Fence picket calculator

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. Dividing a run by the nominal width instead of the dressed width is the one arithmetic slip that turns up as bare fence rather than as a spare board.

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. A count that returns 13 — one post per section — has dropped the post at the far end, because the last section needs a post at both of its ends and only one of them is shared.

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 — which is why the spacing field on this page stops there.

Use 2 rails to about 4 ft of height, 3 rails at 5–6 ft, and 4 rails above that.

The rail count here is not total lineal feet divided by 8, which is the shortcut the lineal-feet method takes. A rail has to reach from post to post in one piece — there is nothing under a mid-span joint to nail to — so the offcuts cannot be added back together. At 7.69 ft bays an 8 ft board yields exactly one rail and wastes the last 3-11/16 in. Thirteen sections × 3 rails is 39 boards, where the lineal-feet method says 38.

The gap widens as bays get shorter. At 6 ft on centre you need 17 sections, so 51 boards — and the lineal-feet method still says 38, a shortfall of 13. If that waste bothers you, buy 16 ft stock: at 7.69 ft bays one 16 ft board makes two rails, halving both the board count and the joints.

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.
• Fasteners and hardware. Picket nails or screws, rail hangers, gate hinges and latches are not in any number here.
• Kickboards and caps. A rot board along the bottom is another run of lumber the full length of the fence, and post caps are one per post.

Four assumptions inside the arithmetic are worth stating outright.

The fence is treated as one straight line. You enter a total, and the picket count and the post count are each rounded up once against it. A fence that turns corners does not behave that way — every straight run ends on a whole picket of its own and needs a post at each end — so an L or a U wants up to one more picket and one more section, with its post, per corner than a single total will give you. Run the page once per straight length and add the answers up; the even spacing then comes out right for each run rather than averaged across all of them.

The rails are counted in 8 ft sticks. That is what the output says and it is all it knows. The point above about 16 ft stock halving both the board count and the joints is real, but it is not reflected in the number — work that case out from the section count and the rails per section.

A rail is assumed to reach post to post in one 8 ft stick, which is why the post spacing field stops at 8 ft. It is not that wider bays are impossible — it is that counting them means counting a different stock length, and a spliced mid-span rail is the joint the section above says you cannot build. If you want 9 or 10 ft bays, buy 12 or 16 ft rails and take the count off the section count times the rails per section by hand; this page will not do that arithmetic for you, and the 8 ft ceiling is there so it cannot pretend to.

Posts are assumed to be placeable anywhere. The even spacing divides the run by a whole number of bays, which is the right answer on open ground. Gates, corners, a tree, a rock or a neighbour's existing post all fix a post position and force the bays either side to be unequal, and none of that is visible here.