Why the diagonals are 41 percent of the cane
The six way pattern has four straight passes and two diagonal ones. If the passes were all equal you would expect the diagonals to be a third of the cane. They are not, and the reason is geometry rather than technique.
Think about how much line a set of parallel strands puts into a seat. A set of vertical strands half an inch apart lays down the seat area divided by half an inch. A set of diagonal strands going through the same holes is still half an inch apart along the rails, but the perpendicular distance between two neighbouring diagonals is that half inch divided by the square root of two, because they meet the rail at 45 degrees. Closer lines mean more of them, and each one is longer. The two effects multiply out to exactly the square root of two.
So four straight passes and two diagonals worth 1.414 each gives 6.83 units where six equal passes would give six. The diagonals are 2.83 of 6.83, which is 41.4 percent, and that ratio is one over the square root of two plus one. It does not depend on the size of the seat or the hole spacing, which is a rare thing in a take-off. Weigh the hank if you doubt it: the first four passes go quickly and then the seat seems to eat cane.
Side rails are longer than the seat is deep
Almost every chair seat is wider at the front than the back, and the side rail therefore runs on a diagonal. On a seat sixteen at the front, thirteen at the back and fourteen deep, each side rail runs 14.08 inches rather than 14, which is only a hair. Widen the taper to five inches and the side goes to 14.22. It is small, but it is in the direction that leaves you a hole short at the back corner, which is the one place a missing hole cannot be hidden.
The other consequence of the taper is that the front rail carries more holes than the back. Those extra strands have to go somewhere, and where they go is into the side rails, fanning slightly. That fan is why a caned seat has a slightly radial look at the front corners and why the hole count on the sides is worth working out rather than copying from the back.
Pressed cane is a groove measurement, not a seat measurement
The other kind of caned seat is a machine woven sheet held in a groove by a reed spline. Nothing about the six way arithmetic applies. What matters is the groove: its perimeter, and above all its width, which is the number that decides which spline fits. Grooves on old chairs are rarely a catalogue size, and the same chair can vary by a thirty-second from front to back because it was cut by hand.
The sheet itself is bought as a rectangle bigger than the opening, because it has to be gripped and pulled while it is being set into the groove, and trimmed after. The overhang is a working allowance, not waste in any meaningful sense.
Where this sits
Under a caned or rush seat there is often nothing else, but on an upholstered chair the layers below the cushion are the seat deck webbing and spring calculator, and the cushion itself is the foam and batting calculator. Loose chair joints usually need attention before a new seat goes in, which is the joint regluing schedule calculator, and the joints themselves are the mortise and tenon calculator. Whether the whole job pays is the furniture refinish breakeven calculator.
Questions people ask
How do I find the hole spacing on a chair that has already been stripped?
Measure across ten holes centre to centre and divide by nine, taking the measurement along the front rail where the run is longest and the error smallest. Doing it on two adjacent holes multiplies any misreading by ten. If the holes are gone entirely because the rail has been filled, the spacing has to be chosen rather than recovered, and the constraint is that the spacing and the hole diameter together have to leave enough wood between holes for the rail to survive being drilled.
Does the calculator allow for the cane going down through one hole and up through the next?
Not as a separate line, no. The passes are worked out as the length lying across the seat, and the dip through each hole plus the tie-offs underneath are covered by the waste percentage, which is why the default sits at twenty rather than the five or ten you would use for a straightforward material take-off. If you want to be precise about it, measure the rail thickness, multiply by twice the hole count, and treat that as a floor under the waste figure.
Why is the answer in feet when cane comes in hanks?
Because a hank is not a standard quantity. Different suppliers sell different lengths, some sell by weight, and the length in a hank of one cane width is not the length in a hank of another. The page gives you feet, which is a real measurement, and converts to hanks only using the figure you type in from your own order. The output line says as entered for that reason.
Can I reuse the old spline groove dimensions for a new sheet?
The groove, yes, and you should, because it is what the sheet has to fit. The spline, no. Old spline comes out in pieces and swells or shrinks with age, so measuring the piece you removed tells you less than measuring the groove it came out of. Run a caliper down the cleaned groove in three or four places, take the narrowest reading, and buy to that. A spline that has to be forced will split the rail and a rail split along a groove is a much bigger repair than a seat.
Is it worth caning a chair that is loose in its joints?
A caned seat pulls the rails together as it dries and tightens, so it puts real load into joints that are already moving. Most restorers deal with the frame first for that reason. Beyond the sequencing, there is a question that comes before either job: on a piece made before about 1980, the finish and any filler are a testing matter because working on them makes dust, and on anything of real value the original surface is most of what the value consists of, so a conservator looks at it before a scraper does.