Fence Post Calculator
Use this fence post calculator to work out how many posts a run needs, how much concrete each hole takes with the post displacement removed, and what it all costs.
Posts in a Run
Concrete Per Hole
The hole, less the post that sits in it.
Fence Post Cost
How to use the Fence Post Calculator
Start with Layout: the run length and the spacing give you the post count, plus the rails and the extra posts that gates and corners need.
Then Concrete: enter the hole size and the post, and you get the concrete per hole with the post displacement already taken out, in bags of each common size.
The usual rules of thumb are built into the field help, hole diameter three times the post width, depth one third of the post length, and always below your local frost line.
Fence Post Cost adds the posts and the concrete. Bag size is worth a thought: 80 lb bags are cheaper per pound but heavier to carry down a fence line, and the hole count decides how many trips that turns into.
Fence Post formula
posts = floor(length ÷ spacing) + 1sections = posts − 1hole volume = π × (diameter ÷ 2)² × (depth − gravel base)concrete per hole = hole volume − post cross-section × concrete depth
A run always needs one more post than it has sections, for the same reason a fence has two ends.
| Post | Actual size | Cross-section | Suggested hole |
|---|---|---|---|
| 4 × 4 wood | 3½ × 3½ in | 12.25 sq in | 10–12 in |
| 6 × 6 wood | 5½ × 5½ in | 30.25 sq in | 16–18 in |
| 4 in round | 4 in dia | 12.57 sq in | 10–12 in |
| 2⅜ in steel | 2.375 in dia | 4.43 sq in | 8–10 in |
Worked example
A 100 ft fence at 8 ft spacing, 10 in holes 30 in deep, 4 × 4 posts on 4 in of gravel:
- Sections: 100 ÷ 8 = 12 whole sections
- Posts: 12 + 1 = 13, plus 2 for one gate = 15 posts
- Concrete depth: 30 − 4 = 26 in
- Hole volume: π × 5² × 26 = 2,042 in³
- Post displacement: 12.25 × 26 = 319 in³
- Concrete per hole: 1,723 in³ = 1.00 ft³
- At 0.375 ft³ per 50 lb bag: 3 bags per hole
Ignoring the post displacement would have called for 1.18 ft³ a hole, about half a bag more each, and eight bags across the fence.
What the result means
The post count is one more than the section count. A hundred feet at eight foot spacing is twelve and a half sections, so you get twelve full bays and a short one, and thirteen posts before gates and corners.
Gate posts are not line posts. A gate hangs off one post and latches on another, and both take a load a line post never sees. Size them up, set them deeper, and expect the gate side to be the part that eventually moves.
Gravel under the post is not optional in most soils. A post set in a sealed concrete cup sits in water. Four inches of gravel lets the hole drain, and it comes off the concrete depth, which is why it is an input here.
Common mistakes
- Forgetting the last post. Thirteen posts, not twelve, for a hundred feet at eight foot centres.
- Not deducting the post from the hole. Over-orders by roughly a fifth on a 4 × 4 in a 10 in hole.
- Setting shallower than the frost line. The fence will lift. Depth is set by your local frost depth, not by the rule of thumb.
- Concrete right to the top, flush with grade. Crown it above grade so water runs away from the post rather than pooling against it.
- Ignoring the gate swing when spacing. Lay out the gate opening first and let the line posts fall where they fall.
Frequently asked questions
How many fence posts do I need for 100 feet?
Thirteen at eight foot spacing, twelve sections plus one, because a run needs a post at each end. Add two more for each gate and allow for corner posts, which are usually heavier than line posts.
How many bags of concrete per fence post?
For a 4 by 4 post in a 10 inch hole 30 inches deep on a 4 inch gravel base, about one cubic foot per hole, which is three 50 pound bags, two 60 pound bags, or one and a half 80 pound bags. Bigger posts and deeper holes rise quickly from there.
How deep should fence post holes be?
A third of the post length is the usual rule, so a 6 foot fence on 9 foot posts sets 3 feet deep. That rule is overridden by your local frost line, the hole has to go below it, whatever the arithmetic says, or the post will heave.
How wide should a fence post hole be?
About three times the post width. A 3.5 inch 4 by 4 wants a 10 to 12 inch hole. Narrower gives the concrete too little collar to resist the leverage of the fence; much wider just costs concrete.
Should I put gravel under a fence post?
Yes in most soils. Three to four inches of gravel lets water drain out of the hole instead of standing against the end grain. It also comes off the concrete depth, so it reduces the number of bags slightly.
What spacing should fence posts be?
Eight feet is standard for wood fencing, since rails come in eight foot lengths. Six feet suits heavier panels, tall privacy fences and windy or sloping sites. Never exceed the rail length you are buying.
Sources and assumptions
Post counts assume one post at each end of the run and even spacing between. Gate and corner allowances are added on top and are user-entered.
Actual post dimensions follow the American Softwood Lumber Standard PS 20 for surfaced lumber.
Bag yields are the standard published figures for pre-mixed concrete: 0.375 ft³ for a 50 lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb.
Hole depth must satisfy the local frost depth and any applicable code, which this calculation does not know. Check before digging.