Here is a the conversion from English to metric.
module = 25.4 / pitch
Here is a table with approximate conversions from pitch to module.
There will be extra wear and some inefficiency due to not having an exact match, however this is good enough for most purposes.
32 pitch = about 0.8 module (actually 0.793 module)
48 pitch = about 0.5 module (actually 0.529 module)
64 pitch = about 0.4 module (actually 0.396 module)
Pinions are sometimes labeled as "metric 48" pitch which is actually 0.6 module.
There are about six different shaft sizes used in electric helicopters today: 1mm, 1.5mm, 2mm, 2.3mm, 3.17mm, and 5mm.
The 1mm shafts are used by the "N20" style tail motors used in micro helicopters. Wes-Technik stocks 0.5 module brass pinions with 6-15T for this shaft size.
The 1.5mm shafts are used by the GWS IPS-style "performance" tail motors used in micro helicopters. Wes-Technik stocks 0.5 module brass pinions with 7-12T for this shaft size.
The 2mm and 2.3mm (3/32 inch) shafts are used by main motors on the micro helicopters, such as the Piccolo, Dragonfly, Hummingbird, etc. The 2mm bore pinions are easy to find, but the 2.3mm bore pinions are much more difficult to find.
The standard Hacker B20-series motors have a 2.3mm shaft, but Aircraft World carries some B20-S motors with a standard 2mm shaft, which makes pinion selection much easier.
Precision Model Products modifies the Team Losi Micro RS4 pinions bored out to 2.3mm for Hacker B20 series motors. These pinions work but are not the best because they are very noisy.
Sparrow Products now sells a pinion adapter (bore reducer) which allows you to use a 3.17mm pinion with a 2mm or 2.3mm motor shaft. This is available from their website.
If you know of another source for 0.5 module metal pinions for 2mm and 2.3mm shafts with setscrews, then please send me a message at RC Groups , because these pinions are difficult to find.
The 3.17mm (1/8 inch) and 5mm shafts are used for nonmicro helicopters. Tower Hobbies carries a wide range of 1/8 inch pinions in both 32 and 48 pitch sizes which are usable in electric helicopters.
Kawada makes 48 pitch 3.17mm pinions up to 40T. RCmart carries a full selection of Kawada pinions.
Please note the Duratrax Mini Quake pinions are NOT 0.5 module and will not work on most micro helicopters. The teeth are too big and the pitch may be 0.6 module, but haven't verified this.
The press-on pinions for GWS EPS gearboxes are 0.4 module and have a 2mm bore. These are usable for the Hornet, but they do not have a setscrew.
If noise is a concern, then the smallest pinions should be avoided because they tend to be noisier then the larger pinions.
The Team Orion Elite Micro Modified motor has a very short shaft, and works best with a tall pinion such as the Align 2.3mm pinions.