Webbing is bought slack and installed tight
This is the arithmetic nobody does. A webbing stretcher pulls the strand before it is tacked, so the piece you cut is shorter than the span it will cover. Pull ten percent into a strand crossing an eighteen inch frame and you cut 16.4 inches of run rather than eighteen, plus the fold-over at each end. Across eight strands on one seat that is about fourteen inches of webbing, and across a set of six chairs it is more than two yards.
How much stretch is right is not something a page can tell you. It depends on the webbing, on the stretcher, and on what the seat is meant to feel like, and the honest position is that the figure on this page is whatever you typed into it. What the page can do is show you the difference between cutting to the tight span and cutting to the relaxed one, which is the mistake that has you a strand short at the end of a roll.
The gap is decided by a whole number
Ask for a one inch gap between three and a half inch webbing on a twenty inch frame and the arithmetic gives 4.67 strands. You get four, and the four spread across twenty inches leave two inch gaps rather than one inch. Ask for five strands and the gaps close to five eighths of an inch, which is a different deck entirely.
There is no way around this and no reason to pretend otherwise, so the page prints the gap that actually results next to the count. That is the number that matters, because the gap is what the layer above bridges. A foam cushion over two inch gaps will find them within a year, and the usual fix is one more strand rather than heavier foam.
The deck is a stack and each layer has its own take-off
Webbing first, then springs tied down to it, then burlap over the springs to stop the stuffing falling through, then whatever the seat is stuffed or foamed with, then the decking cloth, then the cover. Each of those is a different measurement, and the turn-under allowance is different for each one because they are fastened to different places: burlap turns under a couple of inches onto the rail, decking pulls under and around, and the cover pulls further still.
The edge roll is the layer that gets skipped and the one that shows. Without it the stuffing runs to the rail and the rail becomes a line under the cover, and that line is where a seat wears through first. It is a perimeter measurement, which is why it sits on its own line here.
Where this sits
This page stops at the decking. The cushion above it is the foam and batting calculator, and cutting the cover out of goods of a given width, with nap and pattern repeat, is the fabric yardage calculator. If the seat is caned or rushed instead of upholstered, that is the chair caning take-off calculator. A frame that moves under the webbing tension needs the joints seen to first, which is the joint regluing schedule calculator. Whether the whole piece is worth the hours is the refinish breakeven calculator.
Questions people ask
Should the webbing run both ways or only front to back?
Both ways is a woven deck and one way is a set of parallel bands, and they behave differently rather than one being a shortcut for the other. Interwoven strands share load between directions, so a point load in the middle pulls on eight strands instead of four, and the deck sags as a dish rather than as a line. One-way webbing is used, mostly under a sprung deck where the springs do the sharing. The page will do either: put zero into one of the count fields.
How much twine does an eight way tie take?
More than most people allow for, and the honest answer is that it depends on your knots and your tails, which is why this page asks you rather than telling you. If you want a figure to start from, tie one spring properly, measure what it consumed including the tails you cut off, and use that. It will be specific to your chair and your habit, which is exactly the point. A simple tie down uses a fraction of an eight way tie, so a single number covering both would be useless.
Why does the decking come out as a rectangle bigger than the seat?
Because it is pulled under the rails and fastened on the underside or the outside face, so every edge needs a tail. The default three inches is a working allowance rather than a specification, and it wants to be bigger on a deep rail and can be smaller on a thin one. If your goods are narrow enough that the cut piece will not fit across the width, the page says so instead of quietly giving a yardage that assumes a seam you had not planned.
Can I reuse the springs that are in the chair?
That is a condition question and this page will not answer it. What the page will do is count them, so you know how many you are dealing with before the twine comes out. The practical warning is worth repeating: the springs in an old deck are held in compression by twine of the same age as the chair, and cutting that twine releases them. Whether a particular spring is still serviceable, and what gauge or height should replace it, is a supplier and material matter.
Is the burlap really necessary if the seat is foam?
On a sprung deck it does a job that foam cannot: it keeps the foam off the coils, which would otherwise print through and abrade it. On a flat webbed deck with no springs, its role is more about spanning the gaps between strands, and whether it earns its place depends on how wide those gaps came out. The page prints the gap for that reason. On an old piece the layer that comes out is also the layer worth looking at carefully, because old stuffing is a health question before it is a materials one.