Measuring your body for jacket fitment is the right place to start, but it is only a guideline. Different shapes of bodies might require a slightly smaller or larger jacket in any given style. Trying on the jacket you are interested in is always recommended.
The measurement of the chest in its maximum extension during normal breathing, measured with the person standing, passing the measuring tape horizontally around the torso over the scapula bones (the shoulder blades) and just under the armpits.
Sleeve length from shoulder (arm straight)
The distance from the most protruding point of the shoulder to the wrist (on the end of the Ulna bone), measured with the arm relaxed at the person’s side.
The circumference measured at 2 cm below the Adam’s apple in the front, and at the 7th cervical vertebra in the back (i.e. NOT horizontally).
The circumference measured horizontally around the waist, just above the highest point of each of the iliac crest bones, during normal breathing, on the person standing, with a relaxed abdomen.
Always remember when sizing safety equipment like helmets and jackets it is always best to try it on first, if possible, to make sure that the fitment is accurate.