Bra Size Calculator: Finding Your Fit Across US, UK, and EU Systems

Most women are wearing the wrong bra size. That's not a provocative claim — it's a finding repeated by lingerie retailers, fit specialists, and independent studies, with estimates ranging from 64% to 85% of women wearing a poorly fitting bra. The most common errors are a too-large band and too-small cup, because that's the combination that seems to "work" even when it doesn't. Getting your size right — and understanding how to convert it between international sizing systems — makes an immediate, noticeable difference in comfort and support.

A Real Scenario: Maria Shops From a European Brand

Maria, 33, lives in Miami and typically wears a 36C US size. She found a French luxury lingerie brand online with beautiful styles, but the website listed sizes only in EU notation. She knew her US size but couldn't confidently convert it.

Her 36C US breaks down this way: 36-inch band = EU 80 band. C cup with a 36 band means a 3-inch bust-to-band difference. In EU notation, that 3-inch difference still equals C. So her EU size was 80C. She confirmed by checking the brand's measurement guide, which suggested an underbust of 77–82 cm for their 80 band (her underbust measured 80 cm) and a overbust of 92–95 cm for 80C (her overbust measured 93.5 cm). The numbers aligned. She ordered 80C and it fit correctly.

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How Bra Sizing Actually Works

Bra size has two components: the band (the number) and the cup (the letter). The band measurement corresponds to your ribcage circumference. The cup size is determined by the difference between your full bust measurement and your band measurement. But the critical thing to understand is that cup size is relative to band size — a 34C cup holds the same volume as a 36B cup and a 32D cup. This is called "sister sizing."

Most bra sizing guides follow a similar measurement process. Measure your underbust — snugly around your ribcage, just below your bust. In the US/UK system, round to the nearest even number (some brands add 4 inches to the underbust, some add nothing or 2 — this is one reason sizes vary by brand). Then measure your overbust at the fullest point of your bust, keeping the tape parallel to the ground. The difference between these two measurements determines your cup size.

A difference of less than 1 inch = AA cup. 1 inch = A. 2 inches = B. 3 inches = C. 4 inches = D. 5 inches = DD (or E). 6 inches = DDD (or F). 7 inches = G. And the letters continue upward from there, with different brands and countries using different notation for larger cups.

Getting the Fit Right: Beyond the Numbers

The band should be horizontal and firm around your ribcage. It should not ride up in the back — if it does, the band is too large. The underwire (if present) should sit flat against your ribcage, not on breast tissue. The cups should contain all breast tissue with no spilling over the top or sides, and no wrinkling or gaping fabric.

If you're spilling over the top of the cup, you likely need a larger cup (try going up one cup size). If the cups wrinkle and gape, you likely need a smaller cup. If the band rides up, try a smaller band size. If the underwire digs into breast tissue rather than sitting on the chest wall, try a larger cup with the same band size — the underwire needs to be wider to accommodate more volume.

Sister sizing is useful for fit adjustments. If you're a 34D and the band feels slightly too tight but the cup is right, try 36C (the sister size with a larger band and adjusted-down cup that holds the same volume). If the band is right but the cup is slightly too small, try 34DD.

International Sizing Differences

Here's where it gets complicated. The US, UK, EU, French, Italian, and Australian bra sizing systems all use different scales — sometimes the same letter means a different cup volume.

The US and UK systems are close but not identical. In the US: D, DD, DDD, G, H, I, J. In the UK: D, DD, E, F, FF, G, GG, H. So a US 34DDD and a UK 34E hold roughly the same cup volume (both are about a 5-inch bust-to-band difference). But the letters are completely different, which confuses people constantly.

EU (continental European) bra sizing converts the band size to centimeters and uses different letter sequences. A US/UK 34 band is EU 75. A US/UK 36 band is EU 80. A US/UK 38 band is EU 85. Each band increment in EU equals 5 cm, while each US/UK increment equals 2 inches (5.08 cm — nearly identical). The cup letters in EU sizing follow A, B, C, D, E, F, G, H, I, J without the double-letter notation used in US/UK.

A US 34C is approximately EU 75C or UK 34C. A US 34DD is approximately EU 75E or UK 34DD. A US 36D is approximately EU 80D or UK 36D.

French and Italian systems use the same band conversion as EU but sometimes use slightly different cup letter mappings. Australian sizing historically followed UK sizing but with some brand variation. Japanese sizing uses centimeters and a letter system where A starts at a smaller cup volume than American A.

The Problem With "Add 4 Inches" Advice

For decades, the conventional bra-fitting advice was to measure your underbust and add 4 inches to get your band size. This practice originated when bras had less elastic and needed that extra room to close comfortably. Modern bras with better elastic and stretch don't need this addition — and following this outdated rule makes most women wear band sizes too large and cup sizes too small.

If your underbust measures 32 inches and you add 4, you end up in a 36 band. The band is loose. To compensate, you fasten on the tightest hook — but the bra is still providing less support than a proper 32 band would. And because you're in a 36 band instead of 32, your cup letter has to be smaller to hold the same volume (sister sizing). You end up in a 36B when you might actually be a 32D — a cup letter that sounds bigger and feels stigmatized, but physically holds exactly the same volume.

Look, lingerie brands have started moving away from the "add 4" rule, and many now recommend measuring underbust directly and rounding to the nearest even number. If you're between, try both and see which band feels snug but comfortable — snug enough that you can slip only two fingers underneath, not the whole hand.

Practical Tips for International Bra Shopping

Before ordering from any international brand, measure yourself carefully with a soft tape measure. Measure underbust on exhalation (relaxed ribcage) and overbust at the fullest point without compressing the tissue. Note measurements in both inches and centimeters.

Compare your measurements directly against the brand's size chart rather than relying solely on a conversion table. Good lingerie brands publish measurement ranges for each size, not just nominal numbers. A brand that says "80C: underbust 77–82 cm, overbust 92–97 cm" gives you real verification.

Check return policies before ordering. Even with correct measurements and careful size chart comparison, fit varies between styles — a plunge bra, a balconette, and a full-cup bra in the same nominal size will feel and fit differently. Free returns or easy exchanges make online international bra shopping practical. Without them, you're taking a larger gamble than the conversion tables suggest.