Structures · Guide
Fence Post Spacing and Depth: Planning the Layout Before You Buy
The posts decide everything else about a fence. Work out how many, how far apart and how deep before you buy a single board.
A fence is a row of posts with something hung between them. The boards and rails are the part you see, but the posts carry the wind, hold the line and decide how long the fence lasts. Three choices about the posts fix everything else: how far apart they are, how deep they go, and how long they need to be.
How far apart
For wood fences, posts go 6 to 8 feet apart, measured from the center of one post to the center of the next.
| Spacing | Use it for |
|---|---|
| 8 ft (2.4 m) | Standard fences up to 6 ft, ready-made panels, rails sold in 8 ft lengths |
| 6 ft (1.8 m) | Tall privacy fences, windy sites, heavy boards, soft ground |
| 4 to 5 ft | Short runs between fixed points, or beside a gate |
Eight feet is the most common because lumber comes in 8 foot lengths and most ready-made panels are 8 feet wide. It is also the widest spacing that works. Rails longer than that sag, and a solid 6 foot fence at 8 foot spacing catches a great deal of wind.
Closer spacing costs more posts and more digging, and gives a stiffer fence. If your site is exposed, or the fence is solid and tall, 6 foot spacing is money well spent.
One caution about panels. A panel sold as “8 feet” fits between posts set exactly 8 feet apart, center to center, or sometimes 8 feet clear between the posts. Measure the actual panel before you dig.
Dividing the line evenly
Fence runs almost never divide exactly by 8. If you simply measure 8 feet, 8 feet, 8 feet from one end, you finish with a short, odd-looking section at the other. Divide the run evenly instead.
Example. A run of 57 feet with a maximum spacing of 8 feet:
- Sections: 57 ÷ 8 = 7.1, so 8 sections
- Actual spacing: 57 ÷ 8 = 7 feet 1½ inches
Eight equal bays of just over 7 feet look intended. Seven bays of 8 feet and one of 1 foot do not. The fence calculator reports this even spacing for you.
Do this separately for each straight run between corners, gates and ends, because those posts are fixed.
Counting posts
Then adjust:
- Corners. Two runs that meet at a corner share the corner post. Count it once.
- Gates. A gate needs a post on each side. If you treat the gate opening as a gap in the run, that adds one post per gate.
- A closed loop. If the fence goes all the way around and joins itself, the number of posts equals the number of sections.
How deep
The traditional rule is that one third of the post should be in the ground. Put another way, the hole is half as deep as the fence is tall.
| Fence height | Hole depth | Total post needed | Post to buy |
|---|---|---|---|
| 3 ft | 2 ft | 5 ft | 6 or 8 ft |
| 4 ft | 2 ft | 6 ft | 8 ft |
| 5 ft | 2 ft 6 in | 7 ft 6 in | 8 ft |
| 6 ft | 3 ft | 9 ft | 10 ft |
| 8 ft | 4 ft | 12 ft | 12 ft |
Two things override the rule.
A minimum of 2 feet. Even a low fence needs 2 feet of post in the ground to stay upright.
The frost line. Where the ground freezes, the bottom of the hole should be below the depth to which it freezes. Wet soil expands as it freezes and grips the post, and each winter lifts it a little, a process called frost heave. Frost depth ranges from nothing in warm regions to 4 feet or more in cold ones. Your building department knows the local figure.
If frost depth is greater than the one-third rule, dig to the frost depth and buy longer posts.
How wide
Make the hole about three times the width of the post.
- 4 × 4 post (3.5 inches actual): a 10 to 12 inch hole
- 6 × 6 post (5.5 inches actual): a 16 to 18 inch hole
Keep the sides straight and the bottom at least as wide as the top. A hole that narrows toward the bottom, like an ice cream cone, is easy for frost to lift. A hole that is slightly wider at the bottom, like a bell, resists it.
Put 4 to 6 inches of gravel in the bottom of the hole before the post goes in. It lets water drain away from the end grain, where rot starts. Dig that much deeper to allow for it.
Concrete, gravel or soil?
Concrete gives the stiffest result and is the standard choice for gate posts, corner posts and tall privacy fences. Fast-setting mix is poured dry into the hole around the post and soaked with water, with no mixing needed. Slope the top of the concrete away from the post so water runs off.
Compacted gravel drains better than concrete and makes a post easier to replace later. Add crushed stone in layers of a few inches and tamp each layer hard with a bar. It suits firm soils and lower fences.
Tamped soil is the traditional method for farm fencing. It works in firm clay, with patient tamping, and poorly in sand or loose fill.
How much concrete? For a 4 × 4 post in a 10 inch hole:
| Hole depth | Concrete per post | 50 lb bags of fast-setting mix |
|---|---|---|
| 24 in | 0.92 cu ft | 3 |
| 30 in | 1.15 cu ft | 3 to 4 |
| 36 in | 1.38 cu ft | 4 |
| 48 in | 1.84 cu ft | 5 |
These figures subtract the volume of the post itself. On a long fence the total is larger than people expect. Twenty posts at 36 inches is about 75 bags.
Gate posts carry more
A gate hangs its whole weight off one post, and swings on it thousands of times. Gate posts should be:
- Larger. Use 6 × 6 where the line posts are 4 × 4, at least on the hinge side.
- Deeper. Go 6 inches to a foot deeper than the line posts.
- Set in concrete, whatever you use elsewhere.
- Left to cure. Give the concrete several days before hanging the gate.
Setting out the line
- Find the boundary. Know exactly where your property line is, and check local rules on how close to it a fence may stand. Many places require the finished face to point outward.
- Have utilities marked. In the US, call 811 a few days before digging. It is free, and fence post holes are just the depth at which cables and pipes are buried.
- Set the end and corner posts first. Get them plumb and braced.
- Run a string between them, near the top and near the bottom. Every other post lines up to the string.
- Mark the line posts along the string at your even spacing.
- Dig, set and brace each post, checking it is plumb in both directions.
- Leave the tops long. Set all posts a little tall, and cut them to a level line once the concrete has cured.
Slopes
On sloping ground you have two choices. A stepped fence keeps each section level and drops at each post, leaving triangular gaps underneath. A racked fence follows the slope, with rails parallel to the ground and boards still vertical.
Either way, measure the run along the ground, not horizontally, when counting boards. And on a stepped fence, the posts on the downhill side of each step need to be longer. Buy a few posts in the next length up.
Buying the posts
- Rated for ground contact. The tag on the end of the post says so. Lumber treated only for above-ground use rots quickly once buried.
- Straight. Sight down each one. A twisted post makes a twisted fence.
- Long enough. Fence height, plus hole depth, plus a few inches to trim.
- Naturally durable alternatives. Cedar and redwood resist rot without treatment, though they cost more and are still best set on gravel.
With the post count, depth and length settled, the rest of the fence is arithmetic: rails per section, boards per foot, and screws per board. The calculator handles that part once the posts are decided.
Written and checked by Bertha
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