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Spacing a row evenly looks simple until the last one lands crooked because the gap count was off by one. This lays out holes, pins, pickets or slats across a span and hands you every mark, or divides a board into equal parts and gives the cut lines. Work in whatever unit you like and the answers come back in it.
The off-by-one that trips everyone
Here is the catch that throws most even-spacing jobs. If you want a mark at each end of a run, like shelf-pin holes or fence posts, the number of gaps is one less than the number of holes: five holes have four gaps. But if you are floating items inside an opening with a gap before the first and after the last, like pickets in a gate, there is one more gap than items: five pickets have six gaps. Pick the wrong one and everything shifts. This page has a mode for each, so you do not have to remember which.
Holes and pins, ends included
Use this when the first and last marks sit right at the ends of your span and you want the rest spread evenly between. The spacing is the span divided by one less than the count. It is the mode for a row of shelf-pin holes between two fixed points, posts along a known fence line, or rivets from corner to corner. The first mark is at zero and the last is at the full span, with the even ones in between.
Even gaps, floating inside
Use this when the items sit inside an opening with an equal margin at each end, like balusters in a railing or slats in a panel. Now the span is divided by one more than the count, so the end margins match the gaps between. The first item sits one gap in from the edge, not at the edge. This is what gives a run of pickets that even, deliberate look instead of one crammed against each post.
Mark from one edge, every time
However you space it, measure every mark from the same datum edge rather than stepping off each gap from the last mark. Tiny errors in a stepped measurement add up, so by the far end you are off by a noticeable amount. Working from one zero keeps them independent, and the list of positions here is written that way, straight from a single end, so you can set a stop or a tape once and mark the whole run.
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