Originally Posted by FBinNY

This is a common source of error. ERD is the misnamed effective RIM diameter. In fact ERD is the diameter of the imaginary circle formed by the ends of the spokes. Essentially it's the diameter at the spoke holes, plus the height of the spokes above that -- into the spoke head.

Since ERD is the desired height of the spokes, spoke calculators will produce a spoke height equal to the ERD entered.

If you measure your own rims, either add 6-7mm for two nipple heads, or add 3mm or so to the calculated spoke length. Either method is fine (I use the second) but be consistent in how you measure and calculate.