Post Hole Concrete Calculator
Find out how many bags of concrete you need to set fence posts, deck posts, mailbox posts or sign posts.
Enter the hole diameter and depth, the number of posts and the post size, and the calculator subtracts the volume the post itself occupies.
That subtraction matters: for a 4×4 post in a 10 inch hole it is roughly 12% of the volume, and most calculators ignore it.
How to use the post hole concrete calculator
Enter the number of posts and the hole dimensions.
Choose the post shape and size so its volume is deducted.
Read bags per post and bags in total.
Why use the Build Tally post hole concrete calculator
Post volume deducted
Most calculators ignore this — we subtract it.
Per post and total
See both figures so you can buy in the right quantity.
Any post shape
Square, round, or an empty hole.
Bag sizing
Five bag sizes including metric.
How deep and how wide
The common rule is that a third of the post's total length should be buried, and the hole should be about three times the post's width. A 6 ft fence post is therefore usually 9 ft long, set 3 ft deep in a 10–12 inch hole. In freezing climates the hole must also reach below the frost line, or the post will lift over winter. Deeper and wider both increase concrete use quickly, since volume scales with the square of the diameter.
Working out the volume
The hole is a cylinder: π × radius² × depth. From that you subtract the post, which is a square or round prism of the same depth. The calculator does both and adds your waste allowance. A typical 10 inch × 30 inch hole with a 4×4 post takes about 1.2 cubic feet, or two 80 lb bags — which is why fence projects consume bags faster than people expect.
Set posts properly
Add 4–6 inches of gravel at the bottom of the hole so water drains away from the post end rather than sitting against it. Crown the concrete slightly above grade and slope it away so rain runs off. If you are using fast-setting mix designed for dry pouring, follow the bag instructions — some are poured dry and watered, others must be mixed.
Read the guide: How Much Concrete Do I Need? A Straight Answer for Slabs, Footings and Posts
Frequently asked questions
How many bags of concrete per fence post?
Typically one to three 80 lb bags per post, depending on hole size. A 10 in × 30 in hole with a 4×4 post takes about two bags.
How deep should a fence post be?
About one third of the post's above-ground height, and always below the frost line in cold climates. A 6 ft fence usually means a 3 ft hole.
How wide should the hole be?
Roughly three times the post width — about 10–12 inches for a 4×4 post.
Do I need gravel at the bottom of the hole?
It is good practice. Four to six inches of gravel lets water drain away from the post end and slows rot.
Can I set posts without concrete?
Compacted gravel works for some fence types and drains better, but concrete gives far more resistance to leaning and wind load.
Should I use fast-setting concrete?
It is convenient for posts and sets in 20–40 minutes. Check whether your product is designed to be poured dry or mixed first.
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