[trains are bad]

Well, there is clearly a difference mathematically since Google says 3mm is .137795276. Is there a distinction at all in the real world? I just bought some plugs labeled 3.5mm, and they plug right into my MicroDAC which is advertised as 1/8.

 

[tracky]

3.5 mm is the correct measure. 1/8 inch is only an approximation.

Read more about it here:

http://en.wikipedia.org/wiki/Jack_plug