I am looking for a translation of an American ring size 25, I know
this is big but all the charts that I have only go to 15. So what
should the diameter/circumferance be?
A size 25 would be 1.259" in diameter, 3.956" in circumference.
Ring sizes vary from 0.493" for size 1 to 0.940" for size 15.
Plugging these into a formula for deriving size versus diameter, a
simple slope calculation, would be done thus:
ratio of size to diameter equals ( 15 - 1 ) / ( 0.940 - 0.493 ) =
diameter of a size 1 is 0.493", so there is an offset of -14.441
diameter = (size - offset) / ratio
Example: diameter of a size “25”
[ 25 - ( - 14.441 ) / 31.32 ] = [ 39.441 / 31.32 ] = 1.259
This is probably more than you wanted to know. The reason I went
through all of this for myself was that I have to use a formula to
figure out how much wire to expect to use for one of my knots, and it
involves the circumference and width of the ring or bracelet. Everyone
uses American sizes, here, so I had to crank out a formula to
translate “size” to “diameter”, which I then incorporated directly
into my overall calculations. If someone tells me they want a ring,
size 11, and it should be half an inch wide, I can set aside 14" for
each complete iteration of the knot and be fairly sure I won’t end up
short. (In practice, of course, it’s better to waste a couple of extra
inches than it is to come up to the last turn and realize I’ve just
wasted the last ten or twenty hours.