How many fence posts for a 100 ft fence?

Why the picket count is higher than you expect

A "1x6" fence picket is not six inches wide. The American Softwood Lumber Standard (PS 20-20, Table 3) sets the minimum dressed dry width of a nominal 6 in board at 5-1/2 in, and a nominal 4 in board at 3-1/2 in. Every board you buy is milled to that standard.

So a 100 ft run is 1,200 in of fence, and 1,200 ÷ 5.5 = 218.2, or 219 pickets. Divide by a nominal 6 in instead and you get 200. That is a shortfall of 19 boards — about five feet of open fence — discovered on the last afternoon of the job. This single error is the most common defect in fence calculators.

One caveat the standard itself flags: green (unseasoned) western red cedar, redwood and northern white cedar are allowed a wider dressed size, 5-5/8 in for a nominal 6. Wet pressure-treated pickets run wide for the same reason and then shrink. Measure one board off the pallet before you commit to an order.

Posts: the off-by-one that bites

Posts are spans + 1. A 100 ft fence at 8 ft on centre is 12.5 spans, which rounds to 13 sections and therefore 14 posts. Calculators that return 13 — one post per section — leave you a post short at the far end.

Rather than build twelve 8 ft bays and one awkward 4 ft bay, divide evenly: 100 ÷ 13 = 7.69 ft between posts. Equal bays look right, and every rail is cut to the same length.

Where the remainder goes

Single sections almost never divide evenly. A 6 ft bay is 72 in, and 72 ÷ 5.5 = 13.09, so it takes 14 pickets. Thirteen full pickets only cover 71.5 in, leaving half an inch of daylight — and opening the joints to close that half inch would take a gap of about 1/24 in each, which you cannot cut consistently. The clean fix is the fourteenth picket, ripped narrow, or two pickets ripped to share the loss evenly at both ends where it reads as intentional.

Rails and stock lengths

Rails are the reason 8 ft is the practical maximum spacing. At 8 ft on centre a 16 ft rail spans three posts, so joints land on posts and stagger naturally between courses. Go wider than 8 ft and a 2x4 top rail visibly droops.

Use 2 rails to about 4 ft of height, 3 rails at 5–6 ft, and 4 rails above that. The rail figure here is total lineal feet converted to 8 ft boards; buying 16 ft stock instead halves your joint count.

What this does not cover

• Gates. Subtract the gate opening from the fence length and order the gate frame separately — its pickets are usually cut narrower to fit.
• Concrete for the posts. That is a per-hole calculation and it dominates the material weight; see the post hole calculator.
• Slope. On a hillside, stepped panels need full-length pickets at every step and racked panels need slightly longer ones. Add height, not count.