http://sheldonbrown.com/rinard/spocalc.htm
There is one rim dimension: ERD
erd
Effective Rim Diameter (ERD) is the diameter on which you want the ends of the spokes to lie. Most people prefer it near the end of the spoke nipple. If you want to measure your own rim (recommended, just to be sure), then follow these instructions:
Insert two old spokes into holes exactly opposite each other on the rim. Count holes to be sure.
Screw some nipples onto the spokes.
Pull them tight and measure dimension A in the figure (the diameter to the edge of the nipples, where the spokes disappears into them). Do this at several spots around the rim and average the measurements.
Measure the length of a nipple (dimension B in the figure) and add it twice (once for each nipple). The result is Effective Rim Diameter (ERD). Thus,
ERD = A + 2B.
Effective Rim Diameter (ERD) is the dimension you type into the spreadsheet for “ERD, effective rim diameter”. Of all the dimensions you actually might measure, ERD is the most critical dimension affecting spoke length, so it makes sense to measure it a few times at different places around the rim. Always count to make sure you use spoke holes that are actually opposite each other!
