figures are rounded.

The post occupies space inside the hole, and concrete only fills the space remaining around it. A calculator that uses the full cylindrical hole volume therefore overstates the concrete needed — noticeably so for a chunky post in a modest hole. BuildCalcWorks subtracts the embedded post volume and shows both figures so you can see the difference.

For a round post, the cross-sectional area is π × radius², using half the post diameter. For a square or rectangular post it is simply width × thickness. Enter the actual measured dimensions of your posts rather than nominal lumber sizes — a post sold as 4 × 4 is often smaller than 4 in on each face, and this calculator uses exactly what you type.

The calculation assumes a constant diameter, vertical sides and a uniform depth. Hand-dug and augered holes often flare, taper or wander, so the real hole can hold more concrete than a perfect cylinder of the same nominal size. This is one reason a waste allowance can be useful.

• Measure a finished hole rather than relying on the auger or digger size.
• Measure your posts — actual dimensions differ from nominal sizes.
• Only enter a gravel-base depth if you are actually placing one.
• Check the yield printed on the product you plan to use, especially fast-setting post mixes.
• Count your holes carefully; concrete scales directly with the quantity.