How the volume is calculated
Hole volume = π × (Diameter ÷ 2)² × Depth
Concrete per hole = Hole volume − Post volume
Bags = Total concrete ÷ Bag yield
Concrete per hole = Hole volume − Post volume
Bags = Total concrete ÷ Bag yield
Example: 10 posts (4×4, 3.5″ actual) in holes 10 inches wide and 30 inches deep. Each hole holds 2,356 cu in, the post takes 368 cu in, leaving 1.15 cu ft of concrete — 1.92 bags of 80 lb. For all 10 posts buy 20 bags of 80 lb concrete.
Setting posts that last
- Put 4–6 inches of gravel in the bottom of each hole so water drains away.
- Dig below your area’s frost line in cold climates.
- Slope the top of the concrete away from the post to shed water.
Need gravel for the holes or a base? Try the gravel calculator.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10-inch hole 30 inches deep, about 1.9 bags of 80 lb concrete (or 3 bags of 50 lb) per post.
How big should a fence post hole be?
A common rule is a diameter three times the post width (about 10–12 inches for a 4×4) and a depth of one third of the post length, plus 6 inches for gravel.
How much concrete does an 80 lb bag make?
About 0.60 cubic feet. A 60 lb bag makes about 0.45 cu ft, a 50 lb bag 0.375 cu ft and a 40 lb bag 0.30 cu ft.
Do I need to mix fast-setting concrete?
Fast-setting post mix can be poured dry into the hole and then soaked with water. Follow the bag’s directions — usually about a gallon of water per 50 lb bag.