Bra Size Calculator
Find your perfect bra size instantly with our accurate bra size calculator. Supports US, UK, and EU sizing standards. 100% client-side processing keeps your body measurements completely private.
Off uses the modern no-add method most fit specialists rely on today. Turn it on to see the older band calculation instead.
Your Sister Sizes
Same cup volume, different band feel. Handy if your usual size is sold out or the fit is a little off.
What is This Tool
Ever measure yourself with a tailor's tape, land on a number, and still walk out of the store with three different sizes in your dressing room? That is not you doing it wrong. Band and cup construction is not built the same way in Boston as it is in London or Berlin, and a size that fits perfectly in one system can be off by two letters in another. This calculator takes your underbust and bust measurements and runs them through the same formulas fitters use, then lets you flip between US, UK, and EU sizing without measuring a second time.
It also settles the debate that never seems to end: the old add-4-inches rule versus the newer no-add method most bra fitters have switched to. Toggle between the two, watch your size update instantly, and see exactly how much the method changes your result, because it usually does more than people expect.
How to Use
Grab a soft measuring tape, stand naturally, and follow these steps for the most reliable read:
- Wrap the tape around your ribcage right under your bust, keep it snug and level all the way around, then drag the Underbust slider or type the number until it matches.
- Measure around the fullest part of your bust with the tape flat but not tight, and set the Bust slider to that reading.
- Pick US/CA, UK, or EU/International at the top of the results card to see your size translated into that country's labeling, no extra math required.
- Check the sister sizes underneath your main result before you buy. If your usual size is out of stock, one of those two will fit almost the same way.
Key Features
- Three sizing standards in one click, switching between US, UK, and EU cup letters instantly, including the DD/DDD and E/F/FF splits that trip up cross-border shopping.
- Inches or centimeters, your call. Every field converts in real time, so you can measure however your tape is marked without doing mental math.
- Two calculation methods side by side, letting you compare the modern no-add formula against the traditional Plus 4/5 approach and see how differently they land.
- Sister size lookup built right in, giving you your two closest equivalent sizes automatically, labeled by whether the band runs tighter or looser.
- A measurement guide you can actually read. The diagram shows exactly where bust and underbust are measured, so there is no guessing before you start.
Common Use Cases
This calculator comes in handy in more situations than most people expect:
- Buying from a UK or European retailer online and needing your size converted correctly before you check out.
- Re-measuring after a weight change, pregnancy, or postpartum, when your old size just does not sit right anymore.
- Finding a backup size to try when your usual number is out of stock in that color or style.
- Shopping vintage or secondhand lingerie tagged in a sizing system you are not used to wearing.
- Helping a teenager get properly fitted for the first time without an awkward trip to a fitting room.
Frequently Asked Questions
How accurate is this compared to a professional fitting?
It is built on the same formulas most fit specialists use, so it is a strong starting point. Fabric stretch, wire shape, and brand cut still vary a lot, so treat the result as your baseline size to try on rather than a guarantee.
Why did my size change when I switched from Plus 4 to the modern method?
The Plus 4 method inflates your band number before figuring cup size, which usually pushes you toward a smaller cup and a bigger band. The modern method skips that padding, so most people land on a smaller band and a bigger cup with it.
What is the actual difference between US, UK, and EU cup letters?
They share the same alphabet through D, then diverge. US sizing often uses DD and DDD, UK sizing uses DD, E, F, and FF, and EU sizing generally tracks UK letters with a centimeter-based band. That is exactly why the standard toggle exists.
Why are my sister sizes not just one size up or down?
A sister size trades band and cup against each other to keep the same cup volume, so you go down a band and up a cup, or up a band and down a cup. It is a swap, not a simple size shift.
Do you store or see my measurements?
No. Everything runs in your browser through JavaScript. Nothing you type here is sent anywhere or saved after you close the tab.
Advanced Tips
A few habits that make the numbers this tool gives you even more useful:
- Measure twice, ideally without a padded bra, and use the lower of the two underbust readings if they are close. A snug but not tight band does most of the support work.
- If a result feels off, try both calculation methods and both sister sizes before assuming the calculator is wrong. Brand cut varies more than the math does.
- Re-measure every six months to a year, especially after any weight change of ten pounds or more, since band size shifts faster than most people expect.
- When a UK or European brand only lists one size on the tag, start with the EU/International toggle since that is the system the garment was actually cut to.