Step 1 — Band: wrap a soft tape around your ribcage just below your bust. Round to the nearest even number. That's your band size (32, 34, 36…).
Step 2 — Bust: measure around the fullest part of your bust while wearing a non-padded bra.
Step 3 — Cup: subtract band from bust. 1 inch difference = A cup, 2 = B, 3 = C, 4 = D, 5 = DD/E, 6 = DDD/F.
Example: 36" bust − 32" band = 4" → 32D.
Bra sizing changes constantly. Re-measure every 6 months, after weight changes, and after pregnancy. Most women wear the wrong size.