Originally Posted by
Retro Grouch
I once actually built a wheel using that formula to calculate spoke lengths. As I recall, my spokes came out a couple of mm short but I built and used the wheel anyway. I suspect my rim measurement was probably off.
The most common problem folks have with the formula is that they forget that the spokes will end on a circle of the same diameter as they entered for rim diameter. You have to add for the nipple. There are also a few fudge factors of a millimeter here and there, such as the loss of a millimeter or so as the spokes deflect to cross over/under each other, or the difference in spoke hole dimension, vs. spoke length measured to inside of elbow.
This is why I always use the same calculator (which tends to come up short so I add 0.5-1.5mm when rounding). Once you get used to the calculator's bias, you can consistently correct for it and end up exactly where you expect to.