How do you measure a turbo size?

[AWD1gTalon]

I'm interested to know how a turbo size is measured. When people talk about trim size, what is that standing for or what wheel is measured. An example, a 50 trim. where does that number come from, which wheel is measured and would it be exactly 50mm? And how does each wheel affect flow and performance.
Thanks.

[Iceman]

50trim means the wheel it uses.

[L2RTSiAWD]

More specifically, the Compressor wheel.

[AWD1gTalon]

so by measuring the compressor wheel i'll get say 50mm is there a taper on the wheel or anything where it would affect a person measuring it or is 50 all the way down. thanks, this is just something i've been wondering.

[kpt4321]

Trim is a ratio of the exducer diamter and the inducer diameter. Trim is equal to the square of the inducer diameter divided by the exducer diameter.

[AWD1gTalon]

thanks, i was wondering where the numbers came from and how they were determined.