Stop guessing.
Know your true UK bra size.
Two measurements, one honest calculator. No mailing list, no app download — just a band and cup size worked out the way independent UK fitters actually do it.
The Calculator
Measure over a well-fitting, unpadded bra for the truest result.
How to measure yourself properly
A calculator is only as accurate as what you feed it. These three steps take about ninety seconds and make the difference between a size that fits and one that just about does the job.
Find the underbust line
Wearing a non-padded bra, run the tape directly under your bust, parallel to the floor. Pull it snug — firm enough to stay put, not tight enough to dig in.
Measure around the fullest point
Standing naturally, wrap the tape around your back and across the fullest part of your bust. Let it rest — don't pull it flat or let it sag.
Record the difference
Enter both numbers into the calculator above. The gap between the two is what decides your cup — the bigger the gap, the larger the cup letter.
Why UK bra sizing feels different from everywhere else
The band comes first, always
In the UK system, the number in front of your size — 30, 32, 34, and so on — describes the ribcage measurement, not the cup. It's the foundation the whole bra hangs from, which is why fitters spend more time getting the band right than anything else. A band that's too loose forces the cup and straps to do work they were never built for, and no amount of cup adjustment fixes that.
Cup letters aren't a scale of "bigger" or "smaller"
This is the part most people get wrong on their first go: a D cup on a 30 band and a D cup on a 38 band hold roughly the same cup volume — they just sit on a narrower or wider ribcage. The letter only means something once it's paired with its band. Judging a cup size on its own, without the band attached, tells you almost nothing.
How it splits from US and EU labelling
UK and US sizing share the same band logic but drift apart once the alphabet gets involved — a UK DD sits close to a US DDD/E, and by the time you reach G and H cups the two systems can be a full letter or more apart. European sizing swaps inches for centimetres entirely: a UK 34 is roughly a EU 75, and the cup letters usually shift down by one as you cross the Channel. None of this is standardised by law, which is exactly why the same labelled size can feel completely different from one brand to the next.
Brands cut to their own idea of "normal"
Two bras tagged 34C can have gores set at different depths, wires angled differently, or cups shaped for a different natural profile — because every label writes its own pattern. Treat any calculator, this one included, as a confident opening estimate, then let the mirror and a short walk around the shop make the final call.
The full UK bra size chart
Find your underbust measurement in the left column, then read across to the difference between your bust and underbust to land on a cup letter.
| Band | Underbust (cm) | 0" diff | 1" AA/A | 2" B | 3" C | 4" D | 5" DD | 6" E | 7" F |
|---|---|---|---|---|---|---|---|---|---|
| 28 | 63–67 | AA | A | B | C | D | DD | E | F |
| 30 | 68–72 | AA | A | B | C | D | DD | E | F |
| 32 | 73–77 | AA | A | B | C | D | DD | E | F |
| 34 | 78–82 | AA | A | B | C | D | DD | E | F |
| 36 | 83–87 | AA | A | B | C | D | DD | E | F |
| 38 | 88–92 | AA | A | B | C | D | DD | E | F |
| 40 | 93–97 | AA | A | B | C | D | DD | E | F |
| 42 | 98–102 | AA | A | B | C | D | DD | E | F |
Bands are shown in even numbers only, as UK brands rarely produce odd-numbered bands. If your underbust measurement rounds to an odd number, add one to reach the nearest available band.
Understanding sister sizes
A sister size holds the same cup volume as your true size but redistributes it across a different band. It's the reason a 34C and a 36B can fit almost identically — useful to know when your usual size sells out, or when a brand's fit runs unusually loose or tight.
Going up a band
Move up one band size (say, from 32 to 34) and you'll need to drop one cup letter to keep the same volume — a 32DD becomes a 34D. This is the move worth trying if your current band leaves red marks or rides up your back during the day.
Going down a band
Move down one band size and go up a cup letter instead — that same 32DD becomes a 30E. Reach for this if your band gapes at the back or you feel like your bust is barely held in place.
Signs your bra is the wrong size
A calculator gets you close. These five checks — done in front of a mirror, arms raised, arms down — tell you if you've actually landed on the right size.
The centre gore floats
The flat panel between the cups should rest against your breastbone. If it hovers or bounces, the cups are too small or the band is too loose to hold it in place.
Straps dig into your shoulders
Straps should sit comfortably without slipping or carving grooves. If they're doing all the lifting, the band is too loose and needs to take on more of the work.
The band rides up at the back
It should sit level, parallel to the floor, all the way round. A band that climbs up your back has stretched out or was too big from the start.
Breast tissue spills over the cup
Bulging at the top or sides means the cup is too small — sizing up a cup, not the band, is usually the fix.
The cup fabric wrinkles
Loose puckering across the cup means there's empty space in there. Try dropping a cup size or moving to a sister size with a smaller band.
You can't wait to take it off
A correctly sized bra should be close to unnoticeable by mid-afternoon. Constant awareness of it, however minor, is usually your body flagging a fit issue.
Haven't measured yet?
The calculator only takes two numbers. Get your estimated UK size, plus sister sizes, in under a minute.
Back to the calculatorHow to Find Your Correct UK Bra Size
Start With Accurate Bra Measurements
The easiest way to find your UK bra size is to start with accurate measurements. You only need a flexible measuring tape and a few minutes. Your underbust measurement is taken directly underneath your bust, while your bust measurement is taken around the fullest part of your chest. These two measurements are the foundation of a UK bra size calculation.
When using a UK bra size calculator, small measurement differences can affect the result. Keep the tape measure level around your body and avoid pulling it too tightly. It should feel secure against your body without pressing into the skin. Taking your measurements carefully gives you a much better starting point when choosing a bra.
Understanding UK Band Sizes
The number in a UK bra size represents the band size. Common UK band sizes include 28, 30, 32, 34, 36, 38, 40 and 42. The band sits around your ribcage and provides much of the support of the bra. It should feel firm and secure while still allowing you to breathe and move comfortably.
If the band rides up your back, it may be too loose. If it feels excessively tight even on the loosest hook, you may need to review your measurement or consider a different band and cup combination. A correctly fitted band should remain approximately level around your body.
Understanding UK Cup Sizes
The letter in your bra size represents the cup size. UK cup sizing includes letters such as A, B, C, D, DD, E, F, FF, G and H. The cup letter should always be considered together with the band number. For example, 32D and 36D are not identical sizes because the band measurements are different.
Your UK cup size is estimated from the difference between your bust and underbust measurements. As this difference increases, the cup letter generally increases. This is why measuring both your bust and underbust is important when using a bra size calculator UK.
Why Your Bra Size Can Change
Your bra size is not necessarily the same throughout your life. Body measurements can change naturally over time. Weight changes, pregnancy, breastfeeding, ageing and other normal changes can affect your bust and ribcage measurements. Even if you have worn the same size for years, it is useful to measure again when your bras no longer feel comfortable.
Instead of automatically buying the same size as your previous bra, use fresh measurements with our UK bra size calculator. This gives you an updated starting point and can help you avoid choosing a size based only on an old label.
Why the Same UK Bra Size Can Fit Differently
A calculator can give you a useful estimated size, but it cannot account for every difference between bra brands and styles. Manufacturers use different fabrics, underwires, cup shapes, padding and patterns. As a result, two bras with the same size label can feel different when you try them on.
This does not necessarily mean that your UK bra size is wrong. A balconette bra, T-shirt bra, plunge bra and full-cup bra can all fit differently even when they have the same size printed on the label. Always check the actual fit rather than relying only on the number and letter.
Use Your Calculator Result as a Starting Point
Your calculator result should be treated as a useful starting size rather than a guarantee that every bra will fit perfectly. After calculating your size, check the band, cups, centre gore and straps. The band should stay level, the cups should contain your breast tissue without spilling, and the straps should feel comfortable.
If your calculated size does not feel right, try checking whether the problem comes from the band or the cup. You may also want to try a sister size. Sister sizes can provide a similar cup volume while changing the band size, which can be helpful when a particular brand feels tighter or looser than expected.
How to Choose a Bra After Using a UK Bra Size Calculator
Once you know your estimated UK bra size, choosing a bra becomes much easier. Start by checking the size guide provided by the brand or retailer, especially if you are shopping online. Make sure the website is using UK bra sizing rather than US or European sizing. International size labels can differ, particularly when you reach larger cup sizes.
When trying a bra on, begin with the band on the loosest hook. A new bra should normally have enough room to become tighter as the fabric naturally stretches with wear. The band should sit firmly around your ribcage without riding up or causing excessive discomfort.
Next, check the cups. Your breast tissue should sit comfortably inside the cups without spilling over the top or sides. At the same time, the cups should not have large areas of empty fabric or obvious wrinkling. If the cups feel too small while the band feels comfortable, try a larger cup rather than automatically moving to a larger band.
Finally, adjust the straps so they sit comfortably on your shoulders. Straps should provide support without digging into your skin or constantly falling down. A good bra fit comes from the combination of the band, cups and straps working together.
UK Bra Size Calculator for Online Bra Shopping
Shopping for bras online can make finding the right size more difficult because you cannot try the bra before ordering. A bra size calculator UK can give you a useful starting point before you browse different styles. Take your measurements at home, calculate your estimated size and then compare that result with the retailer's own UK size chart.
If the retailer offers a detailed measurement guide, use it alongside your calculator result. Some brands may recommend slightly different sizing depending on the design of the bra. Checking both your measurements and the manufacturer's guide can reduce the chance of ordering the wrong size.
When to Recalculate Your UK Bra Size
You do not need to measure your bra size every day, but it is worth recalculating when your current bras stop fitting as they once did. If the band starts riding up, the cups begin spilling over, the straps become uncomfortable or the bra feels noticeably different, take fresh measurements.
Regularly checking your measurements can also be useful when buying a completely new collection of bras. Your estimated size gives you a starting point, while the final choice should always be based on comfort, support and the individual fit of the bra.
Frequently asked questions
How accurate is a bra size calculator?
It's a strong starting point, not a guarantee. A calculator works from two measurements, but breast shape, tissue density, and each brand's own cut all affect how a labelled size actually fits. Use the result as your first try-on size, then adjust based on the fit signs above.
Why do I measure differently in every shop?
Because there's no legal standard behind bra sizing. Each brand designs around its own fit model and pattern, so a 34C from one label can sit closer to a 32D or 36B from another. This is completely normal — treat your calculated size as a starting size, not a fixed number.
Should I round my measurement up or down?
Round to the nearest whole number in both directions using standard rounding — 0.5cm or more rounds up, anything less rounds down. If you're exactly between two band sizes, it's worth trying both and seeing which one keeps the band level and comfortable.
What's the difference between UK and US cup sizes?
The band numbers match, but the cup letters drift apart as they get larger. A UK DD is close to a US DDD or E, and by G and H cups the systems can sit a full letter or more apart, so always check a brand's own size guide when shopping from overseas.
Can my bra size change over time?
Yes — weight changes, hormonal shifts, pregnancy, and even the natural stretch of an old bra can all move your size. It's worth remeasuring every six to twelve months, or any time a bra that used to fit well suddenly doesn't.
What if my calculated cup size doesn't exist in a brand's range?
Try your sister size instead — the same cup volume on a different band, using the sister-size guide above. It's often the fastest way to find a comfortable match when your exact size isn't stocked.