Rayons

Rayons (spokes in French) accurately calculates the proper spoke length given a set of measurements that you provide. Hub and rim manufacturers may provide the measurements or you can measure them yourself. Detailed instructions are provided in the app.
The app uses simple and accurate controls to input your measurements. The spoke length for your wheel set is displayed in real-time.
You can save the calculation in the app as a wheel set by typing in a text field and giving it a name.
Formula
The spoke length formula used by the Rayons app has a set of limitations based on the mathematics and the nature of the drawings. The formula was designed for use on conventionally laced wheels with j-bend spokes. These types of wheels are most commonly custom made by hand. The limitations outlined here may not be all inclusive.
A wheel has:
An even number of spokes
The same number of spokes on each side of the wheel
Equal spacing between spoke holes, on the rim or the hub
Spokes that are crossed in pairs on each side of the wheel which dictates a spoke count divisible by four

App Minimum and Maximum Dimensions
The Rayons app can be used to calculate spoke lengths for a variety of component types. This covers the majority of Road, Track, Mountain Bike, Touring, Tandem, Fat Bike, Downhill, Generator and Internally Geared wheels. At this time, the app is not recommended for use with E-Bike components. Check your component dimensions and compare them with this chart. Be sure the component dimensions are supported by the app. Read the Terms of Service for more information.
All of the values are in millimeters.
Front Hub Flange Diameter Minimum: 28
Front Hub Flange Diameter Maximum: 150
Front Hub Offset Minimum: 15
Front Hub Offset Maximum: 48
Rear Hub Flange Diameter Minimum: 35
Rear Hub Flange Diameter Maximum: 150
Rear Hub Left Offset Minimum: 20
Rear Hub Left Offset Maximum: 60
Rear Hub Right Offset Minimum: 10
Rear Hub Right Offset Maximum: 52
Hub Spoke Hole Diameter Minimum: 1.5
Hub Spoke Hole Diameter Maximum: 3.0
Effective Rim Diameter (ERD) Minimum: 300
Effective Rim Diameter (ERD) Maximum: 700
Rim Spoke Offset Minimum: 0.0
Rim Spoke Offset Maximum: 5.0
Fat Bike Rim Spoke Offset Minimum: 0.0
Fat Bike Rim Spoke Offset Maximum: 20.0
Counts
Minimum Number of Spokes: 12
Maximum Number of Spokes: 48
Minimum Number of Crosses: 0
Maximum Number of Crosses: 4