Fence Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

Worked example

A 100 ft privacy fence, 6 ft tall, on 8 ft post spacing with 4x4 posts in 10 in holes. Inputs: Fence length 100 ft, Fence height 6 ft, Post spacing 8 ft O.C., Post size 3.5 in, Post hole diameter 10 in, Picket width 5.5 in, Gap between pickets 0 in, Rails per section 3, Number of gates 0. Result: 219 (about 24 × 80 lb bags).

Materials for common fence lengths

Calculated for a 6 ft privacy fence on 8 ft post spacing with 4x4 posts in 10 in holes. Tap a length to load it in the calculator above.

Method & assumptions

A fence run is divided into sections between posts: the number of sections is your fence length divided by the post spacing, rounded up, and the post count is one more than that (there is a post at both ends of the run). Eight feet on center is standard for wood fences; shorter spacing adds stiffness on taller privacy fences or in high-wind areas.

Post length follows a simple site rule: buy a post about 1.5 times the finished fence height, so a 6 ft fence uses 9 ft posts, since roughly a third of the post's length gets buried. Concrete per hole is a cylinder (the hole) minus the post's own volume over that buried depth: π times the hole radius squared times the depth, minus the post's cross-section squared times the same depth. That is the same formula behind our how many bags of concrete per fence post guide, and the numbers cross-check against it: a 10 in hole for a 4x4 post matches almost exactly, and a 14 in hole for a 6x6 corner or gate post does too.

We add a 10% waste allowance to the concrete total before converting to bags, since real holes are never a perfect cylinder, and round every bag count up because you cannot buy a fraction of a bag. Picket count comes from the total run of fence that still needs pickets (a gate opening removes that section's pickets and rails but keeps its two posts) divided by the picket width plus any gap between boards. Rail count is one multiple of sections: roughly one rail for every 2 ft of fence height, so a 3 ft picket fence needs two rails and a 6 ft privacy fence needs three.

Always check your local frost depth before setting the final hole depth: in cold climates the bottom of the hole should sit below the frost line, which can be deeper than the 1/3-of-height rule of thumb calls for. Your local building department or extension office publishes the frost depth for your area.

Pro tips and common mistakes

• Check your local frost depth. The 1.5x-height post rule is a starting point, not code. In freezing climates the hole bottom needs to sit below the local frost line or the post can heave out of level after winter, regardless of what the height formula suggests.
• Go bigger on corner and gate posts. Corner, end and gate posts take more sideways load than line posts. Step up to a 6x6 in a wider, deeper hole for those positions even if the rest of the run uses 4x4s.
• String a line before you dig. Run a mason’s line between end stakes and mark every post location off it. A fence that drifts an inch per section looks crooked over a long run even though each individual post is fine.
• Use ground-contact rated lumber for posts. Posts sitting in soil and concrete need a higher preservative retention (UC4B ground-contact rating) than the pickets and rails above grade. Mixing in standard treated lumber for posts shortens their life significantly.
• Slope the concrete away from the post. Crown the top of each concrete collar so water sheds away from the post instead of pooling around it. Standing water at the post base is one of the most common causes of early rot.

Frequently asked questions

References

• Fence (Wikipedia)
• QUIKRETE Fast-Setting Concrete Mix (product data)
• American Wood Council: ground-contact lumber and span guidance

Related calculators

Related guides

Cite or embed this calculator

Using this in an article or on your own site? Copy a citation, or embed the live calculator for free.