Bra Size Calculator
Find your bra size from bust and band measurements, with sister sizes.
About Bra Size Calculator
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
Worked example
With the default values below, Bra Size Calculator returns US/UK Bra Size of 32D. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Measurement Unitone of Inches (US/UK), Centimetres
- Inches (US/UK)
- Under-bust (Band) Measurementaccepts 20–180
- 32
- Full Bust Measurementaccepts 20–200
- 36
Results
- US/UK Bra Size
- 32D
- EU Size
- 90D
- Band Size
- 32 in
- Cup Letter
- D
- Cup DifferenceBust minus band
- 4.0 in
- Sister Size (looser band)
- 34C
- Sister Size (tighter band)
- 30DD
Step by step
- 1Round under-bust to nearest even band= Band 32
- 2Cup difference= 4 in → Cup D
- 3US/UK size= 32D
- 4Sister sizes (same volume)= 34C • 32D • 30DDA larger band with a smaller cup encloses the same volume.
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.