Post Hole Concrete Calculator
Find how much concrete sets fence or deck posts, in cubic yards and bags, less the posts themselves.
Change the values and press Calculate to work out your own figures.
Enter the post's shape and width, the hole's size and the number of posts.
Show the working
- concrete = (hole area − post area) × depth × posts
- hole: 10 1/2″ across, 2′ 0″ deep
How to use it
Enter the post's shape and width, the hole's size and the number of posts.
Lengths can be decimals, fractions or tape entries such as 12' 6 1/2" (in feet or inches). Prices are optional.
Key facts
concrete = (π × (hole diameter ÷ 2)² − post area) × depth × posts
Bags
Premixed bags yield about 0.30 cu ft (40 lb), 0.45 cu ft (60 lb) and 0.60 cu ft (80 lb) of concrete. Ready-mix trucks are usually cheaper above about 1 cubic yard.
Holes
Dig about three times the post's width and a third of its height above ground, below the frost line; put a few inches of gravel in the bottom for drainage.
Questions
How much concrete for 20 fence posts?
About 0.79 cubic yards (36 bags of 80 lb) for 3.5 in round posts in 10.5 in holes 2 ft deep.
Formulas
concrete = (π × (hole diameter ÷ 2)² − post area) × depth × posts
Sources
Limitations
- Material densities are typical values (the midpoint of the commonly published range); real loads vary with moisture, grading and compaction, so check with your supplier or type in their density.
- Tank results are the geometric volume; real tanks have wall thickness and fittings.