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If you know your size, that's fine. However I have found that most people have a problem area e.g. long, short waisted, narrow or wide shoulders, that is why I suggest you have a friend measure you to get a proper fit. Keep your measuring tape snug but not tight. Never use a string to measure because the measurements are never accurate. Let me know your problem area if you have one.
How to measure male
Overarm A for lab coats and jackets
Chest B around fullest part of chest
Waist C over shirt but not over pants or belt
Inseam D crotch to ankle, allow for shoes
Sleeve Length, male and female:
With forearm parallel to chest, measure from back center of neck around the elbow to wrist bone.
Neck (collar), male and female:
Take a well fitting shirt and lay the collar flat and measure from center of neck button to button hole.