Bra Size Converter Formula

Understand the math behind the bra size converter. Each variable explained with a worked example.

Use the Bra Size Converter

Formulas Used

EU Band Size

eu_band = (us_band - 32) * 2.54 + 63

UK Band Size

uk_band = us_band

French Band Size

french_band = us_band + 51

Bust Circumference (approx in)

bust_circumference = us_band + us_cup

Variables

VariableDescriptionDefault
us_bandUS Band Size34
us_cupUS Cup (A=1, B=2, C=3, D=4, DD=5)3

How It Works

Bra Size Conversion

Band Size Conversions

US/UKEUFrench 327085 347590 368095 3885100

Cup Sizes

US and UK cup letters generally align through D/DD but diverge at larger sizes. EU uses A, B, C, D, E, F progression.

How to Measure

  • Band: Measure snugly around the ribcage just below the bust
  • Cup: Difference between bust measurement and band measurement (each inch = one cup size)

    • Worked Example

    Convert US 34C to other systems.

    us_band = 34us_cup = 3
    1. 01EU band = (34-32) x 2.54 + 63 = 68 (round to 75)
    2. 02UK band = 34 (same as US)
    3. 03French band = 34 + 51 = 85
    4. 04Cup C is the same letter in US, UK, and EU

    Frequently Asked Questions

    Do bra sizes vary between brands?

    Yes, significantly. Sizing is not standardized across brands or even within the same brand across styles. Always try on or check the specific brand size guide.

    What is sister sizing?

    Sister sizes have the same cup volume but different band sizes. For example, 34C, 32D, and 36B all have the same cup volume. Go up a band and down a cup (or vice versa).

    Ready to run the numbers?

    Open Bra Size Converter