FitCard

Size charts tell you who a size is meant for, not how the shirt will fit. Most brands publish body charts ("L is for a 41–44″ chest"), and those can't tell two shirts apart. Six Uniqlo Larges, all "for a 41–44″ chest", measure 41½″ to 50½″ around.

FitCard works from shirts you already own. Name 3–5 shirts that fit you well (brand, item and size), plus any that don't and where they're off (tight in the belly, rides up). FitCard compares their real garment measurements with the shirt you're looking at. You get one size and why, in terms of your own shirts ("fits like your Buck Mason L, 1″ roomier"). You also get how it will fit at the chest, belly and length, and how sure it is.

Ask it:

• Which size to buy in a specific shirt.
• Which shirts will fit you, each in the right size, with a link to the shop and to your full FitCard on fitcard.ai.
• A shirt's garment measurements in every size.

The measurements come from brand charts, wholesale spec sheets and shirts measured by FitCard users with their phones. If one of your shirts isn't known yet, FitCard says so, and you can measure it with your phone at fitcard.ai.

No sign-in. FitCard only sees the shirts you name and never asks for your body measurements. Sizes are a best estimate, so check the shop's return policy. Some shop links are affiliate links; a commission never changes the size.