If you use DT spokes and nipples (these nipples are about half as long as normal) add 1 to 1.5mm to the calculated spoke length.

Spoke length is measured from the inside of the "elbow" to the end of the spoke (including the threaded part).

Calculations for radially spoked wheels (straight-laced or zero crossed) have not been confirmed.

To Convert:
Inches to mm: divide by .03937
mm to inches: multiply by .03937

Spoke sizes

British (U.S.) 12 gauge is 2.6 mm
British (U.S.) 13 gauge is 2.3 mm
British (U.S.) 14 gauge is 2.0 mm
British (U.S.) 15 gauge is 1.8 mm
British (U.S.) 16 gauge is 1.6 mm

British (U.S.) 14 gauge is the same as French 13 gauge
British (U.S.) 13 gauge is the same as French 15 gauge

Below is a brief explanation of which measurement used in the calculator is which. If you need to enter your own measurements, but are unsure of exactly what to measure, give the following a read and all shall be revealed.

Rim Diameter or "Effective Rim Diameter" (ERD) is the diameter on which you want the ends of the spokes to lie. Most people prefer it near the end of the spoke nipple. If you want to measure your own rim (recommended, just to be sure), then follow these instructions:

1. Insert two old spokes into holes exactly opposite each other on the rim. Count holes to be sure.
2. Screw some nipples onto the spokes.
3. Pull them tight and measure dimension A in the figure (the diameter to the edge of the nipples, where the spokes disappears into them). Do this at several spots around the rim and average the measurements.
4. Measure the length of a nipple (dimension B in the figure) and add it twice (once for each nipple). The result is Effective Rim Diameter (ERD). Thus,
   
   ERD = A + 2B.

Effective Rim Diameter (ERD) is the dimension you type into the calculator for the Rim Diameter. Of all the dimensions you actually might measure, ERD is the most critical dimension affecting spoke length, so it makes sense to measure it a few times at different places around the rim. Always count to make sure you use spoke holes that are actually opposite each other!

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Hub diameter is measured between centres of opposite holes in the hub flange. It is usually between 38 and 67 millimeters. Note that it is NOT the outside diameter of the hub's flange.

Spoke hole diameter, is the through diameter of any spoke hole. It is usually 2.6mm.

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Width from centre of flange to centre of flange may differ between left and right sides of the hub. In this illustration, W are the dimensions you enter into the calculator for the left and right sides of the hub, respectively. You can use a hub listed in our database, or measure your own hub by entering the distance from one flange to the other. Not a lot of precision is needed in the width dimensions. A few millimeters error in width will only lead to a fraction of a millimeter error in spoke length.

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Some useful internet links for spokes and wheel building:

Spoke Manufacturers
DT Swiss
Sapim Spokes

Wheel Building
Building Bicycle Wheels by Sheldon Brown A complete walk through of how to build a wheel. Highly recommended!