I broke a drive-size spoke on my PowerTap wheel. I need to order a replacement CX-Ray.

How can I determine what spoke length to order by measuring other spokes on the wheel?

If it's a factory PowerTap wheel, I'd call them and ask.

It's also pretty straightforward to take another spoke out and measure it. Since you're going to be working on it anyway to put the new spoke in, this should be too much of an issue. If you're careful and have both pieces, you can piece the broken spoke together and measure it. If it's a J bend spoke, the length is measured from the inside of the bend (surface of the spoke adjacent to the head) to the very end.

The last option is to calculate it if you have or know the relevant parameters to plug into a spoke length calculator.

To measure a spoke in place you take a ruler and place it against a similar but good spoke. You measure from the rim's inside surface to the hub's outer surface. THEN you determine the amount of the spoke that is hidden in the rim and overlap/is inside the hub shell. At the rim end this can be a few MM's to many depending on the rim/spoke/nipple design. At the hub end this can be the same, a few MM's to the head's inside to many MM's to the end of a threaded into hub design.

Because the amounts that you can't see are hard to determine the best method is to remove a good spoke and do a direct measurement. Some bike brands have this spec listed in their technical sheet (if the wheel was an original spec), some wheel builders keep the spoke lengths on record. Andy.

Modifying AS's method to elimiante guess work about the rim's thickness at the nipple.

Measure any spoke from the inside of the elbow, (or the center of the head, less 1mm) out to the outer edge of the rim. Then remove the tire, lay something across the rim and measure down to the top of any nipple. Subtract to find the length to the top of the nipple, and subtract 1mm because you want spokes ending short of there.

Since spokes stretch (lengthen) when properly tensioned, will FB's and AS's method yield an accurate measurement? I'm not sure how much of a difference being under tension will create, but it might be something to think about.

Actual elongation under load is minimal. In any case spoke length calculations have never factored it, so measuring this way is equally accurate to mathematical calculation. Mechanics have been measuring replacement spokes this way for a century or so. You'll also note that I subtracted 1mm targeting the height below the top of the nipple, which would leave room for elongation.

The only difference between Andrew's method, and mine is the elimination of the need to guess the rims thickness. Either is fine.

Got it. Subtracting the 1mm is also nice because it makes lacing the new spoke through all the existing spokes and rim and touch easier.

Thanks, everyone! This has been very interesting.

In the end I took it to an LBS. They took out another spoke, measured it, checked stock and said they'd have to order it. No worries.

And as I said this is the best method. Andy.