Post Hole Concrete Calculator
Count the bags of concrete needed to set fence or deck posts.
What you'll need to order
- fast-setting concrete mix
- gravel for the hole base
- post
- post level
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
volume = (π × (hole dia ÷ 24)² × depth − post area × depth) × quantity
Subtracting the post volume matters more than it looks — a 4x4 in a 12 inch hole displaces about 14% of the concrete. Put 4 to 6 inches of gravel in the bottom first so the post end can drain.
Worked example
With hole diameter 12 in, hole depth 36 in, post width 4 in, number of posts 1 , this calculator returns 5.39 bags.
Frequently asked questions
How many bags of concrete per fence post?
A 4x4 post in a 12 inch hole 36 inches deep takes about 2 cubic feet of concrete — five and a half 50 lb bags, or three and a half 80 lb bags.
How deep should a fence post hole be?
A third of the total post length, or below the frost line, whichever is deeper. A 6 ft fence on an 8 ft post means a 24 to 30 inch hole in mild climates and 42 inches or more up north.
How wide should the hole be?
Three times the post width. A 4x4 post takes a 12 inch hole, a 6x6 takes 18 inches. Narrower holes leave too thin a concrete collar and the post works loose.
Should I put gravel under a fence post?
Yes — 4 to 6 inches of compacted gravel lets water drain away from the post end instead of pooling in a concrete cup. It is the single biggest factor in how long a wooden post lasts.