A wood picket fence around a backyard, its posts and rails showing

Reference

How deep should a fence post be: depth, spacing and post size by fence height

A fence fails at the posts, not the boards. Depth decides whether it leans after the first winter; spacing decides whether the rails sag; post size decides whether an 8 ft privacy fence survives wind.

How deep should a fence post be? A third of the post's length — half the fence height — and below the frost line: 36 in for a 6 ft fence on a 9 ft post (buy 10 ft and trim), in a 12 in hole dug 42 in deep for the gravel under it. Posts go 8 ft apart for most styles.

How deep should a fence post be: depth chart by fence height

Fence height	Post depth	Hole depth, with gravel	Post length to buy	Hole diameter
4 ft	24 in · 61 cm	30 in	6 ft	12 in
5 ft	30 in · 76 cm	36 in	8 ft	12 in
6 ft	36 in · 91 cm	42 in	10 ft	12 in
8 ft	48 in · 122 cm	54 in	12 ft	16 in

The rule behind the chart: a third of the post's overall length goes in the ground — half the fence height — with 6 in of gravel under the post, so the hole is 6 in deeper than the post sits. Where the ground freezes, go below the frost line: 36 to 60 in or more across the northern states — check your county's frost depth. Many codes accept 24 in for a fence up to 6 ft; that is a minimum, not a target.

Fence post spacing by style

Style	Post spacing	Rails, 4 / 6 / 8 ft
Privacy (solid board)	8 ft on center	2 / 3 / 4
Picket	8 ft on center	2 / 3 / 3
Shadowbox (board on board)	8 ft on center	2 / 3 / 4
Split rail	10 ft on center	2 / 3 / 3
Chain link	10 ft on center	1 / 1 / 1

Eight feet matches the rail lumber: a 16 ft 2×4 spans two bays with no waste. Go to 6 ft in open, windy country or for a solid 8 ft fence: each post then carries less of the wind.

Post size by height and wind

Fence height	Post	Note
4 ft	4×4	4×4 at 8 ft spacing
6 ft	4×4	4×4 at 8 ft; 6×6 in open, windy sites or for gate posts
8 ft	6×6	6×6 — an 8 ft solid fence is a sail

Gate posts carry the gate's weight on a lever arm: use a 6×6 (or a 4×4 set 6 in deeper in concrete) for any gate over 4 ft wide, and set both gate posts in concrete even on a gravel-set fence.

How much concrete per fence post

Hole volume minus the post, over 6 in of gravel at the bottom for drainage. Per hole, before waste — the figures the post hole calculator gives for the same inputs.

Fence	Post	Hole	Concrete	40 lb bags	60 lb bags	80 lb bags
4 ft	4×4	12 in × 30 in deep	1.4 cu ft	5	4	3
5 ft	4×4	12 in × 36 in deep	1.75 cu ft	6	4	3
6 ft	4×4	12 in × 42 in deep	2.1 cu ft	8	5	4
8 ft	6×6	16 in × 54 in deep	4.74 cu ft	16	11	8

Count posts, rails and pickets →

How long a wood fence lasts

Material	Years
Pressure-treated pine	15–20
Western red cedar	15–30
Redwood	20–30
Untreated pine or spruce	5–10
Vinyl	20–30
Chain link, galvanized	20–30

The posts go first, at the ground line where wet and dry alternate. Gravel under the post, concrete crowned to shed water, and sealing the cut end of each post add years to any of these.

Post depth and hole size: Quikrete Setting Posts project guide — a third of the post's overall length in the ground, plus 6 in of gravel under it, in a hole about three times the post width, and below the frost line; many local codes set a 24–30 in minimum for fences. Spacing and rails: fence-manufacturer installation guides (privacy and picket 8 ft, split rail and chain link 10 ft). Lifespan: university extension services (pressure-treated pine 15–20 years, cedar 15–30, untreated pine 5–10 in ground contact). Concrete per post computed with the post hole calculator on this site from Quikrete bag yields (40 lb 0.3 cu ft, 60 lb 0.45, 80 lb 0.6). Accessed 2026-10-17.

Fence post questions

How long should a fence post be for a 6 foot fence?

9 ft — 6 ft above ground plus 36 in in the ground — so buy 10 ft and cut the top after setting rather than trying to set to an exact height. An 8 ft post set 24 in deep is the code minimum, and leans sooner.

How many bags of concrete per fence post?

About 4 bags of 80 lb (5 of 60 lb) for a 4×4 in a 12 in hole 42 in deep; 8 bags of 80 lb for a 6×6 in a 16 in hole. See the table above.

Should fence posts be set in concrete or gravel?

Concrete for privacy fences, corners and gates; gravel drains better and suits split rail and picket where wind load is small. Either way, gravel under the post.

How far apart should fence posts be?

8 ft for board fences, 10 ft for split rail and chain link. Closer in high wind.

Put these figures to work

These numbers are the inputs every concrete calculator on the site runs on — slabs, footings, steps and the posts set in them. Each tool applies them to one part of the job.

Tools for the job

What makes this job easier

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