[Phishin Phool]

I used to wonder this and see it asked all the time so here is a link to a  page that will answer that - it shows you how to calculate it based on what info you have and since the math can be tough there is a d/l spreadsheet that will do it for you. Good page to bookmark until you get a feel for it.
 
http://www.apexhifi.com/specs.html

 

[KG Jag]

This might be more helpful for most:
 
http://www.audiobot9000.com/

 

[Phishin Phool]

Wow that is a great reference - undfortunately my favorite amp from Garage1217 (Project Sunrise)  isn't listed there and neither is my music hall DAC/AMP combo. Also no signs of Decware or Creek Audio