The returns are a third of the zipper
Measure the back of a 22 by 20 in cushion and you get 22 in. Order a 22 in zipper and the cover will fight you, because a straight slot along one edge means the foam has to go in past two corners that do not open at all.
The usual answer is to carry the zipper round both back corners by a few inches, so the opening becomes a U and the back panel folds back out of the way. Six inches at each corner turns a 22 in zipper into 34, and those 12 in of return are 35 per cent of the whole thing. That is the number people leave out.
On the default pair of cushions it is 68 in of chain instead of 44 — about 55 per cent more. It is not a rounding error and it is not optional.
What the opening actually is
The page prints the geometry rather than a verdict, because whether an opening is workable is a matter of how compressible your foam is and how much your cover fabric gives. What it can tell you is the shape of the problem: a 34 in opening in a wall 4 in deep, turning two corners, with a 22 by 20 by 4 in block that has to pass through it.
Longer returns open the cover further. That is the whole trade, and the return length is a field rather than a fixed figure for exactly that reason. A firm block in a tight cover wants more; a soft one in a cover with some give wants less.
Chain by the yard against made-up zippers
Made-up zippers come in steps. Ask for 34 in on a 2 in step and you get 34; ask for 33 and you get 34 as well, with an inch you cannot use. Across a handful of cushions that rounding adds up, and it is the usual reason workrooms move to continuous chain.
Chain by the yard cuts to whatever the opening is, and a slider goes on with a pair of pliers. It fits exactly rather than to the next step up, and it costs you the ten minutes of putting sliders and stops on. Whether it also works out cheaper is a question of your own two prices, which is why the page prints the equivalent chain yardage in stock-length mode as well — take that figure to your supplier and compare. The default here is chain because that is what a boxed cushion normally gets; switch to stock lengths and the page shows you what the rounding adds.
The placket is half the boxing
A zipper sits in the middle of the boxing wall, so the zipper section of the boxing is two strips rather than one. Each strip is half the boxing height, plus the tape beyond the teeth, plus a seam allowance — on a 4 in boxing with a half inch tape and a half inch allowance that is 3 in a strip.
Notice what happens as the boxing gets shallower. On a 2 in boxing the same strip is 2 in tall and the tape is a quarter of it. There is no rule stated here about how shallow is too shallow, because that depends on the chain and on how the seam is finished, but the arithmetic shows you where the room runs out.
The strips are cut long — four inches over the zipper length on the defaults — so the plain boxing has something to join onto past the zipper ends. Then the rest of the perimeter, 50 in on this cushion, is made as ordinary boxing.
What is deliberately absent
Any statement about size numbers, strength or outdoor life. Coil, moulded and metal chains behave differently, size numbering is not consistent enough to quote, and how a tape holds up under sun is a question for the supplier of the specific product. This page counts inches and leaves the specification to the people who publish one.
Questions people ask
How long should a zipper be for a cushion?
The back edge plus a return round each back corner. For a 22 by 20 in cushion with 6 in returns that is 34 in, not 22. The returns turn a straight slot into a U so the back panel folds back and the foam goes in without forcing the corners of the cover. Ordering to the back measurement alone is the commonest reason a cover will not go on.
How much of a cushion zipper is in the corner returns?
About a third on a typical boxed cushion. Six inches at each of two corners is 12 in out of a 34 in zipper — 35 per cent. Across a pair of cushions it is the difference between 44 in of chain and 68 in, so it is worth having in the order rather than discovering it at the bench.
Is zipper chain by the yard cheaper than made-up zippers?
That depends on your two prices and this page will not guess at either. What chain definitely does is fit the opening exactly rather than rounding to the next stock step, at the cost of your time putting sliders and stops on. The page prints the equivalent chain yardage even in stock-length mode, and what the rounding adds, so you can put both figures to your supplier and see where the crossover falls for your own job.
How wide do I cut the zipper placket strips?
Half the boxing height, plus the tape beyond the teeth, plus a seam allowance, for each of the two strips. On a 4 in boxing with half an inch of tape and half an inch of allowance that is 3 in a strip. Cut them a few inches longer than the zipper so the plain boxing has something to join to past the ends.
Can the zipper just run along the back edge?
It can, and the page will cost it that way. What you get is a straight 22 in slot in a 4 in wall with a 20 in deep block behind it, and the cover has to be worked over the foam through that. Whether that is practical depends on how compressible the foam is and how much the cloth gives, which is a judgement for whoever is stuffing it rather than something this page will rule on.