Fence Post Calculator

How to estimate fence posts and concrete

Two separate calculations that people usually merge and get wrong. First, how many posts the run needs. Second, how much concrete each hole swallows. The second one is where the published rules of thumb fall apart.

posts = ceiling(run ÷ spacing) + 1 + (gates × 2)
concrete per hole = 3.1416 × radius² × fill depth − post volume
bags = total concrete ÷ yield per bagFill depth is the hole depth minus any drainage gravel. Bag yields are Quikrete's stated figures.

Counting posts

A straight run always needs one more post than it has spans. A 100 foot fence at 8 foot spacing has 12.5 spans, which rounds up to 13, so 14 posts. The plus-one is the far end, and forgetting it is the classic error.

Gates do not share posts with the run. Each gate needs its own hinge post and latch post, so add two per gate. Hinge posts also carry a swinging load all day, so many people set those deeper or in a wider hole than the line posts.

Spacing itself is a tradeoff. Eight feet is the common maximum and matches most prefab panels. Six feet is better on exposed sites or for solid privacy fence, which acts like a sail. Wire and rail can go to ten.

The concrete number, and why the rule of thumb fails

This is the interesting part, and it is worth showing our work.

Quikrete's Fast-Setting Concrete Mix data sheet gives a worked example: two 50 lb bags will set a 3 inch square post in a 9 inch diameter hole 30 inches deep. Two 50 lb bags at their stated 0.375 cubic feet each is 0.75 cubic feet.

Now do the geometry. A 9 inch hole 30 inches deep is a cylinder of 1.10 cubic feet. A 3 inch square post through that depth displaces 0.16 cubic feet. That leaves 0.94 cubic feet of concrete needed, against the 0.75 the manufacturer says will do it.

Those numbers do not agree, and the gap is about 25 percent. The reconciliation is that a real post hole is not filled with concrete from the very bottom to the very top. Put 6 inches of drainage gravel in the base, so concrete fills only the top 24 inches, and the requirement drops to 0.76 cubic feet. That matches Quikrete's example within one percent.

So their figure quietly assumes a gravel base, which is also what most fence-building guidance recommends anyway. This calculator makes that assumption visible as a field you can change, rather than baking it into a constant. Set the gravel to zero and watch the bag count climb, because that is the honest number for a hole filled to the bottom.

Hole size

The common guidance is a hole about three times the post width, and a depth of a third to a half of the post's height above ground. A 4x4 post for a 6 foot fence lands at roughly a 9 to 12 inch hole, 24 to 30 inches deep.

Note that a nominal 4x4 is actually 3.5 inches, and a 6x6 is 5.5. Use the actual dimension, which is what the post size selector does, otherwise you overstate how much concrete the post displaces.

Frost depth overrides all of this. In cold regions, a post that does not reach below the frost line will be lifted by freezing ground no matter how much concrete is around it. Your local building department publishes the depth; use it.

A quick cost check

A 100 foot fence at 8 foot spacing with one gate is 16 posts. At 9 inch holes 30 inches deep with 6 inches of gravel, that is 0.76 cubic feet each, about 12.2 cubic feet total, so 33 bags of 50 lb fast-setting mix. At $7 a bag that is about $230 of concrete on top of the posts.

Drop the gravel and fill to the bottom and it becomes 41 bags, roughly $290. That single field is a $60 decision on a modest fence.

Common estimating mistakes

• Forgetting the last post. Spans plus one, always.
• Sharing gate posts with the run. A gate needs two posts of its own.
• Using nominal lumber dimensions. A 4x4 is 3.5 inches. It matters for the displacement.
• Taking two bags per post as universal. It is correct for one specific hole size. A 12 inch hole needs nearly twice the concrete of a 9 inch one, because area scales with the square of the radius.
• Ignoring frost depth. The only item on this list that can undo the entire fence.

When to call someone

Call 811 before you dig, every time, no exceptions. It is free, it is the law in most places, and buried gas and electrical lines do not announce themselves. This is the one step on this page that can seriously hurt you, and no amount of careful measuring substitutes for it.

Beyond that: property lines. A fence set even a few inches over a boundary is a legal problem that gets more expensive the longer it stands. If you are not certain where the line runs, a survey costs far less than moving a finished fence. Local rules on height, setback, and which side faces out are worth a call to the building department before you buy anything.