A coil is a spiral, and spirals grow slowly
Wrap a curtain of length L onto a barrel of diameter D, adding t of radius per wrap, and the outside diameter comes out as the square root of D squared plus 4Lt over pi. The square root is the whole story. Each new wrap runs round a bigger circle than the one beneath it, so it absorbs more curtain for the same radial growth, and the coil gets progressively harder to make bigger.
The default 10 ft door has 138 in of curtain and makes a 13.11 in coil on a 10.75 in barrel: 1.18 in of build, in 3.68 wraps. Double the curtain to 276 in and the coil goes to 15.10 in — 15 per cent bigger for twice the door. The next foot of opening height adds 0.19 in to the coil, and the foot after that adds slightly less.
Checking the arithmetic without trusting it
The page prints a check line, and it is worth understanding rather than skipping. The wraps run from a first circumference of 34.78 in to a last of 40.17 in, and the mean of the first and last circumferences times the number of wraps has to come back to the curtain length. At the defaults that is 37.47 in times 3.68, which is 138.0 in — the curtain, to within rounding.
That identity holds because the radius grows linearly with the wrap number, so the mean circumference is the circumference at the mean radius. If the check line ever drifts away from the curtain length by more than rounding, something in the inputs is out of the range the model behaves in, and the note under the table says so.
The last wrap is not the first wrap
Fifteen per cent more curtain goes into the outermost wrap than the innermost one at the defaults, for the plain reason that it is going round a bigger circle. This is why counting wraps by eye and multiplying by one circumference gives an answer that is always short, and why the arithmetic has to integrate rather than multiply.
A fatter barrel makes a fatter coil
This one gets argued about, so it is worth being exact. Going from a 10.75 in barrel to 12.75 in on the same curtain drops the build from 1.18 in to 1.02 in, because the curtain is wrapping in a thinner layer round a bigger circle. But the coil outside diameter goes from 13.11 in to 14.79 in, because the two inches of extra pipe outweigh the 0.32 in of diameter that shallower build gives back.
That direction never reverses. The coil diameter is the square root of the barrel diameter squared plus a fixed term, and a square root of something bigger is bigger. A larger barrel buys a shallower wrap and a smaller ratio of coil to pipe, and it always costs overall size. Whatever else decides the barrel — deflection over the span, the counterbalance inside it, what the manufacturer builds — it is not going to be a smaller coil.
What the build number really is
The single input everything depends on is the radial build per wrap, and the common mistake is to put the slat thickness in. Interlocking slats nest, so the curtain builds less per wrap than the metal is deep. The measurement is easy and settles it: wrap one turn, measure across the coil, wrap another, measure again, and halve the difference in diameter. Insulated and perforated curtains behave differently enough from a plain slat that borrowing a figure from one for the other is not worth doing.
Questions people ask
How do you calculate the coil diameter of a rolling door?
The coil outside diameter is the square root of the barrel diameter squared plus four times the curtain length times the build per wrap, all over pi. At the defaults — 138 in of curtain, a 10.75 in barrel, 0.32 in of build — that is 13.11 in, in 3.68 wraps. The square root is the important part: coils grow much more slowly than curtain length does.
How much bigger is the coil on a taller door?
Much less than proportionally. Doubling the curtain from 138 in to 276 in takes the coil from 13.11 in to 15.10 in, a 15 per cent increase for twice the door. Another foot of opening height adds 0.19 in of coil, and the foot after that adds slightly less again, because each wrap runs round a bigger circle than the one under it.
Does a bigger barrel give a smaller coil?
No, the opposite, though the build does get shallower. Going from a 10.75 to a 12.75 in barrel on the same curtain drops the build from 1.18 to 1.02 in but takes the overall coil from 13.11 to 14.79 in. The two inches of extra pipe outweigh the saving, and that never reverses — the coil diameter is a square root of the barrel diameter squared plus a fixed term.
What is the build per wrap and how do I measure it?
The amount the coil radius grows for each turn of curtain, and it is not the slat thickness — interlocking slats nest into each other. Wrap one turn onto the barrel and measure across the coil, wrap another turn and measure again, then halve the difference in diameter. Insulated and perforated curtains build differently from plain slats, so the figure does not transfer between them.
How much headroom does a rolling door need?
The coil radius plus wherever the barrel centreline sits above the opening plus clearance for the hood — 9 plus 6.55 plus 2, or 17.55 in at the defaults. The page compares that with the headroom you measure and prints the difference. It is a comparison of two of your own measurements and nothing more; whether a particular door fits a particular opening is a question for the door drawing.