Guide
How much concrete per fence post?
The hole, minus the post, divided by what one bag yields. Here is the calculation, then how many bags to buy.
The calculation
- Volume of the hole = π × (diameter ÷ 2)² × depth.
- Minus the post = post width × post thickness × depth.
- Bags = concrete volume ÷ the yield of one bag, printed on the bag.
Example in inches
A 10 in hole, 30 in deep, with a 4×4 post (3.5 in actual): π × 5² × 30 ≈ 2,356 in³, minus 3.5 × 3.5 × 30 ≈ 368 in³, leaves about 1,989 in³, or 1.15 ft³. An 80 lb bag yields about 0.6 ft³: 2 bags per post.
Example in centimeters
A 25 cm hole, 80 cm deep, with a 10 × 10 cm post: π × 12.5² × 80 ≈ 39,270 cm³, minus 10 × 10 × 80 = 8,000 cm³, leaves about 31 liters. A 25 kg bag yields about 12.5 liters: 2.5 bags, so 3 bags per post.
Round for the whole job
Add up the holes first, then round to whole bags: rounding post by post buys too much. Fourteen holes of 1.15 ft³ make about 16 ft³ of concrete.
How big a hole?
A common rule of thumb: a hole about three times as wide as the post, and deep enough to bury a third to a half of the post's height above ground, below the frost line where the ground freezes. Local practice and your post supplier come first.
In the app
Fence Calc does this hole by hole, with your hole depth and diameter, your post width and the weight of your bags. End, corner and gate posts can have bigger holes, and line posts can be driven without concrete. See Posts and concrete.
