Originally Posted by FBinNY

For wheel building, you need to know the spoke hole diameter, or if using a spoke calculator the ERD (effective rim diameter). The easiest way to measure this is to drop a nipple into a hole, measure the overall diameter to the edges of the rim. lay a straightedge across the rim and measure the drop to the nipple. Subtract 1-2mm from this to have an ERD between the slot and top of the nipple.

Whenever measuring rim diameters, it's best to take two measurements at right angles in case the rim is slightly. The round diameter is the average of these.