For the DIY or gear maker, it's nice to know how much gross grain, or tape, or whatever else is left on a roll, without unwinding it. I found these two formulas online and they agree with each other. I have not measured them, but they agreed with what was printed on one of my rolls of grossgrain.
L=pi*(r2^2 - r1^2)/t where
L= length of tape remaining in inches
r2 = outer radius of roll in inches
r1 = radius of the cardboard core in inches (typically about .072")
t = thickness of the grossgrain in inches (typically about .0165")
Another one using diameters is:
D= outer diameter of roll in inches
d= diameter of cardboard core in inches (typically 1.45")
I use the formula in excel and enter the diameters and thickness and let Excel do the rest. I convert the the total length measurement to feet and yards. Actually having the measurement in feet is more beneficial to me making gear.