Home › Fence post depth calculator · Updated November 3, 2026 · Sources
Fence Post Depth Calculator
Enter the fence height, the post's actual width and how many posts. You get the hole depth and width, the post length to buy, the concrete per post, and a drawing to check it against.
Order options waste, bags, prices, gravel, rebar
Fence post depth chart
One post, no frost line entered. Concrete stops 3 in below grade; bags are before any extra.
| Fence height | Post | Post buried | Hole depth | Hole width | Post length | 80 lb bags | 50 lb bags |
|---|---|---|---|---|---|---|---|
| 3 ft | 4×4 | 18 in | 24 in | 10.5 in | 4 ft 6 in (buy 6 ft) | 1.1 | 1.7 |
| 3 ft | 6×6 | 18 in | 24 in | 16.5 in | 4 ft 6 in (buy 6 ft) | 2.7 | 4.2 |
| 4 ft | 4×4 | 24 in | 30 in | 10.5 in | 6 ft (buy 6 ft) | 1.5 | 2.4 |
| 4 ft | 6×6 | 24 in | 30 in | 16.5 in | 6 ft (buy 6 ft) | 3.7 | 5.9 |
| 5 ft | 4×4 | 30 in | 36 in | 10.5 in | 7 ft 6 in (buy 8 ft) | 1.9 | 3.1 |
| 5 ft | 6×6 | 30 in | 36 in | 16.5 in | 7 ft 6 in (buy 8 ft) | 4.8 | 7.6 |
| 6 ft | 4×4 | 36 in | 42 in | 10.5 in | 9 ft (buy 10 ft) | 2.4 | 3.8 |
| 6 ft | 6×6 | 36 in | 42 in | 16.5 in | 9 ft (buy 10 ft) | 5.8 | 9.3 |
| 8 ft | 4×4 | 48 in | 54 in | 10.5 in | 12 ft (buy 12 ft) | 3.2 | 5.2 |
| 8 ft | 6×6 | 48 in | 54 in | 16.5 in | 12 ft (buy 12 ft) | 8.0 | 12.7 |
Lumber sizes are nominal: a 4×4 post is 3½ × 3½ in and a 6×6 is 5½ × 5½ in. Enter the actual width.
How it is worked out
- Buried depth = the larger of half the height above ground, or your frost depth.
- Hole depth = buried depth + 6 in of gravel under the post for drainage.
- Hole width = 3 × the post width.
- Concrete = the round hole minus the post, from the gravel up to 3 in below grade. The top is backfilled with soil.
Check: QUIKRETE's fast-setting mix sheet says two 50 lb bags set a 4 in post in a 10 in hole 2 ft deep. The same formula gives 1.93 bags.
Setting the posts
- Dig to the hole depth on the ticket, about three times the post width across.
- Add 6 in of gravel and tamp it.
- Stand the post, plumb it on two faces and brace it.
- Fill with concrete to about 3 in below grade, following the bag instructions.
- Top up with soil once the concrete has set.
Questions
How deep should a 6 ft fence post be?
For a 6 ft fence on 4×4 posts, bury 36 in of post and dig the hole 42 in deep, which leaves room for 6 in of gravel underneath. That makes the post 9 ft long, so buy 10 ft posts. Go deeper if your frost line is lower.
What is the rule for fence post depth?
Bury one third of the post’s overall length, which works out to half the height above ground, plus 6 in of gravel under the post. The hole should also reach at least 6 in below the frost line. This follows QUIKRETE’s post-setting guide.
How wide should a fence post hole be?
About three times the post width. A 4×4 post is 3½ in actual, so the hole is about 10½ in: a 10 or 12 in auger. A 6×6 (5½ in actual) needs about 16½ in.
How many bags of concrete per fence post?
For a 6 ft fence on a 4×4 post, about 2.4 bags of 80 lb mix, or 3.8 bags of 50 lb fast-setting mix, per post, with the concrete stopping 3 in below grade. Round up for the whole fence; the ticket does this for you.
What is the frost line and where do I find mine?
It is how deep the ground freezes in winter. Posts set above it can heave. Your local building department can tell you the frost depth for your area; enter it in the calculator and the hole depth adjusts.
Can I pour the concrete dry?
Fast-setting mixes are made for that: QUIKRETE’s instructions say to pour the dry mix to 3–4 in below the top, then add about a gallon of water per 50 lb bag. Standard mix should be mixed with water first.
Guides: How to set fence posts in concrete
Sources
Depth, gravel, hole width and fill level: QUIKRETE, Setting Posts project guide. Fast-setting yield and the two-bag example: QUIKRETE Fast-Setting Concrete Mix data sheet. Standard mix yields: QUIKRETE Concrete Mix No. 1101. Depth range of one third to one half: HGTV.