Bra Size Calculator
Enter your underbust and bust measurements to get your band size, cup size, and nearby sister sizes — a measurement-based starting point for a better fit.
How this calculator estimates your bra size
Two measurements drive a measurement-based bra size estimate: your underbust (band) and your bust at its fullest point. The gap between those two numbers determines your cup letter, while your underbust measurement, rounded to a standard band number, sets the foundation everything else builds on. This calculator applies the same band-rounding and cup-difference logic used across the US, UK, and EU sizing systems, so the number you get back lines up with what you'd see on a standard size chart.
It's worth saying upfront: this gives you a strong, evidence-based starting size, not a guaranteed perfect fit. Cup shape, wire width, fabric stretch, and how your tissue is distributed all affect how a given size actually feels, which is exactly why sister sizing and fit-check adjustments exist.
The band and cup formula, explained
Band size (US/UK, inches)
Band = Underbust, rounded to nearest even number
A 31" underbust rounds up to a 32 band. A 33" underbust rounds up to a 34 band. Odd underbust measurements always round up to the next even band size.
Band size (EU, centimeters)
Band = Underbust (cm), rounded to nearest 5
A 73cm underbust rounds to a 75 EU band. EU sizing works in 5cm steps rather than the 2-inch steps used in US and UK band sizing.
Cup size (the "golden formula")
Cup = Bust − Band, in inches
A 1-inch difference is an A cup, 2 inches is B, 3 inches is C, 4 inches is D, and each additional inch moves one letter further along — though the exact letters used after D vary between US and UK systems.
Cup letter progression
1"=A · 2"=B · 3"=C · 4"=D · 5"=DD/E · 6"=DDD/F
US sizing typically uses DD then DDD/F; UK sizing uses DD then E, F, FF, G. The underlying difference in inches is identical — only the label changes by region.
US, UK, and EU sizing — why the same body gives different labels
Band sizes are identical between US and UK systems — a 34 band is a 34 band on both sides of the Atlantic. Where they diverge is the cup letters used past D. US sizing commonly continues DD, then DDD or F. UK sizing instead continues DD, then E, F, FF, and G. That means a US 34DD equals a UK 34E, and a US 34DDD matches a UK 34F — the actual cup volume is identical, only the letter differs.
EU sizing changes the band number entirely, since it's based on centimeters rather than inches, rounded to the nearest 5cm rather than the nearest even inch. A brand using EU sizing will show a noticeably different number than the same measurements would produce on a US size chart, even though the physical fit is meant to be equivalent.
Sister sizing: the fix for "almost right"
Sister sizes share the same cup volume but spread it across a different band size. Moving up one band size while dropping one cup letter keeps the same total volume but loosens the band — useful if your band digs in or leaves marks. Moving down one band size while increasing one cup letter keeps the same volume but tightens the band — useful if your band rides up or your straps are carrying too much weight, since the band is meant to provide roughly 80% of a bra's support.
| Main Size | Sister Size (Larger Band) | Sister Size (Smaller Band) |
|---|---|---|
| 34D | 36C | 32DD |
| 36C | 38B | 34D |
| 32D | 34C | 30DD |
| 38B | 40A | 36C |
Sister sizing is the standard move when your size is sold out, or when the band and cup don't feel right at the same time — try the sister size before assuming you need a completely different size range.
Signs your calculated size needs a small adjustment
The band should stay level and snug all the way around. If it rides up, try a smaller band with a larger cup (a sister size) before assuming the whole size is wrong.
Empty space or wrinkled fabric at the top of the cup usually means the cup is too big for that band. Try one cup size down, or a different cup shape.
Tissue overflowing the cup edges, sometimes called "quad-boobing," usually means the cup is too small. Size up one cup letter before changing the band.
Straps should only provide about 20% of total support. If they're cutting in, the band likely isn't doing its job — tighten the band or try a firmer-banded sister size.
More everyday life and health calculators on CalcMora
Bra sizing is just one piece of overall clothing fit, and a couple of related tools on CalcMora round out the picture. If you're also sorting out bottoms, the pant size calculator by height and weight uses a similar measurement-based approach to estimate waist and inseam sizing. For dresses specifically, the dress size measurements calculator combines bust, waist, and hip figures the same way this tool combines underbust and bust to land on a size. And since overall frame and proportion play a role in how clothing fits more broadly, the body shape calculator can help identify your general shape category, which is useful context when comparing cup styles and coverage options across different brands.
Bra size calculator — FAQ
How do I measure for a bra size calculator?
You need two measurements. For your band, wrap a soft tape measure snugly around your ribcage directly under your bust, keeping it level all the way around your back. For your bust, measure loosely around the fullest part of your chest, usually at nipple level, without compressing any tissue. Take both measurements wearing a well-fitting, non-padded bra or no bra at all for the most accurate starting point.
How is cup size calculated from my measurements?
Cup size comes from the difference between your bust measurement and your band measurement, in inches. A 1-inch difference is an A cup, 2 inches is a B cup, 3 inches is a C cup, 4 inches is a D cup, and each additional inch moves one letter further, though the letters used after D vary by country. The band number itself comes from your underbust measurement, rounded to the nearest even number in US and UK sizing.
What is a sister size and when should I use one?
A sister size is a different band-and-cup combination that holds roughly the same cup volume as your main size. Moving up one band size while dropping one cup letter (for example, 34D to 36C) keeps the same volume but loosens the band. Moving down one band while increasing one cup letter (34D to 32DD) tightens the band while keeping the same volume. Sister sizing is the standard fix when your size is sold out, or when the cups feel right but the band doesn't.
Why do US and UK bra sizes differ after a D cup?
Band sizes are identical between US and UK sizing, but the cup letters diverge once you pass D. US sizing typically continues with DD, then DDD or F, while UK sizing uses DD, then E, F, FF, and G. This means a US 34DD is equivalent to a UK 34E, and a US 34DDD matches a UK 34F — the cup volumes are the same, only the letter used to label them changes.
How does EU bra sizing work compared to US and UK?
EU band sizing is based directly on your underbust measurement in centimeters, rounded to the nearest 5cm, rather than the inch-based system US and UK brands use. A 73cm underbust typically rounds to a 75 EU band. EU cup letters also use a different stepping pattern than US or UK, so the same physical measurements can produce different-looking labels depending on which region's sizing system a brand follows.
What if my band-to-bust difference is more than 10 or 11 inches?
Standard cup letter charts typically run out around K or L for very large differences, but the underlying logic still applies — each additional inch of difference moves one letter further up the alphabet. At this range, brand-specific sizing varies more than usual, since not every brand manufactures the full range of larger cup sizes, so checking a specific retailer's size chart becomes more important than relying on a generic letter alone.
Why might a bra in my calculated size still not fit well?
A measurement-based calculation gives a strong starting point, not a guarantee, because cup shape, wire width, breast projection, and tissue distribution all vary independently of the two measurements used here. If the band digs in or rides up, or the cups gape or show spillage at the edges, your best next step is usually a sister size or a different cup shape (full-coverage, balconette, or plunge) rather than a completely different size altogether.
This tool is for educational purposes only. Always verify important results with a qualified professional.
