Lay your jacket flat and buttoned on the floor. Measure across the chest at the highest point, armpit seam to seam.
Measure straight across the bottom of the jacket, edge to edge.
Flip your jacket over, and measure straight across from shoulder seam to shoulder seam.
Measure straight down the jacket from the top seam of the jacket where it meets the collar down to the bottom of the jacket.
Measure the arm of the jacket from the shoulder seam straight down to the bottom of the sleeve.
Measure straight across the cuff opening.