Two numbers, one equation
A pleated panel starts flat and ends narrow. The difference between those two widths is the fabric that goes into the pleats, and it is fixed the moment the panel is cut. Call it the take-up. Then:
take-up = pleat count x fabric per pleat.
That is the whole thing. You can set the depth and let the count fall out, or set the count and let the depth fall out, but you cannot choose both, and neither can a workroom. This is why a made-to-measure curtain has an odd number of pleats at an odd depth rather than a round eight at exactly five inches: the panel width was decided by how many widths of fabric were joined, and the pleats had to absorb whatever was left over.
The spacing is set separately and it is the surprising part. The spaces between pleats have to cover the face of the rod, so:
space = rod face per panel divided by (pleat count minus one).
Notice that the pleat depth is not in that equation. Fewer pleats means wider spaces, regardless of how much fabric each pleat swallows. That is the trade you are actually making when you change the count.
Returns and overlaps, and why the finished width is not the rod width
A rod stands off the wall on brackets. The distance from the wall to the centre of the rod is the return, and a curtain that does not wrap back to the wall leaves a gap that light comes through and that looks unfinished from the side. So the outer end of each panel needs enough width to turn the corner: that is the return, usually three to four inches on a standard bracket, and more on a double rod or a bay.
Where a pair meets in the middle, one panel has to cross the other or there is a vertical stripe of daylight down the centre. That crossing is the overlap, and three to four inches is the usual working figure. Both the return and the overlap are finished panel width that never shows on the face of the rod, which is why the finished width of a panel is always more than half the rod.
The mark-out list on this page starts at the return edge, because that is the edge you measure from in practice: the return is the first thing you set, then the pleats, then whatever is left is the overlap.
Marking out, and why you do it from one end
The temptation is to work from the middle out, so any error splits evenly. Do not. Measure from the return edge and mark every pleat position cumulatively along the panel, because that is how the arithmetic was built and because it puts any accumulated error at the leading edge, where an extra quarter inch of overlap is invisible. Split the error to both ends and you get a return that does not reach the wall on one side.
Mark the start and the end of each pleat, not just its centre. A pinch pleat is formed by folding the fabric so that the two marks meet, then dividing that fold into three or two smaller folds and tacking at the base. If you only mark centres you have to measure the depth at every pleat instead of matching two marks.
The table on this page gives the positions in feet and inches to the sixteenth, which is what a tape measure reads. The decimal is beside it for anyone marking with a rule.
Fullness, and where it comes from
Fullness is the ratio of flat cloth to rod width, and it is decided long before pleating, when the widths of fabric were joined into panels. Two panels of 60 inches each across a 52 inch rod is 2.3 times fullness, which is a normal working figure. The same two panels across an 84 inch rod is 1.4 times, which will hang nearly flat. The pleats do not create fullness; they organise it.
If the number this page reports is far off what you intended, the problem is upstream in the cutting, not in the pleating. The curtain size calculator works out panel width and yardage from the window and a target fullness, and the flat panel width it produces is what you bring here. For the fabric itself, including the way a pattern repeat forces every cut to the same point in the design, the fabric yardage calculator handles the buying.
Where this stops
This page assumes evenly spaced pleats across a straight rod. It does not handle a bay, where each section has its own rod face and the pleats have to land clear of the corners; a bay is best worked out one straight section at a time and then checked at the joints. It does not know about the pleat style — a two fold, three fold and goblet pleat all take up different amounts of fabric for the same visual weight, which is why the depth is a field here rather than a constant. And it says nothing about how the panel stacks when it is drawn open, which depends on fabric thickness as much as on pleat depth.
Questions people ask
How much fabric should each pleat take?
Four to six and a half inches covers most three-fold pinch pleats in a medium weight fabric, and this page takes your figure rather than choosing one. Deeper pleats look richer and stack wider when the curtain is open, which means the rod needs to run further past the window. Shallower pleats sit closer together and stack tighter. The practical constraint is at the base of the pleat: past about seven inches of take-up the fold gets bulky enough that tacking it down neatly through several layers becomes a fight.
Why does the pleat count have to be a whole number when the depth does not?
Because you cannot sew two thirds of a pleat. The take-up is a fixed quantity of fabric that has to be divided among a whole number of folds, so the count rounds and the depth absorbs the remainder. That is why a workroom sets the depth as a target, computes the count, and then reports the depth that actually results. Setting the count directly, which this page also allows, is the same arithmetic run the other way.
Should the first pleat sit right at the return?
Yes, at the inner edge of the return. The return is the flat section that wraps back to the wall, so it carries no pleat, and the first pleat sits where the return ends and the face of the curtain begins. The same applies at the other end: the overlap is flat, and the last pleat sits at its inner edge. That is why the spacing formula divides by one less than the pleat count rather than by the count.
What happens if I add one more pleat?
Two things at once, in opposite directions. The depth of every pleat drops, because the same take-up is divided among more of them. And the space between pleats narrows, because the same rod face is divided among more gaps. On a panel with 27 inches of take-up over a 26 inch rod face, five pleats gives 5.4 inch pleats 6.5 inches apart, and six pleats gives 4.5 inch pleats 5.2 inches apart. Run both counts and compare rather than guessing which looks better.
Does the overlap need to be as wide as the return?
It does not have to be, and it is a different job. The return exists to close the gap between the curtain and the wall at the side. The overlap exists to close the gap between two panels at the centre, and it only has to be wide enough that the two leading edges stay crossed when the curtain is drawn and the fabric moves. Three inches is a common figure; more on a heavy fabric that swings, less on a fixed pair that is never actually closed. Both are set as fields here because both depend on the hardware you have.