Fence Calculator
Posts, rails, pickets and bags of concrete for a fence run, with the post hole volume worked out properly rather than guessed at.
How many fence posts do I need?
Divide the total run by your post spacing to get the number of sections, then add one. A straight fence always needs one more post than it has sections, because there is a post at both ends. A 100 foot fence at 8 foot spacing is 13 sections and 14 posts.
Then add two posts for every gate. Gates hang off dedicated posts on both sides, and those posts want to be heavier and set deeper than line posts because they carry a swinging load.
Standard spacing is 8 feet for wood fences and 10 feet for chain link. Going wider than 8 feet on a wood privacy fence means the rails start to sag in the middle, especially with a 6 foot height catching wind.
Pickets depend on the gap
Picket count comes from the pitch, meaning the picket width plus whatever gap you leave between them. A standard dog ear picket is 5.5 inches wide.
- Solid privacy with no gap: pitch is 5.5 inches, so 100 feet of fence needs 1,200 divided by 5.5, which is 219 pickets.
- Spaced picket with a 2.5 inch gap on a 3.5 inch picket: pitch is 6 inches, so 100 feet needs 200 pickets.
- Shadowbox alternates pickets on both sides of the rails. The pitch looks wide from either side but you are buying roughly the same number as a solid fence, since you are covering both faces.
Buy about 5 percent extra. Fence pickets are a natural product and a few in every bundle will be split, badly cupped or have a knot in exactly the wrong place.
Concrete per post, done properly
Most fence calculators just say two bags per post. The real number depends on your hole size, and getting it right on a long fence is worth real money.
The volume you need is the hole minus the post that sits in it. A 10 inch diameter hole 30 inches deep holds about 1.36 cubic feet. A 4x4 post, which is actually 3.5 inches square, occupies about 0.21 cubic feet of that depth. So you need roughly 1.15 cubic feet of concrete per post.
An 80 pound bag of concrete mix yields about 0.6 cubic feet, so that is just under two bags per post. A 60 pound bag yields 0.45, so about two and a half. For a 14 post fence you are looking at around 27 of the 80 pound bags.
| Hole size | Volume | Concrete per post | 80 lb bags |
|---|---|---|---|
| 8 in wide, 24 in deep | 0.87 cu ft | 0.70 cu ft | about 1.2 |
| 10 in wide, 30 in deep | 1.36 cu ft | 1.15 cu ft | about 1.9 |
| 12 in wide, 36 in deep | 2.36 cu ft | 2.10 cu ft | about 3.5 |
| 12 in wide, 42 in deep | 2.75 cu ft | 2.45 cu ft | about 4.1 |
How deep do posts go?
Two rules, and you follow whichever gives the deeper hole.
One third of the post length. A 6 foot fence uses 8 foot posts, so 2.5 to 3 feet in the ground. This is the structural rule and it is about resisting wind load.
Below the frost line. This is the one people in northern states skip and regret. When saturated soil freezes it expands and lifts anything in it. A post set above the frost line gets jacked out of the ground a little more every winter until the fence is visibly uneven. Frost depth runs from almost nothing in the Gulf states to 42 inches or more across the northern tier. Your local building department publishes the number.
Hole diameter should be about three times the post width, so a 4x4 post wants a 10 to 12 inch hole. Narrower than that and there is not enough concrete around the post to develop any resistance.
Before you dig
Call 811 at least a few days before you start. It is free, it is the law in every state, and hitting a gas line or fibre trunk with a post hole digger is a genuinely serious event. Mark your dig area and wait for the utilities to come out and paint.
Also worth doing before you buy anything: confirm the property line, ideally from a survey rather than from where the last fence was. Fences built a foot onto a neighbour's land come down at the builder's expense, and that is a conversation nobody wants to have after the concrete has cured.
Common questions
How many posts do I need for a 100 foot fence?
At 8 foot spacing, 13 sections and therefore 14 posts for a straight run, since you need a post at both ends. Add two more for each gate. Chain link at 10 foot spacing needs 11 posts for the same run.
How many bags of concrete per fence post?
For the common 10 inch wide, 30 inch deep hole with a 4x4 post, about 1.15 cubic feet, which is just under two 80 pound bags or about two and a half 60 pound bags. Bigger holes need considerably more, a 12 by 42 inch hole takes about four 80 pound bags.
How deep should fence posts be?
One third of the post length, or below your local frost line, whichever is deeper. For a 6 foot fence on 8 foot posts that is 2.5 to 3 feet minimum. In northern states the frost line can push that to 42 inches or more.
How far apart should fence posts be?
8 feet for wood fences and 10 feet for chain link. Going wider on a wood privacy fence lets the rails sag in the middle, particularly at 6 feet tall where the fence catches a lot of wind.
How many pickets do I need for a privacy fence?
Divide the fence length in inches by the picket pitch, which is the picket width plus any gap. With standard 5.5 inch pickets butted tight, a 100 foot fence needs about 219 pickets. Buy 5 percent extra for splits and bad boards.
These figures are planning estimates. Bulk material weight shifts with moisture and screen size, and suppliers round differently, so confirm the final quantity with your yard before you order.