By Rehan Khatri, RK Fashion Zone | Last updated: September 2026
Find your bra size in under a minute. Enter your underbust and bust measurements below to get your band size, cup size, and sister sizes in US, UK, and EU sizing. It’s free, and you only need a soft measuring tape.
Your estimated US bra size
—
| Region | Your size |
|---|---|
| US | — |
| UK | — |
| EU / FR | — |
Your sister sizes
| If the band feels… | Try (US) |
|---|---|
| Too loose: go down a band | — |
| Too tight: go up a band | — |
How to Measure Your Bra Size at Home
You need two measurements. Wear an unpadded or lightly lined bra, stand straight, and use a soft tailor’s tape.
- Underbust: wrap the tape around your ribcage directly under your bust. Keep it level and firm, but not digging in. This sets your band size.
- Bust: wrap the tape around the fullest part of your chest. Keep it parallel to the floor and don’t pull it tight.
- Difference: subtract your underbust from your bust. Each inch (2.5 cm) equals one cup size.
- Calculate: enter both numbers above to see your US, UK and EU sizes.
For the best result, measure in front of a mirror and take each measurement twice. If the two readings differ, use the average.
How Bra Size Is Calculated
A bra size has two parts: a number (the band) and a letter (the cup). The band is your underbust measurement rounded to the nearest even number. The cup comes from the difference between bust and underbust.
| Bust minus underbust | US cup | UK cup | EU cup |
|---|---|---|---|
| Under 1 in (2.5 cm) | AA | AA | AA |
| 1 in (2.5 cm) | A | A | A |
| 2 in (5 cm) | B | B | B |
| 3 in (7.5 cm) | C | C | C |
| 4 in (10 cm) | D | D | D |
| 5 in (12.5 cm) | DD (or E) | DD | E |
| 6 in (15 cm) | DDD (or F) | E | F |
| 7 in (17.5 cm) | G | F | G |
| 8 in (20 cm) | H | FF | H |
| 9 in (22.5 cm) | I | G | I |
| 10 in (25 cm) | J | GG | J |
Cup size is relative to band size. A 36C holds more volume than a 32C, which is why sister sizes exist.
Bra Size Charts (US, UK and EU)
Bust measurement (inches) for each size
Find your band in the left column, then read across to the bust measurement closest to yours. The column heading shows your cup.
| Band | A | B | C | D | DD / E | DDD / F | G | H |
|---|---|---|---|---|---|---|---|---|
| 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 |
| 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 |
| 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 |
| 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 |
| 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 |
| 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 |
| 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 |
| 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 |
Band size conversion
| US / UK band | Underbust (in) | Underbust (cm) | EU / FR band |
|---|---|---|---|
| 30 | 29 – 30 | 74 – 77 | 65 |
| 32 | 31 – 32 | 79 – 82 | 70 |
| 34 | 33 – 34 | 84 – 87 | 75 |
| 36 | 35 – 36 | 89 – 92 | 80 |
| 38 | 37 – 38 | 94 – 97 | 85 |
| 40 | 39 – 40 | 99 – 102 | 90 |
| 42 | 41 – 42 | 104 – 107 | 95 |
| 44 | 43 – 44 | 109 – 112 | 100 |
What Are Sister Sizes?
Sister sizes have the same cup volume but a different band. Go down one band and up one cup, or up one band and down one cup. For example, 34C, 32D and 36B are sister sizes.
Use them when the cups fit but the band is too loose or tight, or when your usual size is sold out. Stretchy fabrics and different brands can also make a sister size the better fit.
How a Bra Should Fit
- The band sits level and snug around your back, on the loosest hook when new. You should fit two fingers under it.
- The center panel (gore) lies flat against your breastbone.
- Your breast tissue is fully inside the cups, with no spilling or gaps.
- The underwire follows your natural crease and sits on the ribcage, not on breast tissue.
- The straps stay put without digging in. The band should carry most of the support.
Signs Your Bra Doesn’t Fit
Band problems
- It rides up your back. The band is usually too big.
- It digs in or leaves marks, or you feel restricted when you breathe.
- You can pull it more than 1 to 2 inches away from your back.
Cup problems
- Breast tissue spills over the top or sides (cups too small).
- The fabric wrinkles or gapes (cups too big).
- The wire sits on breast tissue, or the gore floats away from your chest.
Strap problems
- Straps dig into your shoulders. This usually means the band isn’t doing enough work.
- Straps slip off. Tighten them, or try a style with closer-set straps.
Choosing a Bra for What You’re Wearing
Once you know your size, the style matters just as much. The right bra can change how an outfit sits, and the wrong one can show through even when the size is correct.
Snug, cropped or thin fabrics show every seam, so a seamless or molded t-shirt bra in your correct size gives the smoothest line. That matters more now that fitted tops are back: the baby tee trend, for example, works best when there are no visible straps or edges, as you can see in this look at how stylists pair baby tees with tailored trousers.
Strapless bras depend almost entirely on the band, so the band should feel firm and secure. Look for silicone grip strips and side boning. Structured, body-hugging gowns such as a mermaid dress often call for a strapless, low-back or multiway bra that keeps its shape without visible lines.
For everyday wear, choose a wireless or lightly lined bra in a comfortable stretch fabric, and keep at least three bras in rotation so each one has time to recover its shape.
Sports Bra Sizing
Many sports bras use S, M, L sizing. Use your calculated size as a guide, but check each brand’s chart because they vary. As a rough guide:
| Sports bra size | Typical band | Typical cups |
|---|---|---|
| XS | 30 | A – B |
| S | 32 – 34 | A – C |
| M | 34 – 36 | B – D |
| L | 36 – 38 | C – DD |
| XL | 38 – 40 | D – DDD |
For high-impact activity, look for encapsulation (separate cups) or compression plus a wide, firm band. If you’re between sizes, size up in the band and check that there’s no bounce.
Common Bra Sizing Myths
- “Add 4 inches to your underbust.” This outdated method usually gives a band that’s too loose. Use your actual underbust, rounded to the nearest even number.
- “Cup size is a fixed volume.” It isn’t. Cup volume changes with the band size.
- “Your size is the same everywhere.” It isn’t. Brands, styles and fabrics fit differently, so expect to vary by a band or cup.
- “Once I know my size, I’m done.” Bodies change. Re-measure every 6 to 12 months and after weight change, pregnancy, breastfeeding or a new exercise routine.
Frequently Asked Questions
How do I find my bra size?
Measure around your ribcage under your bust and around the fullest part of your bust. Round the underbust to the nearest even number for your band, then subtract it from your bust. Each inch of difference equals one cup letter.
Is this bra size calculator accurate?
It gives a reliable starting size based on standard measurement methods. Fit still depends on the brand, style, fabric and your breast shape, so try a few sizes and check each brand’s size chart.
What is the difference between US and UK bra sizes?
Band sizes are the same. Cup sizes match up to D, then the UK uses DD, E, F, FF, G while the US commonly uses DD (or E), DDD (or F), G, H.
Should I add 4 inches to my underbust measurement?
No. That method is outdated and often gives a band that’s too big and unsupportive. Use your actual underbust measurement, rounded to the nearest even number.
What is a sister size?
A sister size has the same cup volume as your size with a different band. Go down a band and up a cup, or up a band and down a cup. For example, 34B, 32C and 36A are sister sizes.
How often should I measure my bra size?
Every 6 to 12 months, and after any major change such as weight gain or loss, pregnancy, breastfeeding, or a new fitness routine.
Why does my bra size differ between brands?
There’s no universal sizing standard. Brands use different patterns, fabrics and stretch, so your size can vary by a band or cup between labels.
What if my bust and underbust measurements are the same?
A difference under 1 inch usually points to an AA cup. If you’re unsure, a professional bra fitting can help.
What is my bra size if I wear a 34B in one brand and a 32C in another?
That’s normal. 34B and 32C are sister sizes with the same cup volume, so the two brands are likely cut differently in the band.
Final Thoughts
The right size improves comfort, posture and how your clothes fit. Use the calculator, confirm the fit with the checklist above, and re-measure regularly. Bookmark this page to come back whenever your size changes.