Material Summary
- Posts—
- Hole size—
- Concrete purchase—
- Gravel base—
- Total bag weight—
- Estimated cost—
Typical Hole Sizes
| Project | Diameter | Depth |
|---|---|---|
| Fence post | 10 – 12 in | 24 – 36 in |
| Deck post | 12 – 18 in | Below frost |
| Mailbox | 8 – 10 in | 24 in |
| Pergola / gazebo | 12 – 18 in | 36 in+ |
| Pole barn | 16 – 24 in | 48 in+ |
A common rule sizes the hole about 3x the post width.
How Much Concrete Per Post Hole?
Concrete for a post hole is the volume of the hole minus the post that sits in it. For a round hole that is the diameter and depth run through the cylinder formula, then the post cross-section is subtracted because the wood takes up space the concrete does not. Multiply by the number of posts and you have the total, which this calculator converts into cubic yards and 80 pound bags. A typical 10 inch wide, 30 inch deep fence hole with a 4×4 post takes a bit over one cubic foot of concrete, or about two 80 pound bags per hole.
How Deep Should a Post Be?
Depth is driven by two things: the load and the frost line. Fence posts commonly go 24 to 36 inches, deck and structural posts go below the local frost line, which can mean 42 to 48 inches in cold climates. The one-third rule is a handy starting point for fences, burying about a third of the post length, but frost depth overrides it. Set the project type or frost depth above and the calculator fills in a sensible hole depth you can adjust.
Should You Use Gravel and How Many Bags?
A few inches of gravel under the post lets water drain instead of pooling against the wood or under the concrete, which matters most where it freezes. For the concrete itself, buy by the bag: count the holes, size them, and let the calculator total the bags plus a waste margin, since the last hole always seems to need one more bag. It also estimates total bag weight so you know whether the load fits in the truck.
Frequently Asked Questions
How deep should a fence post be? Usually 24 to 36 inches, or below your frost line, whichever is deeper.
How much concrete per post? About 1 to 2 bags of 80 lb concrete for a typical fence post hole.
Should I use gravel? Yes, 2 to 3 inches at the bottom for drainage and to reduce rot and frost damage.
Do I subtract the post? Yes, the post displaces concrete; this calculator subtracts it for a tighter estimate.