Fence post concrete calculator
Enter how many posts you are setting and the size of each hole. The calculator subtracts the post and the gravel at the bottom to give the concrete and bags you need.
Estimates round up. Check product labels and supplier figures before ordering.
Fence post hole sizes
| Post | Hole diameter | Typical depth (6 ft fence) |
|---|---|---|
| 4x4 wood | 10-12 in | 24-36 in |
| 6x6 wood | 16-18 in | 30-36 in |
| Chain link terminal, 2-3/8 in | 8-10 in | 30-36 in |
| Chain link line, 1-5/8 in | 6-8 in | 24-30 in |
How it's calculated
Each hole is a cylinder. The calculator takes the part above the gravel layer, subtracts the space the post takes up, and divides by the yield of one bag.
Concrete per hole (cu in) = (3.1416 x (diameter / 2)2 - post area) x (hole depth - gravel depth)
Cubic feet = cubic inches / 1,728
Bags = total cubic feet / bag yield
Example: a 10 in hole 30 in deep with 6 in of gravel leaves 24 in of concrete. The hole holds 3.1416 x 25 x 24 = 1,885 cu in, and a 4x4 post (3.5 x 3.5 in) takes 294 cu in, leaving 1,591 cu in, or 0.92 cu ft. That is about 2.5 bags of 50 lb fast-setting mix per post, or 25 bags for 10 posts.
Bag yields assumed: 50 lb fast-setting mix about 0.375 cu ft, 60 lb concrete mix about 0.45 cu ft, and 80 lb about 0.60 cu ft. A common rule is a hole about three times the post width, buried one third to one half of the post's height above ground, and below your frost line.
What you'll need
- Fast-setting concrete mix
- Post hole digger
- Fence post level
- Gravel
Common questions
How many bags of concrete per fence post?
For a 4x4 post in a 10 in hole with 24 in of concrete, about 2.5 bags of 50 lb fast-setting mix or about 1.5 bags of 80 lb mix.
How big should a fence post hole be?
About three times the post width across, so 10 to 12 inches for a 4x4. Depth should be one third to one half of the post's height above ground, and below the frost line.
Do I need gravel under a fence post?
It helps. About 6 inches of gravel at the bottom lets water drain away from the end of a wood post, which slows rot.
Can I pour fast-setting concrete in dry?
Fast-setting mix is made to be poured dry into the hole and then soaked with about a gallon of water per 50 lb bag. It sets in roughly 20 to 40 minutes.