ie the stretched length where it is too restricted to pass a thread around to measure. Or to measure without taking apart a worker, to obtain a spare, before having to take apart to replace. Using callipers, pair of rods and ruler or whatever to take measurements of diameters and so radii and gap, templates, whatever intrusions will allow.
2 pulleys , small diameter "a" and big diameter b and gap between the 2 pulleys c. So distance between pulley centres is a +b +c Straight belt section between the two pulley rims (doubley tangential) is x N is the angle, in degrees, between the line-of-centres and the b radius to the tangent point of the large pulley. Also the angle between this extended line and the radius a of the small pulley.Parallelogram with parallel sides a, b and the other two sides x and (a+b+c) and two right angles. So reducing the parallelogram by lenghth "a" down to a right angle triangle of lengths x, b, (a+b+c) and N between b and (a+b+c)
x = sqrt [(a+b+c)^2 - (b-a)^2]
and N= cos(-1) [ (b-a)/(a+b+c)]
Then band length = 2x + 2*Pi*a (2N/360) + 2*Pi*b[1 - (2N/360)]
Confirmed practically in one case anyway
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on