Calculating proper spoke length
Five things you need to know in order to use online spoke length calculator for your wheel. If the hub is unevenly dished and each side of the wheel will require different length spokes, run the spoke length calculator separately for each side of the wheel. This spoke length calculator asks for the hub spoke hole diameter, I always leave this at the default 2.5 mm

Effective rim diameter (ERD) is the interior diameter of the rim. If you're lucky, the ERD is printed on the rim. Otherwise, I think the best way to find the ERD is by hunting on the manufacturer's website or Googling the rim model. The ERD can be measured with a tape measure or by spanning the rim with two long spokes, but I find this difficult and inaccurate.

Number of spokes on the wheel make sure that the number of spoke holes in the hub and in the rim are the same!

Hub flange diameter is the distance between the center of spoke holes on opposing sides of the rim.
FIG Measure hub flange diameter using calipers (shown in gray).

Hub center to flange center (W_left and W_right) for both sides of the hub
FIG Calculate the centertoflange distances by measuring the over locknut distance (OLD) and the distance from the locknut to the flange on both sides of the hub. The centertoflange is then the half the OLD minus the distance from the locknut to the flange.

Lacing pattern: Spokes can be laced in a few different patterns. Crossthree is the standard.