Screw Spacing Calculator

Calculate even spacing for screws, bolts, or other fasteners along a board, accounting for a margin at each end.

Board Length (in)

Edge Margin, each end (in)

Number of Screws

How the spacing is worked out

The first and last screws sit on the margins, so the gaps are one fewer than the screws - the fence-post count:

usable length = board length - 2 × edge margin
spacing       = usable length / (screws - 1)
screw n       = edge margin + (n - 1) × spacing

With the defaults, a 48" board with 2" margins leaves 44" usable, and five screws make four gaps: 44 / 4 = 11.00" centre to centre, landing at 2", 13", 24", 35" and 46". Divide by five instead and you get 8.8", running the last screw off the end. Margins that swallow the board are rejected, as is any count below two.

Choosing the margin and the count

Edge margin. Keep at least five screw diameters from an end - about 3/4" for a #8 - and drill a pilot. A screw is a wedge.
Round numbers beat exact ones. Spacing rarely lands on a tape mark. Mark straight from the position list rather than stepping the spacing along, which accumulates error.
More screws is not more strength along the grain. Close fasteners on one fibre line concentrate a split; stagger them either side of centre.

Screwing across the grain, as in a solid top fixed to a rail, pins wood that has to move - a 12" oak panel shifts around 1/8" in width between a damp summer and a dry winter. Slot the outer holes.

Frequently asked questions

Why isn't the spacing just the length divided by the number of screws?

Because screws sit at both ends of the run. Five screws leave four spaces, so the usable length is divided by four. Dividing by five gives gaps too small and leaves a stub of board past the last screw.

How close to the end of a board can I put a screw?

About 3/4" for a #8 in softwood, more in hardwood or thin stock, and always with a pilot drilled to the root diameter. Nearer than that and the end splits as the threads pull the fibres apart.

How do I space screws for a deck board?

Deck boards are fastened at every joist rather than at even spacing - two screws per joist, 16" on centre. Use this page for trim, cleats and ledger strips, where the pattern is yours to choose.