Enter the height of your fence to see how deep the posts go, how wide to dig the holes, what length of post to buy and how many bags of concrete the whole run needs.
How to use the fence post depth calculator
- Enter the finished fence height above the ground.
- Pick the post shape and enter its real width. A 4×4 measures 3.5 in, not 4.
- Keep the one-third rule for line posts. Switch to one-half for gate posts, corner posts, tall privacy fences in windy spots and loose or sandy soil.
- If your area has a frost line, enter it. The calculator uses whichever depth is greater.
- Enter the number of posts and the bag size to get the concrete order.
Formulas
Depth in ground = fence height × 1/3 (or 1/2), never less than 24 in or the frost depth Hole diameter = 3 × post width Hole depth = depth in ground + gravel base Post length = fence height + depth in ground, rounded up to the next 2 ft Concrete per post = hole volume − post volume Bags = concrete for all posts ÷ bag yield, rounded up
The one-third rule and the three-times-the-post-width hole are the long-standing rules of thumb used by fence builders and repeated on concrete bag instructions. They are a starting point: a frost line, a local code or a manufacturer’s instructions always override them.
Worked example: a 6 ft privacy fence on 4×4 posts
- Depth: 72 in × 1/3 = 24 in, which meets the 24 in minimum, so the posts go 24 in into the ground.
- Hole: 3 × 3.5 in = 10.5 in across, and 24 + 6 in of gravel = 30 in deep.
- Post length: 6 ft + 2 ft = 8 ft posts.
- Concrete: the hole holds 1.20 ft³ over 24 in, the post takes up 0.17 ft³, leaving 1.03 ft³ per post.
- Ten posts: 10.3 ft³ ÷ 0.375 ft³ per 50 lb bag = 27.5, so buy 28 bags.
Quick reference
| Fence height | Depth at 1/3 | Depth at 1/2 | Post to buy at 1/3 |
|---|---|---|---|
| 4 ft | 24 in | 24 in | 6 ft |
| 5 ft | 24 in | 30 in | 8 ft |
| 6 ft | 24 in | 36 in | 8 ft |
| 8 ft | 32 in | 48 in | 12 ft |
Questions
How deep should a fence post be for a 6 foot fence?
At least 24 in for line posts by the one-third rule, and 36 in for gate and corner posts or windy sites. In cold climates the bottom of the post should also reach the local frost line.
How wide should a fence post hole be?
About three times the width of the post: 10 to 12 in for a 4×4 and around 16 in for a 6×6. That leaves enough concrete around the post to resist leaning.
Do fence posts need gravel at the bottom?
A few inches of gravel under the post lets water drain away from the end grain, which is where wood posts rot first. Six inches is a common allowance.
How many bags of concrete per fence post?
A 4×4 post set 24 in deep in a 10.5 in hole takes about 1 ft³ of concrete: roughly three 50 lb bags of fast-setting mix or two 80 lb bags.
Sources
- QUIKRETE Fast-Setting Concrete Mix (No. 1004) and Concrete Mix (No. 1101) product data sheets, The QUIKRETE Companies. Used for: bag yields of 0.375 ft³ (50 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb).