Fence Calculator

What the fence calculator does

A fence is a handful of different materials that all have to add up: posts, rails, pickets, and the concrete to set the posts. This works out the lot. You give it the length of the run, the number of rails, and the picket width and spacing, and it returns the posts, rails, pickets, bags of concrete, and fasteners.

It turns a length of fence into a full material list. Below is how it gets there and the assumptions behind each part.

How to use it

1. Enter the length of the fence run, with its unit.
2. Enter the number of rails per section, the horizontal pieces between posts, usually two or three.
3. Enter the picket width and the spacing between pickets, the gap you want between them.
4. Press Calculate for the material list, or Reset to clear it.

How the materials are worked out

Each material follows from the length. The posts are spaced along the run and the rails fill the sections between them, while the pickets are counted by how much width each one takes with its gap:

Pickets = fence length ÷ (picket width + spacing)

The calculator sets the posts at 8 foot spacing, counts the rails by section, works out the pickets from their width and gap, and adds concrete and fasteners from the post count. Each result is rounded up to whole pieces.

Posts, rails, and pickets

The posts go in at 8 foot spacing, with one extra for the far end, since a run of fence needs a post to finish on as well as start from. Eight feet is the usual maximum span for a timber fence; closer is stiffer, but eight is the standard the calculator works to, using 8 foot 4x4 posts.

The rails are the horizontal 2x4s that span each section between posts, and the count is the rails per section times the number of sections. The pickets are the vertical boards on the face, and the count comes from how much of the run each picket plus its gap eats up. A tight gap means more pickets, a privacy fence with the boards almost touching means the most of all.

Concrete and fasteners

The calculator sets each post in concrete and counts one bag per post, which suits a typical post hole. Deeper or wider holes, the kind you want for a tall or exposed fence, can take two bags or more each, so treat the concrete as a starting figure and step it up if your holes are big. As a rule the holes want to be deep enough to put about a third of the post in the ground, and below the frost line where the ground freezes.

The fasteners are estimated from the post count as a rough guide to the screws or nails you will get through. If you are fixing a lot of pickets, each one screwed to every rail, it is worth buying to the picket count rather than the bare figure, since a picket fence goes through far more fasteners than a rail-and-post frame alone.

A worked example: a 100 ft fence

Say the fence is 100 feet long with 3 rails per section and pickets that take about 6 inches each with their gap.

At 8 foot spacing that is 13 sections plus an end post, so 14 posts. Three rails across each of the 13 sections is 39 rails. The pickets are 100 feet divided by 6 inches each, about 200 pickets. And one bag of concrete per post is 14 bags, with fasteners on top.

So a 100 foot fence comes out at 14 posts, 39 rails, around 200 pickets, and 14 bags of concrete, give or take the picket spacing you choose.

Questions people ask

How many posts do I need for a fence?

Space them about 8 feet apart and add one for the end. A 100 foot fence needs roughly 14 posts.

How many pickets do I need?

Divide the fence length by the width of a picket plus the gap you want between them. A 100 foot fence with pickets taking 6 inches each is about 200 pickets.

How much concrete per fence post?

Often about one bag per post for a typical hole, but a deep or wide hole can take two or more. Set roughly a third of the post in the ground, below the frost line.

How far apart should fence posts be?

Eight feet is the usual maximum for a timber fence. Closer spacing makes a stiffer fence, which is worth it for tall or exposed runs.

References

A quick note on the numbers. The post spacing, rail-by-section count, and picket spacing are standard timber fence practice, and the counts here are geometry, the run divided by the post spacing and the picket width. Post-hole depth and setting posts below the frost line are governed by local building codes, which also set property-line and height rules worth checking before you build. The unit conversions follow the US National Institute of Standards and Technology guide.

1. Local building codes for fence height, setback, and post footing depth (frost line). Check with your local building department. https://codes.iccsafe.org
2. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811