See Sizing Charts for the following brands:
Ariat: click here.
Bella Canvas: click here.
Carhartt: click here.
Cornerstone: click here.
Cutter & Buck: click here.
District: click here.
Gildan: click here.
New Era: click here.
Nike: click here.
OGIO: click here.
Port Authority: click here.
Red House: click here.
Sport Tek: click here.
Under Armour: click here.