How to Measure
Two measurements are all you need to calculate bra size at home, both taken with a soft measuring tape and no padding — no fitting room required.
- Underbust (band) — wrap the tape directly under your bust, snug and level, following the same line all the way around your ribcage. It should feel firm but not tight enough to restrict breathing.
- Bust — wrap the tape loosely around the fullest part of your bust, keeping it parallel to the floor. Wearing a well-fitting (non-padded) bra while measuring bust gives a more consistent reading than measuring bare.
Round both measurements to the nearest whole inch or half-centimeter. Studies and industry surveys are frequently cited suggesting a large share of women wear an incorrectly sized bra — usually a band that's too big and a cup that's too small, which is why a genuine measurement-based starting point matters more than guessing from a label.
How the Calculation Works
This calculator uses the traditional US sizing method, still the most widely referenced starting point across fitting guides. The band size comes from your underbust measurement, rounded to an even number, plus 4 inches. The cup size comes from the difference between your bust and underbust measurements — each inch of difference steps up one cup letter (0" = AA, 1" = A, 2" = B, 3" = C, 4" = D, and so on).
It's worth knowing this traditional method isn't universally agreed upon — some modern fit specialists argue for methods that add less (or nothing) to the underbust measurement, on the reasoning that band construction and elastic have changed significantly since the traditional formula was popularized decades ago. Because of this, and because manufacturing varies by brand, treat any calculator's output — including this one — as an evidence-based starting point rather than a guaranteed fit.
A Guide to Breast Shapes
Cup letter alone doesn't capture how tissue is distributed, which is why the same letter can fit very differently depending on shape. Here's what each profile in the tool above generally means, the mechanical fitting challenge each one tends to create, and which bra styles and features tend to work best — with more on matching a style to what you'll actually be doing in the activity guide below.
Round
Similar fullness above and below the nipple line. Round shapes are usually the most forgiving to fit, since volume is evenly distributed and most cup constructions accommodate them well. Best for: full-cup, balconette, or plunge styles — this shape has the most flexibility of any profile.
Teardrop
Less fullness on top, more on the bottom — the classic sloped profile. A plunge or balconette cut, which is cut lower at the top of the cup, tends to follow this natural slope better than a full, rounded cup shape. Best for: plunge and balconette styles; a molded full-cup can leave empty space at the top.
East-West
Breast tissue points outward toward the sides rather than straight forward. Bras with side support panels or "wings" help center tissue forward, reducing both side spillage and gapping at the center. Best for: full-coverage styles with reinforced side panels; avoid plunge cuts, which offer little side containment.
Side Set
A wider gap between the breasts, with tissue positioned toward the outside of the chest. A narrower center gore (the panel between the cups) and side support help bring the profile together visually and physically. Best for: plunge styles with a narrow, low-set gore; wide-set balconette styles tend to widen the gap further.
Slender
Less overall volume relative to chest size, often with a flatter profile at the top of the cup. Lightly padded or push-up styles can add shape without the bulk of a heavily structured full-coverage cup. Best for: push-up or lightly lined styles; unlined styles can look less filled at the very top of the cup.
Relaxed / Pendant
Breast tissue sits lower on the chest wall with less firmness at the top — a natural result of tissue composition and gravity over time, not something "wrong" with the body. Full-coverage styles with a wide band and supportive straps generally give the most comfortable, secure fit for this profile. Best for: full-coverage styles with a wide band and wide-set straps; balconette and plunge styles typically under-support this profile.
What Are Sister Sizes?
A sister size is a different band-and-cup combination that holds roughly the same cup volume as your calculated size — useful when a size is close but not quite right. Moving to a sister size with a looser band and smaller-lettered cup (for example, 34C → 36B) keeps the same volume in a bigger, more relaxed band. Moving to a tighter band and bigger-lettered cup (34C → 32D) keeps the same volume in a smaller, firmer band. This is the mechanism behind most "if it's not quite right, try this instead" fitting advice — the cup letter isn't a fixed volume, it only means something relative to its band number.
Which Bra Type Is Best for Each Activity?
The same calculated size can call for a genuinely different bra depending on what you're doing in it — support needs, fabric, and construction all change with the activity, not just the size on the label.
Gym & Sports
High-impact movement pulls tissue in multiple directions at once, so support matters more here than in daily wear. Encapsulation styles (individual molded cups, similar to a regular bra) support each side independently and generally suit higher-impact activity and larger cup sizes better than compression alone. Compression styles, which flatten tissue against the chest, work well for lower-impact activity and smaller cup sizes. Wire-free construction avoids underwire movement during motion, and moisture-wicking fabric keeps sweat from sitting against skin through a longer session.
Everyday & Regular Wear
For all-day comfort under regular clothing, a seamless or t-shirt-style molded cup avoids visible lines under fitted tops, and a wider, more structured band distributes weight across the ribcage rather than the shoulders — which is what actually reduces strap dig by the end of a long day, more than the straps themselves.
Fashion & Going-Out
A plunge style works under lower necklines by angling the cups inward without a center panel showing. A strapless style relies almost entirely on band snugness for support, since there are no straps to help — sizing accuracy matters more here than in any other category. A backless style trades some support for open-back coverage, and tends to work best at smaller-to-mid cup sizes where less structure is needed to begin with.
Sleep
A wire-free bralette or soft, unstructured style avoids the pressure points an underwire can create while lying down for hours. Going without a bra entirely overnight is also a perfectly reasonable, comfortable choice for many people — there's no medical requirement to wear one while sleeping, so this comes down entirely to personal comfort.
Can One Bra Really Do Everything?
Not entirely — and honest guidance here matters more than a tidy sales pitch. A single well-made bra, most often a seamless, medium-support t-shirt bra in your correct size, can reasonably cover light daily wear, errands, and casual outings. What it typically can't do well is everything at once: it won't give a strapless outfit the support a strapless-specific bra provides, it won't manage sweat the way real sports-bra fabric does during an actual workout, and it won't disappear under a low, plunging neckline the way a plunge-cut style will. If budget or drawer space only allows for one or two bras, a correctly sized t-shirt bra is the most broadly useful starting point — just expect to add a second, purpose-built option for sport, formal wear, or plunging necklines specifically, rather than asking one bra to do a job it wasn't built for.
US vs. UK vs. EU/AU Bra Sizes: What's the Difference?
Band numbers and cup letters don't mean the same thing in every country, which is a common source of confusion when shopping internationally or from an overseas brand.
- US vs. UK — band numbers match directly (a US 34 and a UK 34 cover the same underbust range), but cup letters diverge above a D cup. UK sizing continues DD, E, F, FF, G...; US sizing more commonly continues DD/E, DDD/F, G, H — and the exact letters used vary by brand on both sides.
- EU (EN 13402) — uses a completely different band number: typically your underbust in centimeters, rounded to the nearest multiple of 5. A US/UK 34 is usually an EU 75 in the same style, with cup letters that generally track the UK sequence.
- France, Belgium & Spain — use a third band-numbering convention again, roughly the EU number plus 15 (so an EU 75 is commonly a FR/BE/ES 90).
- Australia & New Zealand — generally follow UK-style cup lettering, though band numbers can differ slightly by brand, so it's worth checking a specific retailer's own size chart when in doubt.
Use the bra size converter below to convert the size this calculator just gave you into US/CA, UK, EU, FR/BE/ES, or AU/NZ sizing.
Signs Your Bra Doesn't Fit
- Spilling — breast tissue overflowing above or around the cup usually means the cup is too small, not that the band needs to change; try the same band with a bigger cup letter.
- Gapping — empty, unfilled space in the cup usually means the cup is too big for that band; try a smaller cup letter or a differently-shaped cup (like a plunge instead of a full cup).
- Riding up the back — the band is usually too big; a properly fitting band stays level and doesn't creep upward during the day.
- Digging straps — since a properly fitting band should provide most of the support, straps digging in often signals the band is too loose, forcing the straps to compensate.
- Underwire poking or resting on breast tissue — the wire should sit flat against the ribcage at the sides and center, not on tissue itself; this usually means the cup size or shape is wrong for your profile.
Frequently Asked Questions
How does breast shape affect my final size?
The number and letter from the formula stay the same regardless of shape, but shape determines which cup construction (plunge, balconette, full-coverage, and so on) will actually contain that volume comfortably — this is why two people with the same calculated size can need very different bra styles.
What's the best bra style for a relaxed or pendant shape?
Full-coverage styles with a wide band, supportive (often wider-set) straps, and a cup cut lower at the top tend to work best, since they follow where the tissue naturally sits rather than trying to lift it into a shape it doesn't hold.
Can a sister size fix gapping or spilling?
Often, yes — if a size gaps, the tighter-band/bigger-cup sister size keeps the same band snugness while giving slightly more room in the cup's shape without changing the underlying volume; if a size spills only slightly, the looser-band/smaller-cup sister size can sometimes resolve it without losing support.
Why do my measurements give a size that doesn't match what I usually buy?
This is extremely common — sizing isn't standardized across brands, and vanity sizing (labeling garments smaller than their actual measurements) is widespread. Use this result as a starting point for trying on, not a guarantee of the exact label you'll need in every brand.
Should I measure over or under clothing?
Under light clothing or directly on skin gives the most accurate reading — avoid measuring over thick sweaters or padded bras, which can add an inch or more to both measurements and skew the result.
What's the best bra for working out?
It depends on cup size and impact level: encapsulation styles (individual molded cups) generally support higher-impact activity and larger cup sizes better, while compression styles suit lower-impact activity and smaller cup sizes. Wire-free construction and moisture-wicking fabric matter more for sport than for everyday wear.
Can I wear the same bra for everyday wear and the gym?
You can, but it's rarely the best option for either — a t-shirt bra lacks the moisture-wicking fabric and directional support a real workout needs, and a sports bra usually lacks the smooth, seamless profile that looks best under fitted everyday clothing. One bra can cover light activity in both categories; it won't excel at either.
Is it bad to sleep in a bra?
No — there's no medical requirement either way. A soft, wire-free bralette can be more comfortable for some people while lying down, since a structured underwire can dig in overnight; going without a bra entirely is equally fine. This comes down to personal comfort, not health.
Why are UK and US bra sizes different?
Band numbers match between the two, but cup letters diverge above a D cup because the two markets extended their labeling systems differently over time — UK sizing generally continues DD, E, F, FF, G, while US sizing more often continues DD/E, DDD/F, G, H. The exact letters used also vary by individual brand.
How do I convert my bra size between US, UK, and EU?
Use the bra size converter above — band numbers shift between US/UK and EU/FR systems (EU typically uses your underbust in centimeters, rounded to the nearest 5), while cup letters generally carry over from UK-style lettering in EU, FR/BE/ES, and AU/NZ sizing.
This calculator provides a measurement-based estimate for general informational purposes. Individual elasticity, brand manufacturing differences, and personal comfort preferences all affect final fit — use this result as a starting point for trying on, not a guarantee of fit in any specific brand or style.