Fence Post Concrete Calculator

Enter your posts and holes to see how many bags of concrete to buy, plus the gravel for drainage under each post.

How the calculation works

Each hole is a cylinder. The concrete fills the cylinder above the gravel layer, minus the space the post takes up:

concrete per hole = (π × (diameter ÷ 2)² − post area) × (hole depth − gravel depth)

The result is in cubic inches, so it's divided by 1,728 to get cubic feet. Then it's divided by the bag's yield. Bag yields are the volumes printed on standard concrete mix bags: 0.30 cu ft for 40 lb, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb.

Worked example

Ten 4x4 posts set in 10" holes that are 30" deep, with 6" of gravel. Each hole holds (78.5 − 12.25) × 24 = 1,590 cubic inches of concrete, or 0.92 cu ft. Ten posts need 9.2 cu ft. That's 16 bags of 80 lb mix, or 31 bags of 40 lb mix.

Tips before you buy

• Buy one or two extra bags. Holes are rarely perfectly round, and a returned bag costs less than a second trip.
• Check your frost line. In cold climates the bottom of the post should sit below the frost depth, or the post can heave. Your local building department can tell you the required depth.
• Fast-setting mix is poured dry into the hole and then water is added, so it's quicker for fences. It yields slightly less per bag than standard mix, so check the bag.
• Slope the top of the concrete away from the post so rain runs off instead of soaking into the wood.

Frequently asked questions

How many bags of concrete per fence post?

A typical 4x4 post in a 10" hole that's 2 to 2.5 feet deep needs about one to two 80 lb bags, or two to four 40 lb bags. Larger posts and deeper holes need more, so use the calculator above.

Do I need gravel under a fence post?

A 4" to 6" layer of gravel at the bottom lets water drain away from the end of the post, which helps wood posts last longer. It's recommended but not required.