About the Post Hole Concrete Calculator
Setting posts in concrete is one of the most common small concrete jobs: fence posts, gate posts, deck and pergola posts, mailbox posts, washing-line poles. It is also the job most often done by guesswork — "two bags a post" — which is right for some holes and badly wrong for others.
The amount of concrete a post needs depends on three things: the size of the hole, how deep it is, and how much of the hole the post itself fills. That last part is the one people forget. A slim post in a wide hole takes up little room, but a big post in a tight hole can fill nearly half of it.
This post hole concrete calculator works out the concrete per hole and in total, subtracts the space the post occupies and any gravel at the bottom, and gives the answer in bags — per hole and for the whole job.
How to Use the Post Hole Concrete Calculator
Choose feet and inches or metres and millimetres.
Enter the number of holes, each hole's diameter and depth, and the post shape and width. Use the post's actual size: a "4×4" timber post is 3½ inches square, and a "100 mm" post is usually about 95 mm.
If you put gravel in the bottom for drainage, enter its depth.
Adjust the waste allowance if you like — 10% is the default.
How the Concrete Is Worked Out
hole = π × (diameter ÷ 2)² × (depth − gravel)
post = width² × (depth − gravel) (square post)
= π × (width ÷ 2)² × (depth − gravel) (round post)
concrete = (hole − post) × holes × (1 + waste %)Step-by-Step Example
Ten 4×4 fence posts in 10-inch holes, 36 inches deep.
Hole: π × 5² × 36 = 2,827 in³
Post: 3.5 × 3.5 × 36 = 441 in³
Concrete: 2,827 − 441 = 2,386 in³ = 1.38 ft³ per hole
Ten posts, + 10%: 15.2 ft³ = 0.56 yd³
Bags: about 4 × 60 lb per hole, or 26 × 80 lb in totalThe post takes up about 16% of each hole here — worth subtracting, but not dramatic. A 6×6 post in the same hole would take up almost half.
Sizing the Hole
Two rules of thumb are widely used for fence and deck posts.
Width: a hole about three times the post's width. A 4×4 (3½ in) takes a 10 to 12-inch hole; a 6×6 (5½ in) about 16 inches. The concrete around the post needs enough body to resist it leaning under wind or load.
Depth: about a third of the post's total length in the ground. A post that stands 6 feet above ground is usually 9 feet long with 3 feet buried. In cold climates the hole must also reach below the frost line, or freezing ground can lift the post over the winter.
Gate posts, corner posts and anything carrying a heavy load usually need bigger and deeper holes than the rest of the fence.
Gravel and Drainage
Many installers put 4 to 6 inches of gravel in the bottom of each hole before setting the post. It lets water drain away from the end grain of a timber post, which is where rot usually starts. Enter the gravel depth and the calculator takes it off the concrete.
It also helps to crown the top of the concrete — shape it into a slight dome sloping away from the post — so rainwater runs off instead of collecting at the base.
Fast-Setting or Standard Mix?
Fast-setting post mix is poured dry into the hole around the post, then water is added on top. It sets in 20 to 40 minutes, so the post does not need bracing for long. It is ideal for fences.
Standard concrete is mixed first and poured in. It takes longer to set but is stronger, and is the better choice for decks, gates and anything structural.
Bag yields differ slightly between the two, so check the bag for its yield per bag and adjust if needed.
Setting a Post Step by Step
A few minutes of care at each hole makes a straight, solid fence.
Dig to depth and clean the bottom. Remove loose soil so the post and concrete rest on firm ground, and add gravel if you are using it.
Stand the post and brace it. Use two temporary braces at right angles, and check it is plumb on two adjacent faces with a spirit level.
Fill in stages. Add concrete a few inches at a time, working it with a stick to remove air pockets, and recheck the post is still plumb.
Finish above ground level. Crown the top so water runs away from the post.
Leave it to cure. Standard mix needs a day or two before the post is loaded; fast-setting mix can take rails within hours.
Understanding Your Result
Concrete needed is the total for all the posts, including waste.
Bags in total gives the count for two common bag sizes.
Per hole gives the bags for a single hole — useful when working post by post.
Space taken by the post shows how much of each hole the post fills.
Sizing gives the rule-of-thumb hole size for your post.
Worth knowing covers narrow holes and drainage.
When Should You Use This Calculator?
Before a fence job, to buy the right number of bags in one trip.
For decks and pergolas, where posts are larger.
When holes are dug by machine, which are often wider than planned.
When comparing hole sizes, to see the concrete each one takes.
Common Mistakes
Using nominal post sizes. A 4×4 is 3½ inches.
Not subtracting the post. It can be a large share of a tight hole.
Holes too shallow. Below the frost line, and about a third of the post length.
Letting water pool at the base. Crown the concrete.
Assuming "two bags a post". It depends entirely on the hole.