Fence Post Concrete Calculator

What is a fence post concrete calculator?

A fence post concrete calculator works out how many bags of concrete you need to set a row of fence posts, based on hole diameter, depth, post size, and whether you use a tube form. It turns “how many bags should I buy?” into a concrete answer.

How this calculator works

• Enter the number of posts, hole diameter and depth, and the post size.
• It computes hole volume minus the post volume for each hole.
• Total concrete is converted to 60 lb and 80 lb bag counts.
• Choose “tube form” when using a cylindrical form — that removes the post-displacement term.

How to use the result

1. Dig holes to the recommended depth — usually at least one-third of the post above ground, below the frost line where it freezes.
2. Buy a bag or two extra per handful of posts — holes are rarely perfectly round and gravel settles.
3. Compare bag costs against a small delivered short-load for long runs of posts.
4. Set the post plumb and braced before the concrete cures — the bag count matters less than a straight line.

How many bags of concrete per fence post?

A common rule of thumb is one 50–60 lb bag per post for a typical 8–10-inch hole, but exact needs depend on hole size. A 10-inch hole 30 inches deep with a 4×4 post holds about 1.1 ft³ of concrete — roughly two 60 lb bags. Deep frost-line holes need more.

Common mistakes

• Digging shallow holes. A post set too shallow heaves in winter and sags under wind load.
• Undersizing holes. The hole’s diameter drives concrete use more than the post does — a wide shallow hole wastes bags.
• Pouring concrete above grade. Concrete should sit below soil level so it doesn’t hold water against the post.

Post-hole concrete is cylinder volume math — the same geometry behind the concrete calculator’s footings. If you’re also buying gravel for the hole bottom, the mulch and soil calculator converts that fill volume too.