AstraCalculator

Post Hole Concrete Calculator

The post takes up part of the hole, so the concrete you need is the hole minus the post. This does that subtraction for you and converts the result into bags.

Timber is sold by its rough size but arrives smaller. A "4x4" really measures 3.5 inches.

A third of the post length is the usual rule, and below the frost line where the ground freezes.

holes

How the math works

Hole volume = pi x radius squared x depth. Subtract the volume of the part of the post that sits in the hole, then divide by what one bag yields: 0.6 cubic feet for an 80 lb bag, 0.45 for a 60 lb, 0.375 for a 50 lb.

Common questions

How many bags of concrete per fence post?
A 4x4 post in a 10 inch wide, 24 inch deep hole needs about 1.5 cubic feet of concrete, which is two to three 80 lb bags. A 6x6 in a 12 inch, 36 inch hole needs closer to four.
How deep should a fence post hole be?
About a third of the post above ground, so a 6 foot fence wants a 2 foot hole. Where the ground freezes, go below the local frost line or the post will lift over winter.
How wide should the hole be?
Roughly three times the width of the post. A 4x4 wants a 10 to 12 inch hole. Narrower than that and there is not enough concrete around the post to hold it.
Do I need to mix the concrete first?
For fence posts many people pour the dry mix into the hole and add water on top. It works for fences, but mixing first gives a stronger, more predictable set and is worth it for gates or anything carrying weight.

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