The rule comes from a concrete maker's instructions. Set the hole's depth at 50 percent of the post's height above ground, and make the hole about 3 times the post width across. Fast-setting concrete comes in bags, and a bag yields 0.375 cubic feet. The calculator adds the concrete for all posts and rounds up to whole bags, so buy the bag size the result names.
How to use this calculator
- Measure the post's height above ground, from the finished ground to the top of the post. Leave out the buried part, because the calculator adds the hole depth.
- Enter the post's actual width, not the name on the tag. A 4x4 is smaller than its name, and the starting value is its actual width.
- Enter the number of posts, counting corners, ends and gate posts. If you have not counted yet, work out how many posts a run needs from its length and spacing.
- Open More options to enter the local frost depth if you know it. The calculator uses whichever is deeper, the rule depth or the frost depth.
- Before you dig, call the utility locate service and wait for every buried line to be marked.
- Round the post length up to the next length the lumber yard sells.
Worked examples
Short garden fence
Post height above ground: 4 ft · Post width: 3.5 in · Number of posts: 8
- Hole depth
- 24 in
- 50 lb bags of fast-setting concrete
- 23
- Hole diameter
- 10.5 in
- Post length to buy
- 6.0 ft
- Concrete per hole
- 1.03 cu ft
Backyard privacy fence
Post height above ground: 6 ft · Post width: 3.5 in · Number of posts: 12
- Hole depth
- 36 in
- 50 lb bags of fast-setting concrete
- 50
- Hole diameter
- 10.5 in
- Post length to buy
- 9.0 ft
- Concrete per hole
- 1.55 cu ft
Corner post in a cold climate
Post height above ground: 6 ft · Post width: 5.5 in · Local frost depth: 42 in
- Hole depth
- 42 in
- 50 lb bags of fast-setting concrete
- 12
- Hole diameter
- 16.5 in
- Post length to buy
- 9.5 ft
- Concrete per hole
- 4.46 cu ft
Tall fence on a windy lot
Post height above ground: 8 ft · Post width: 5.5 in · Number of posts: 6
- Hole depth
- 48 in
- 50 lb bags of fast-setting concrete
- 82
- Hole diameter
- 16.5 in
- Post length to buy
- 12.0 ft
- Concrete per hole
- 5.10 cu ft
Bags of fast-setting concrete per post for common heights and widths
| Post height above ground | Post width: 3.5 in | Post width: 5.5 in |
|---|---|---|
| 3 ft | 3 | 6 |
| 4 ft | 3 | 7 |
| 6 ft | 5 | 11 |
| 8 ft | 6 | 14 |
Each cell: 50 lb bags of fast-setting concrete. Inputs not in the table use the calculator's starting values.
Why hole depth matters
Wind pushes on every panel a post holds. A shallow hole lets the post lean and rock, and soft soil makes that worse.
Gate posts often need a deeper or larger hole than line posts, because the gate's weight and swing pull on them. For a gate, follow the gate maker's instructions and treat this result as a starting point.
How the calculation works
The depth is the larger of the rule's share of the height above ground and the frost depth you enter. The post length to buy is the height above ground plus that depth.
Concrete per hole is the hole's volume less the square post in the middle. The bag count multiplies that by the number of posts and rounds up. For round forms rather than dug holes, use the round form calculator.
Frost and the building department
Where the ground freezes, the hole must reach below the frost line, or freezing soil can lift a post over winter. The local building department knows the frost depth for your area, so ask before you dig.
Some installers put gravel in the bottom of the hole. That layer sits inside the depth shown, so work out how much gravel it needs.
Mistakes to avoid
- Entering the name on the tag instead of the actual width, which makes the hole and the concrete too large.
- Buying the post at the height above ground. Buy the post length the calculator shows instead.
- Skipping the frost depth in a cold region. The rule alone can leave the bottom of the post above the frost line.
- Digging before the utility locate service has marked the lines.
- Using a line post hole for a gate without checking the gate maker's instructions.
Common questions
What size hole do I need for a 4x4 post?
Measure the post itself, because a 4x4 is smaller than its name. The hole diameter is about 3 times the post width, and the depth is 50 percent of the post's height above ground.
How many bags of concrete do I need per fence post?
Enter the height, the actual width and any frost depth. Each bag of fast-setting concrete yields 0.375 cubic feet, and the calculator rounds the total for all posts up to whole bags.
How are fence posts measured?
A post is sold by its full length and named by its nominal size, such as a 4x4. Enter only the height above ground, the part you can see, and the calculator adds the hole depth to give the length to buy.
How deep should a wood fence post be?
A concrete maker's rule sets the depth at 50 percent of the post's height above ground. Where the ground freezes, the hole must reach below the frost line, so enter that depth under More options.
Is the one-third rule for fence post depth right?
Quikrete's guide says to bury 33.3 percent of the post's overall length, plus a 6 inches gravel base. A third of the full length works out to 50 percent of the height above ground, which is this calculator's rule. Where the ground freezes, use the frost depth if it is deeper.
What size hole do I need for a chain link line post?
Measure across the pipe and enter that as the post width. The hole is about 3 times the post width across, and chain link line posts are typically 1.625 to 2 inches across. For footing depth, the industry minimum is 24 inches, plus 3 inches per foot of fence height over 4 ft.