About this calculator
Setting fence posts, mailbox posts or deck posts in concrete? This calculator works out the volume of each hole, subtracts the part the post itself fills, and converts the result into bags of concrete — per post and for the whole job.
Fast-setting mixes (poured dry into the hole, then watered) and standard mixes (mixed first) have different yields per bag, so pick the product you plan to use.
How to use it
- Enter how many posts you are setting and choose the post type.
- Enter the hole diameter. Holes are typically about three times the post width — 10–12 in for a 4×4.
- Enter the concrete depth: the hole depth minus any gravel you put in the bottom for drainage.
- Choose the bag type. Fast-setting 50 lb bags are the most common for fence posts.
The formula
Hole volume = π × (diameter ÷ 2)² × depth
Concrete per post = hole volume − post cross-section × depth
Bags per post = concrete per post ÷ bag yield
A 4×4 post is actually 3.5 × 3.5 in, which takes up about 12 sq in of each hole’s cross-section. In a 10 in hole that is about 16% of the volume — enough to change the bag count on a long fence.
Worked example
These are the calculator’s starting values and the results they produce:
| Number of posts | 10 |
|---|---|
| Post type | 4×4 wood post (3.5 × 3.5 in) |
| Hole diameter | 10 in |
| Concrete depth in the hole | 24 in |
| Bag type | 50 lb fast-setting (0.375 cu ft) |
Tips before you buy
- A common guideline is to bury one-third to one-half of the post’s above-ground height, and below the frost line in cold climates.
- Put 4–6 in of gravel in the bottom of each hole so water drains away from the end grain.
- Crown the concrete at the top so rain runs away from the post.
- Brace each post plumb in two directions before the concrete sets — fast-setting mix firms up in 20–40 minutes.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in hole with 24 in of concrete, you need about 0.92 cubic feet — roughly two and a half 50 lb bags of fast-setting mix or one and a half 80 lb bags of standard mix.
How deep should a fence post hole be?
A common rule is one-third to one-half of the post’s above-ground height, so a 6 ft fence needs holes about 2–3 ft deep. In cold climates the hole should reach below the frost line; check local requirements.
Can I use fast-setting concrete for fence posts?
Yes. Fast-setting mix is designed for posts: pour the dry mix into the hole around the braced post and add water. It sets in under an hour, while standard mix needs to be mixed first and takes longer to set.
Sources
Areas and volumes are standard geometry. The published figures this calculator relies on come from these sources; other defaults (waste allowances, typical densities and prices) are common trade figures worth checking with your supplier.
- Quikrete Concrete Mix data sheet 1101: bag yields: 80 lb 0.60 cu ft, 60 lb 0.45 cu ft, 40 lb 0.30 cu ft
- Quikrete Fast-Setting Concrete Mix data sheet 1004-50: a 50 lb bag of fast-setting mix yields about 0.375 cu ft; setting time