There is a reasonable degree of expansion in the Smart Jacket, and it should fit under / over most items based on your standard chest size. The only variance could be over a big, bulky touring jackets, for example. If you are planning to wear the Smart Jacket under your riding jacket, measure your chest size first wearing what you will wear when riding (base layers, t-shirt, fleece, etc.). If you are planning to wear the Smart Jacket over your riding jacket, measure your chest over the jacket.

Dainese Men's D-Air Smart Jacket Sizing

Alpha Size	Euro Size	US Size	Chest (cm)	Waist (cm)
XS	42	32	82 - 86	70 - 74
SM	44-46	34-36	86 - 94	74 - 82
MD	48-50	38-40	94 - 102	82 - 90
LG	52-54	42-44	102 - 110	90 - 98
XL	56-58	46-48	110 - 118	98 - 106
2XL	60	50	118 - 122	106 - 110

Sizing information is provided by the manufacturer and does not guarantee a perfect fit.

How to Measure

C Chest
Measure around the fullest part of your chest, under the armpits, keeping the measuring tape parallel to the ground.
W Waist
Measure around your natural waistline, around the top of the hip bones, inline with the navel while keeping the measuring tape parallel to the ground.