Lay the jacket flat on a smooth surface
Zip or button the jacket closed
Measure chest width from armpit to armpit
Double the chest width for full chest measurement
Measure shoulder width from shoulder seam to shoulder seam
Measure sleeve length from shoulder seam to cuff
Measure center back length from the base of the collar to the hem
Measure waist width at the narrowest part of the jacket
Measure hem width straight across the bottom
Measure armhole opening if needed
Compare all measurements to a size chart