Measuring your hub and rim for spoke length
How to measure ERD, flange diameter, and center to flange distance, plus why dish makes the two sides of a wheel need different spokes.
Effective rim diameter, the number that matters most
ERD is the diameter of the circle that the seated spoke ends form inside the rim, measured to where the nipple meets the spoke. It is not the tire bead diameter and not the outer rim width. The cleanest way to find it is to thread two spare spokes with nipples into opposite holes, mark them, measure between the marks, and add the known offset. If you trust the maker, their published ERD is a fine starting point, but a measured value protects you from spokes that end up two or three millimetres off.
Flange diameter and hole circle
The flange diameter this tool wants is the pitch circle of the spoke holes, meaning the diameter of the circle passing through the centers of the holes. Measure across the flange from the center of one hole to the center of the hole roughly opposite it. On many hubs the two flanges share a diameter, but disc and track hubs frequently differ side to side. Recording each flange separately avoids a subtle error that grows with larger flanges.
Center to flange and wheel dish
Center to flange is the distance from the hub's centerline, the midpoint between the locknuts, out to the plane of each flange. Because a cassette or a disc rotor pushes one flange inboard, the two distances rarely match, and that difference is what wheelbuilders call dish. A larger center to flange distance on one side lengthens the spokes there. This is why a rear wheel almost always needs two spoke lengths, one for the drive side and one for the non drive side.
Cross pattern and the lacing angle
The cross count sets how far around the flange each spoke reaches before meeting the rim, and the tool turns it into an angle of crosses times 720 divided by spoke count. Radial lacing at 0 cross gives the shortest spokes and the most direct pull. Two cross and three cross wrap further, adding length and letting spokes support torque from braking or driving. Higher spoke counts shrink the per spoke angle, so a 36 hole three cross wheel laces at a smaller angle than a 24 hole three cross wheel.