How do I calculate concrete for a post hole?

For a round hole, multiply pi by the radius squared and the depth, using feet for all dimensions. If the post sits in the hole, subtract the embedded post volume, multiply by the number of holes, then add waste.

How many 80 lb bags of concrete do I need per fence post?

It depends on hole diameter and depth. A 10 inch diameter hole that is 30 inches deep around a 4x4 post uses about 1.0 cubic foot of concrete, or about 2 bags of 80 lb concrete mix before extra waste.

Should the concrete calculator subtract the post volume?

Yes, for a closer estimate when the post is set inside the wet concrete. The difference is small for oversized holes but can matter across a long fence line.

Fence & post calculator

Post Hole Concrete Calculator

Estimate how much concrete you need for round post holes. Enter the number of posts, hole size, depth, post size, and bag yield to get cubic yards and concrete bag counts.

Concrete needed 16 bags 8 holes, 10 in wide, 30 in deep, with 10% waste.

Post holes

Number of holes Hole diameter (inches) Concrete depth (inches)

Use the depth that will be filled with concrete. If you add gravel at the bottom, do not include the gravel depth.

Post and bags

Post width in concrete (inches) Waste / overage (%) Bag size

Common dry concrete yields vary by brand. Use the package yield if it differs from these defaults.

How to use the post hole concrete calculator

Measure the hole diameter and the depth you plan to fill with concrete. Add the number of holes, choose your bag size, and enter the post width if the post will sit in the wet concrete.

The calculator treats each hole as a round cylinder, subtracts the square post volume, adds your waste percentage, then converts the total into cubic feet, cubic yards, and bag counts.

Post hole concrete tips

Round up: buy whole bags and keep an extra bag available for uneven holes or spillage.
Check depth: fences, decks, mailboxes, and pergolas may have different depth and frost requirements.
Keep dimensions realistic: a common 4x4 post is about 3.5 inches wide, while a 6x6 post is about 5.5 inches wide.

FAQ

Can I use this for fence post concrete?

Yes. It is designed for fence post holes, gate posts, mailbox posts, deck posts, pergola posts, and similar round holes.

Why subtract the post volume?

If the post is inside the concrete-filled part of the hole, the post takes up space that concrete does not fill. Subtracting it makes the bag estimate more accurate.

How much waste should I add?

For small post projects, 5–10% is a practical starting point. Use more if holes are rough, wider at the bottom, or dug by hand in loose soil.