How jacket sizing works
Men’s jacket and suit sizes in the US and UK are based directly on chest circumference measured around the fullest part of the chest, under the arms, rounded to the nearest even number.
The formula
Jacket Size = Chest Circumference rounded to the nearest even inch, which is also the number printed on most US/UK suit jacket labels (e.g., a 40-inch chest is generally a size 40 jacket). This calculator also maps that to common S-XXL letter sizing and an approximate EU size, which typically runs about 10 higher than the US/UK number.
Fit also depends on shoulder width, jacket cut (slim vs. classic), and brand-specific patterns, so this is a starting-point estimate — trying on a jacket or checking a specific brand’s size chart is the most reliable method. For general reference purposes only.