Every fence, whatever it is made of, stands or falls on its posts. Panels can be replaced and boards can be swapped, but a post that leans, rots at ground level or lifts in the frost takes the whole run with it. This guide works through the three numbers that decide whether a fence stays put: how many posts you need, how deep each one goes, and how much concrete holds it there. If you would rather have the arithmetic done for you, our fence calculator works out posts, spacing and concrete for a whole fence in one go.
How many posts do you need?
The rule is the same one used for panels: posts = sections + 1. Divide the length of each straight run by your post spacing to get the number of sections, round any fraction up, then add one post for the end of the run. A 60 ft run at 6 ft spacing has 10 sections and needs 11 posts. The same run built at 8 ft spacing has 7.5 sections, which rounds up to 8, so it needs 9 posts — one of the quiet attractions of wider spacing is simply buying and digging for fewer posts.
Spacing is set by what goes between the posts. Pre-built panels dictate 6 ft spacing, because the panel itself is 6 ft wide and has to land on a post at both ends. Stick-built board fences, where individual boards are nailed to two horizontal rails, can stretch to 8 ft spacing because the rails carry the span. Going wider than the system allows is the single most common cause of sagging fences: rails bow, panels rack out of square and gates stop closing. In very windy or exposed sites, do the opposite and halve the spacing instead — drop an 8 ft layout to 4 ft centres, for example — because wind load, not weight, is what a fence post spends its life fighting.
How deep should a fence post be?
The working rule is to bury about a third of the post. For the most common job of all — a 6 ft fence built on 8 ft posts — that means about 2.5 to 3 ft of post in the ground and the rest above it. Shallower than a third and the post has too little to lever against: it works loose in the soil and starts to lean within a few winters. Deeper is rarely wasted effort, particularly at corners and gates, but the one-third rule is the minimum to plan around.
One thing can overrule the one-third rule: frost. The bottom of the hole must sit below your local frost line, so that freezing ground cannot grip the concrete collar and jack the post upward a fraction at a time. That depth ranges from a few inches in the mildest southern states to 3 to 4 ft in the coldest northern regions, so check your local code or a frost-depth map before you dig. Dig the hole about three times as wide as the post — roughly 12 in across for a 4×4 — and tip 4 to 6 in of gravel into the bottom before the post goes in, so water drains away from the end grain instead of pooling against it.
How much concrete per post?
Concrete fills the space between the post and the sides of the hole, so the amount you need is the hole volume minus the post itself. Bag yields are printed on the sack: an 80 lb bag makes about 0.60 cu ft of concrete, and a 60 lb bag about 0.45 cu ft. Divide the volume you need by the yield and round up, because part bags cannot be bought, and a hole that takes more concrete than you have mixed is a miserable surprise halfway through a job.
| Hole size (diameter × depth) | Concrete needed | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 in × 30 in | ≈ 1.2 cu ft | 2 | 3 |
| 12 in × 36 in | ≈ 2.1 cu ft | 4 | 5 |
So a standard post hole takes about two 80 lb bags, and a big hole of the sort dug for a corner or gate post takes about four. On a long run those bags add up fast — ten standard holes is twenty bags before you have started — which is why it pays to total them in advance. Our fence calculator adds up the concrete for the whole fence, not just one hole.
Setting a post that stays straight
Dig the hole, add the gravel, then stand the post in and brace it plumb in both directions, with a level held on two adjacent faces and temporary battens screwed to stakes either side. Fill the concrete in stages rather than one pour, tamping each layer down with a spare length of wood to chase out the air pockets, and check for plumb again after every stage, because a post drifts as the weight goes in around it. Finish by sloping — crowning — the top of the concrete away from the post on all sides, so rainwater runs off instead of sitting against the wood at ground level, which is the exact spot where posts rot.
UK readers will usually meet a different product: fast-set postcrete, sold per post, typically one 20 kg bag per post according to the label, with the water added in the hole. For a single-post repair on a Saturday morning it is hard to beat. For a long new run, though, ordinary bulk concrete works out cheaper per post and gives you more working time before it goes off in the barrow.
Corner and gate posts do the heavy work
Not all posts carry the same load. Line posts mostly just stand there, but a corner post is pulled in two directions at once, a gate post carries the full weight and swing of the gate, and the tension of the whole fence concentrates at exactly these points. Give them a bigger section of post, a deeper and wider hole, and more concrete than the line posts get. Wide gates deserve diagonal bracing too, running from the gate post back into the fence line, so years of swinging cannot slowly lever the post out of plumb.
Counting panels at the same time as posts? Our guide to how many fence panels you need works through both counts together, run by run. And once your layout is settled, run the whole job through the fence calculator one last time — posts, depth and bags — before the delivery arrives.
Frequently asked questions
How deep should a fence post be?
About a third of its length. An 8 ft post in a 6 ft fence needs roughly 2.5 to 3 ft in the ground, and the bottom of the hole must sit below your local frost line — check local code, because that depth runs from a few inches in mild states to 3 or 4 ft in the coldest regions.
How many bags of concrete per fence post?
About two 80 lb bags for a standard hole of 10 in diameter by 30 in deep. A big hole — 12 in by 36 in, the sort used for corner and gate posts — takes about four 80 lb bags, or five 60 lb bags.
Is 8 ft post spacing OK?
Yes, for a stick-built board fence with two rails to carry the span. Pre-built panels need 6 ft spacing because the panel is 6 ft wide and must land on a post at both ends. Spacing wider than the system allows is the top cause of sagging fences.
Can I set posts in gravel instead of concrete?
It is possible for some fences, and gravel does drain well, but posts set in gravel can work loose over time and are harder to set dead plumb. For panel fences, and anything carrying a gate, concrete is the durable standard and the one worth paying for.