The fastest approach: use the size chart above and match it to your suit, jacket, or shirt size. For suits and jackets, that means your US jacket size (38, 40, 42, and so on). For shirts and sweaters, match it to the size you'd normally wear (Small, Medium, Large).
The catch: a size 40 jacket from one brand can fit noticeably differently than a size 40 from another, cut, country of origin, and even the decade it was made all affect it. This method gets you in the right neighborhood fast, but it's the least precise of the three if your wardrobe spans a lot of different brands.
▶ Watch: Kirby on Matching Hanger Size to Clothing Size