How to Calculate Fence Materials
Building a fence requires precise math to ensure you don't end up with half a panel left over at the end of your property line. Our calculator determines your exact post and panel count, and then uses advanced cylindrical geometry to tell you exactly how many 80lb bags of concrete you need to set your posts.
The Concrete Math (Post Hole Geometry)
Most generic calculators just guess "2 bags per post." But a 3-foot deep hole requires significantly more concrete than a 1.5-foot hole. We calculate the exact volume of a standard 10-inch diameter hole, subtract the volume of the 4x4 post (which actually measures 3.5" x 3.5"), and divide by 0.60 cubic feet (the yield of one 80lb bag of concrete).
Post Spacing: Why 8 Feet Is the Standard
Most wood fence panels and rail sections are pre-cut for 8-foot post spacing because that's the longest span a standard 2x4 rail can carry without sagging over time. Going wider (10–12 ft) requires upgrading to 2x6 rails or adding a mid-span post. This calculator automatically rounds your total fence length up to the next full 8-foot section so you never end up needing a custom-cut partial panel at the end of the run.
How Many Pickets Do I Need?
For a standard 6" wide picket with a 1/4" gap (privacy fence style), each 8-foot section needs roughly 15 pickets. For a "shadowbox" or board-on-board style, add 50% more pickets since boards alternate front and back. Going wider than a 1" gap moves you into a "semi-privacy" style, which uses fewer pickets but offers less screening.
Frequently Asked Questions
How deep should fence post holes be?
As a general rule, post holes should be 1/3 to 1/2 the height of the above-ground fence. For a 6-foot tall fence, dig your holes at least 2 to 3 feet deep.
Do I need gravel at the bottom of the post hole?
Yes, adding 6 inches of gravel at the bottom of the hole before setting the post improves drainage and prevents the bottom of the wood post from rotting.
How much does a full fence cost compared to just materials?
See our Fence Cost Guide for a full 10-year cost comparison across wood, chain-link, vinyl, and aluminum, including maintenance costs most people forget to budget for.
How many fence posts do I need for 100 feet?
At standard 8-foot spacing, 100 linear feet needs 13 posts (12 full sections plus the starting post), plus 1 additional post for every gate. Always round up to the next full post when your total length doesn't divide evenly by 8.
What size post do I need for a 6-foot privacy fence?
A 4x4 post is standard for residential 6-foot fences. For fences taller than 6 feet, gate posts, or corner/end posts carrying extra load, upgrade to a 4x6 or 6x6 post for added stability.