We follow US sizing guidelines 

Here is an example of how our ideal customer would measure themselves:

A potential Ninja wants to buy a hoodie.  But he's not sure what size to pick. He makes the right choice:  
  1. He finds a measuring tape and works out his chest circumference, shoulder width and arm length.  This gives him the opportunity to compare the specifications of his body to the bespoke sizing table of the hoodie he wants to buy.
  2. Once he knows his own body's measurements, he would measure a similar item of clothing (a hoodie) that he owns to compare how tight or loose those specific measurements fit on him.
  3. Once he's done these two steps of the process, he can now know with relative certainty how loose, long, tight or short this specific hoodie is going to be.

All you need is a measuring tape.