Bra Size Calculator

Frequently Asked Questions

How is bra size calculated?

Band size comes from the snug underbust measurement (US/UK round to the nearest even inch). Cup size is bust − band: 1 in difference = A, 2 in = B, 3 in = C, 4 in = D, 5 in = DD/E, and so on.

How do US/UK and EU/FR sizing differ?

US/UK bands are in inches; EU bands are underbust cm rounded to the nearest 5 (a 34 US band ≈ 75 EU). EU/FR cup letters also start at A but increase in different bust-minus-band increments - convert via a sizing chart rather than guessing.

What are sister sizes?

Sister sizes are bras with the same cup volume but a different band: going down a band size means going UP a cup (and vice versa). A 34C is sister to 32D and 36B - useful if the band fits poorly but the cup is right.

My measured size doesn't fit - what now?

Try the closest sister sizes and styles from different brands. Cut, wire shape and stretch vary widely, so a "perfect" measurement is just a starting point. Many people wear a band one size too big and a cup two sizes too small.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A bra size is two numbers fused into one label - a band size (the snug measurement around your ribcage just under the bust) and a cup letter (the difference between the full bust and the band). Get either one wrong and the bra will ride up, dig in, gape, or spill, and you will blame the brand instead of the math. This calculator takes a fresh underbust and bust measurement (in inches or centimetres), then converts the two numbers into US/UK sizing and the metric EU/FR conventions. It also lists your sister sizes - equivalent volumes at adjacent band widths - because the same cup volume is sold under several labels (32D = 30DD = 34C) and knowing all three triples the styles you can shop from.

Formula / Method

Modern band sizing uses the underbust measurement directly: round it to the nearest even inch (the old "add 4 to the underbust" rule is widely abandoned and gives bands four inches too loose). EU bands are taken in centimetres and rounded to the nearest 5 cm step (60, 65, 70, 75, 80, 85, 90, 95, …). French sizing equals the EU band + 15. The cup is the rounded difference, in inches, between full bust and band: a 1″ difference is an A cup, 2″ a B, 3″ a C, 4″ a D, 5″ a DD, 6″ a DDD (or E), then F, G, H, I, J, K. The same cup letter is used across US, UK, EU and FR labels in this tool. A sister size keeps the cup volume constant: drop the band one step (two inches) and add one cup letter, or raise the band one step and drop one letter - the cup itself contains the same fabric, just attached to a different band.

Worked Example

Underbust 32″, bust 36″. The band is 32 (already even). The cup difference is 36 − 32 = 4″ - a D cup. So US = 32D, UK = 32D. In centimetres, 32″ ≈ 81 cm, which rounds to EU band 80; the French equivalent is 80 + 15 = 95. EU = 80D, FR = 95D. Sister sizes: 30DD (down one band, up one cup) and 34C (up one band, down one cup) all hold the same cup volume, so if the 32D band feels tight try 34C; if it gapes try 30DD.

Common Mistakes

Using the old "add 4" or "add 5" rule and ending up two band sizes too large - if the back of the bra rides up, the band is too loose, not too tight.

Measuring the bust loosely or while leaning forward - stand upright, breathe normally, tape parallel to the floor across the fullest point.

Wearing a padded bra during measurement, which inflates both the band and the cup.

Assuming a D cup is "large" - a 30D is a small cup volume on a small frame; cup letters only make sense paired with their band.

Sticking to one band when the cup feels wrong - the right fix is almost always a sister size, not a different cup letter on the same band.

Tips & FAQ

How tight should the band be? Snug enough that you can just slip two flat fingers under it horizontally, on the loosest hook - the band provides about 80% of the support, the straps only 20%. Why do I fit different sizes across brands? Cup volume is not standardised; one brand's 34D might equal another's 34DD, especially across UK, US and EU labels. Use sister sizes to widen the search. What if my measurements fall between sizes? Try both and choose the band that stays put when you raise your arms. How often should I re-measure? Every 6–12 months, after weight changes of 5 lb or more, after pregnancy, and after starting any new training program - the ribcage and breast tissue both change. Sports vs everyday bras? The same band/cup math applies, but compression-style sports bras often fit one band size up and unstructured cups blur the letter; size up the band, not the cup, for high-impact wear.

Frequently Asked Questions

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