Bra Size Calculator
Find your bra size from bust and band measurements, with sister sizes.
Inputs
Measure snugly around your ribcage directly under your breasts.
Measure around the fullest part of your bust, keeping the tape parallel.
US/UK Bra Size
32D
EU Size
90D
Band Size
32in
Cup Letter
D
Cup Difference
4.0in
Bust minus band
Sister Size (looser band)
34C
Sister Size (tighter band)
30DD
Step by step
Round under-bust to nearest even band
32 in → nearest even
= Band 32
Cup difference
36 (bust) − 32 (band)
= 4 in → Cup D
US/UK size
= 32D
Sister sizes (same volume)
= 34C • 32D • 30DD
A larger band with a smaller cup encloses the same volume.
How it works
The band size is your under-bust measurement rounded to the nearest even number. The cup is determined by how many inches your full bust exceeds the band: 1 in = A, 2 in = B, 3 in = C, etc. Sister sizes share the same cup volume: moving up one band size requires moving down one cup letter to keep the same fit, and vice versa. The EU band is your under-bust in centimetres plus roughly 10 cm, rounded to 5.
Formulas
Band size
Band = under-bust in inches, rounded to nearest even number
- U_in
- Under-bust measurement in inches
Cup letter
Cup index = round(bust inches − band); A=1, B=2, C=3 …
- B_in
- Full bust measurement in inches
Frequently Asked Questions
What is a sister size?
Sister sizes share the same cup volume but use a different band. For example, 34C, 36B, and 32D all have the same amount of cup space. If a 34C doesn't fit well in the band, a 36B gives you more band room with the same cup.
Should I add inches to my under-bust measurement?
The old 'add 4 inches' method is largely obsolete with modern bra construction. This calculator uses the contemporary approach: round directly to the nearest even number. If you prefer a looser band, size up by 2.
Why is my EU size different?
EU band sizes are based on the under-bust in centimetres (plus a fitting allowance), typically 10–15 cm larger than US. So a US/UK 34B is roughly an EU 75B.
I'm between sizes — which do I choose?
Try both and see which gives you a more level band and natural cup shape. Bra fit experts recommend starting with a snug band and adjusting cup size first, since a loose band cannot properly support the bust.