Panel widths for roll, gate and Z folds. The tucked panels of a roll fold must be narrower, and equal panels will not close.
Equal panels do not close
A three-panel leaflet from an A4 sheet looks like three 99 mm panels. It is not, if it rolls. On a roll fold each panel tucks inside the one before it, so it has to be shorter — by around 2 mm on light stock and 4 mm on heavy or coated paper.
Cut them equal and the innermost panel fouls the spine. The piece will not lie flat, it buckles along the fold, and on a coated stock the fold itself may crack. The folding machine will still run it; the result is just wrong.
What makes this expensive is when you find out. Folding is the last operation on the job, after printing, after drying, sometimes after lamination. There is no proof stage that catches it, because a flat proof folds by hand well enough to look fine.
A Z fold is the exception, and that is the trap
An accordion or Z fold reverses direction at each crease, so nothing is ever tucked inside anything else and all the panels are genuinely equal. A gate fold is different again: two flaps meet at the centre and each needs shortening once so they do not meet under tension.
So the same three-panel leaflet needs three different sets of artwork depending on which way it folds. "Tri-fold" is not a specification — it describes at least two different objects. Tell your printer roll or Z, and tell them which panel is the cover.
Where the shortening figure comes from
No printer publishes a number for this, which is why no calculator exists. It is a function of the caliper of the stock and the number of folds it has to accommodate, and shops carry it as a rule of thumb.
Two to four millimetres per tucked panel covers ordinary work: two for a light uncoated text stock, three as a general default, four for heavy or coated. If you are running something unusual — a heavy board, six panels, a laminated finish — ask the printer for their figure rather than trusting this one.
Asked often
Do the panels have to be different widths?
On a roll fold, yes. On a Z fold, no — it folds outward each time so nothing tucks inside. On a gate fold the two flaps are shortened once each.
How much narrower?
About 2 mm per tucked panel on light stock, 3 mm typically, 4 mm on heavy or coated paper. No printer publishes a figure, so ask if the job is unusual.
What happens if I get it wrong?
The innermost panel fouls the fold, the piece will not lie flat, and on a coated stock the fold can crack. It is invisible until the job is folded, which is the last operation.