What You'll Need
All you need is a soft measuring tape (the kind used for sewing) and a mirror. If you don't have a soft measuring tape, use a piece of string and measure it against a ruler. Wear your best-fitting unpadded bra for the most accurate results.
Step 1: Measure Your Band Size
Wrap the measuring tape snugly around your ribcage, directly under your bust. The tape should be level all the way around and firm but not tight — you should be able to fit one finger underneath comfortably.
Round to the nearest whole number. If the number is even, that's your band size. If it's odd, round up to the next even number. For example, if you measure 33 inches, your band size is 34.
Step 2: Measure Your Bust
Wrap the tape around the fullest part of your bust. Keep the tape level and don't pull it tight — it should rest gently against your body. Stand straight and breathe normally.
Round to the nearest whole number.
Step 3: Calculate Your Cup Size
Subtract your band measurement from your bust measurement. The difference determines your cup size:
0-1 inch = AA or A cup. 1 inch = A cup. 2 inches = B cup. 3 inches = C cup. 4 inches = D cup. 5 inches = DD/E cup. 6 inches = DDD/F cup. 7 inches = G cup. 8+ inches = H cup and above.
For example: if your band measures 34 inches and your bust measures 38 inches, the difference is 4 inches, making you a 34D.
Sister Sizes: Your Secret Weapon
Sister sizes are bra sizes that share the same cup volume but with different band sizes. If your usual size doesn't feel right, try a sister size. For example, a 34C has the same cup volume as a 32D and a 36B.
If the band feels too tight but the cups fit, go up a band size and down a cup size (34C to 36B). If the band feels too loose but the cups fit, go down a band size and up a cup size (34C to 32D).
When to Re-Measure
Your bra size can change due to weight fluctuations, hormonal changes, pregnancy, nursing, aging, and even changes in muscle mass. We recommend re-measuring every 6-12 months, or whenever your bras start feeling consistently uncomfortable.