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.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ICC International Residential Code 2021, R403.1.4 — minimum footing and pier embedment depth below frost line
The edition matters as much as the number. Jurisdictions adopt code revisions on their own schedule, so verify which one applies where you are building.
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.
Bags needed for common post settings
Open any row to load it into the calculator above — the link carries the numbers with it, so it can go straight to a supplier or a crew.
| Job | Size | 50 lb bags needed |
|---|---|---|
| Single fence post | 10 in × 30 in × 4 in × 1 | 2.90 bags |
| Whole fence line | 10 in × 30 in × 4 in × 14 | 40.54 bags |
| Gate post | 12 in × 42 in × 6 in × 2 | 9.99 bags |
| Deck posts | 12 in × 36 in × 6 in × 6 | 25.70 bags |
| Mailbox post | 8 in × 24 in × 4 in × 1 | 1.27 bags |
Four to six inches of gravel in the bottom first, so the post end drains instead of sitting in water.
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.