Fence Post Calculator
Posts are the part of a fence that fails first, and almost always because the hole was too small or too shallow. Enter the run and the spacing, get the post count and the exact number of concrete bags.
Reviewed · Method: measured area + industry waste allowance, rounded up to real package sizes · How the numbers are calculated
Enter your dimensions and hit Calculate my posts. You will get the quantity, the buyable quantity, a shopping list, and a budget range.
Figure. Post count comes from the run length divided by spacing, plus one end post. Concrete comes from hole volume, not from post count.
Figure. One third of the post goes in the ground, and the bottom sits in gravel so water drains away from the wood.
The one-third rule
A post needs roughly a third of its length in the ground to resist lateral load. For a six foot fence that means a 24 to 30 inch hole and an eight foot post. For an eight foot fence it means a 30 to 36 inch hole and a ten foot post. Using an eight foot post for an eight foot fence leaves four inches of burial, which is why so many fences lean after the first windy winter. The burial depth also has to clear the local frost line; in cold climates the hole must be deeper than the frost penetrates, or the ground heaves the post upward every spring.
Spacing: eight feet is standard, and why
Eight feet is the default because rails and panels are commonly sold in eight foot lengths, so spacing at eight feet produces no waste. Six feet gives a stiffer fence and is worth it for privacy panels, gate posts, and anything exposed to wind. Ten feet looks lighter and saves posts but visibly sags unless you use thicker rails. Whichever you pick, the calculator adds one, because a run needs a post at both ends, not just at each interval.
Hole size and concrete quantity
A hole should be about three times the width of the post, so a 4x4 post wants a 10 to 12 inch hole. That gives room to tamp the concrete around all four faces rather than only two. An 80 lb bag of concrete mix yields about 0.6 cubic feet, which fills a 10 inch hole about 30 inches deep with a little to spare. Set the first four inches of the hole with gravel so water drains away from the base of the wood instead of sitting against it.
Bracing before the concrete sets
Wet concrete holds a post only as well as the post is held while it cures. Brace every post with two diagonal props, check plumb on two adjacent faces rather than one, and leave the braces for at least 24 hours. Corner posts and gate posts carry the most load and are worth setting in a wider hole with more concrete than the run posts. If a gate is going in, hang it after the concrete has had a full week, not the next morning.
Frost depth decides your hole
In warm climates 24 to 30 in is enough. In cold climates the hole has to go below the frost line, which can be 36 to 48 in. A post set above it moves with the freeze-thaw cycle and takes the fence with it.
A deeper hole means more concrete, and that is the line item people underestimate most. Twelve inches of extra depth across 14 posts is several extra bags.
Concrete, foam or tamped gravel
Concrete is the default for a reason: it is strong, cheap and does not move. Expanding foam is faster and easier to carry but costs more per hole and relies on the hole being dry. Tamped gravel drains best and is the traditional choice where frost heave is severe.
Whichever you use, crown the fill slightly above grade so water runs away from the post rather than pooling against it.
Worked example: 100 ft privacy fence on 8 ft centres
| Fence run | 100 ft |
| Bays | 100 / 8 = 12.5, so 13 bays |
| Posts | 13 + 1 = 14 |
| Hole volume | 10 in diameter x 30 in deep, about 1.4 cu ft each |
| Concrete | 22 cu ft = 37 bags of 80 lb |
| Rails at 3 per bay | 39 pieces of 8 ft |
| Budget band | $663 low, $1,070 mid, $1,752 high |
Mistakes that cost real money
Setting posts without gravel at the base
Concrete holds the post, gravel drains the water. A post sitting directly on soil or on a concrete plug collects water at the end grain, which is exactly where rot starts. Four inches of gravel at the bottom of every hole is cheap insurance.
Using an 8 ft post for an 8 ft fence
If 30 in goes in the ground, an 8 ft post leaves about 5.5 ft above grade. Buy the post length for the finished height plus the embedment, not the other way around.
Skipping diagonal bracing on corners
Corner and gate posts take far more load than line posts. Brace them diagonally before the concrete sets, or use a thicker post and a larger hole. A leaning gate is almost always a corner post problem.
Ignoring the frost line
In cold regions, a 30 in hole can sit above the frost line and heave. Check the local frost depth and set the hole below it; otherwise the fence lifts a little every winter and never comes back down straight.
Forgetting gates and terminals
Every gate needs two posts sized for the span, and a gate wider than 4 ft needs a frame, not just hinges on a 4x4. Gates are the highest-load point on any fence and the first thing to fail.
Frequently asked questions
How deep should a fence post be buried?
About one third of the post length, and below the local frost line. A six foot fence typically uses an eight foot post with 24 to 30 inches in the ground. Shallow holes are the single most common reason fences lean.
How many bags of concrete per fence post?
Roughly one and a half to two 80 lb bags for a 10 inch hole 30 inches deep. That is about 0.9 to 1.2 cubic feet, and each bag yields about 0.6 cubic feet. A wider or deeper hole needs proportionally more.
Can I use gravel instead of concrete for fence posts?
For a light garden fence, yes, and it drains better. For anything over four feet tall, or a privacy fence with wind load, concrete is the safer choice. Gravel-packed posts can be adjusted later, which is its one real advantage.
How far apart should fence posts be?
Eight feet is standard because rails and panels come in eight foot lengths. Six feet is stiffer and better for privacy fences and windy sites. Ten feet saves posts but needs heavier rails to avoid visible sag.
Do I need to wait before hanging a gate?
Yes, about seven days. Concrete reaches most of its working strength within a week. Hanging a gate on a green post puts a lever load on concrete that has not cured and is a common cause of a gate that will not close properly a month later.