Post Hole Concrete Calculator
Get bags of concrete per hole and for the whole fence or deck, with the post volume subtracted so you buy the right amount the first trip.
Rule of thumb: 3 times the post width.
Below frost line; 1/3 of post height is typical.
A 4x4 is 3.5 in; a 6x6 is 5.5 in.
Total bags
10
Bags per hole
2
Total concrete
5.52 cu ft
Each hole takes 0.92 cu ft after subtracting the post. Round bags up per hole so you never come up short mid-pour.
How the math works
The hole is a cylinder, so its volume is pi times the radius squared times the depth. The post displaces concrete, so the calculator subtracts the post's volume over the buried depth: width squared for square posts, or pi r squared for round ones.
Dividing the remaining volume by the yield of your bag size gives bags per hole, rounded up per hole rather than across the total, because you mix and pour one hole at a time and cannot split a partial bag between distant holes.
FAQ
- How many bags of concrete per fence post?
- A typical fence post hole 10 inches in diameter and 24 inches deep with a 4x4 post takes about 1 cubic foot of concrete, which is two 80 lb bags or three 60 lb bags. Bigger holes and 6x6 posts go up fast, so calculate per hole.
- How deep should a fence post hole be?
- A common rule is one third of the above-ground post height, with a minimum around 24 inches, and always below your local frost line so freeze-thaw cycles cannot heave the post. Add 6 inches of gravel below the post for drainage.
- How wide should the hole be?
- Three times the post width is the standard rule. For a 4x4 post (3.5 inches actual) that is a 10 to 12 inch diameter hole, which leaves room for concrete to surround the post on all sides.
- How much does one 80 lb bag of concrete yield?
- An 80 lb bag yields about 0.60 cubic feet of mixed concrete. A 60 lb bag yields 0.45, a 50 lb bag 0.375, and a 40 lb bag 0.30 cubic feet.