Get Help from someone to take your measurements with normal shirt. 
Try all measurements in centimeters.

- (A) Chest Round ?
 - (B) Back Shoulder (Left Point to Right Point) ?
 - (C) Collar Round ?
 - (D) Sleeve (Top Shoulder Point to Wrist) ?
 - (E) Waist Round ?
 - (F) Hip Round ?
 - (G) Front Length (According your Coat Style) ?
 - (H) Back Length (if Regular Coat) ?
 - (K) Back Length (if Long Coat) ?
 
Here are some tips on how to get your size, but please follow the current item's size guide.

Metallic Shearling Military Jacket
Loved the jacket it's everything I expected and more perfect fit