Most people buy bras by guessing, or by repeating whatever size was on the last one that did not hurt. The trouble is that a bra label is only a code for two measurements, and the code changes depending on where the bra was made. A free bra size calculator removes the guesswork: you give it two numbers and it does the conversion work for you. This guide explains how the calculator above works, how to take your own measurements properly, and how to read a result in any country.

What a bra size calculator does

A bra size has two parts. The band is the number, and it describes the size around your ribcage. The cup is the letter, and it describes how much bust volume sits in front of that ribcage. The letter is never fixed on its own: a C cup on a 30 band is a smaller cup than a C on a 40 band, because the letter only means “this much bigger than the band”.

That is why a correct bra size calculator needs two inputs, not one. It takes your underbust and your bust, works out the band from the first, then works out the cup from the difference between the two. The bra band and cup size calculator above does exactly that, and it keeps going: once it has your size, it converts it to the labelling used in the US, UK, EU, France, Italy, Australia, Japan and China. If you have ever stared at a foreign size chart in a shop or an online basket, you will know why that last step matters.

How to use this calculator

The tool looks simple on purpose, but there is more inside it than the two boxes suggest. Here is every part, in the order you will meet it.

How to measure your bra size at home

This is how to measure bra size at home with nothing more than a soft tape and a mirror. Wear an unpadded or lightly lined bra, or none at all, and stand relaxed with your arms by your sides.

This is the practical answer to how to find your bra size, and it takes about two minutes. If you would rather see the process in the tool itself, press How to measure inside the calculator.

How to calculate bra size by hand

If you like to check the result yourself, here is how to calculate bra size with a pencil. Take a snug underbust of 76 cm and a bust of 91 cm.

Run the same body through the Classic chart and you get a different answer, because the rules are different. Here is the full list for this example, which you can check against the tool.

Modern fit versus Classic chart

Two questions decide which method fits you: which store are you buying from, and how does that brand label its bras? The Classic chart is the EN 13402 European standard: the band is your underbust rounded to the nearest 5 cm, with the cup taken from the difference in 2 cm steps. The older UK and US habit of adding four inches to the underbust, which the Classic chart mirrors when it converts to UK/US labels, was designed for non-stretch bands. Modern fit assumes today’s elastic band and uses your underbust directly.

That is why the same underbust gives a UK band that is consistently four sizes higher on Classic. The table and chart below show it for seven underbust measurements.

Neither is wrong. If you are shopping at a UK or US brand and want a size that matches what the label means today, start with Modern fit. If a brand quotes EN 13402 sizes or you are buying from a European store, Classic chart is the one to follow. When in doubt, try the Modern result first and use the sister sizes if it is close but not right.

Bra size chart and converter

Here is a bra size chart and bra size converter you can scan in a few seconds. Band sizes first, then cups.

Band size conversion

Find your band in the first column and read across for every system. The second column shows the snug underbust range, in centimetres, that matches each band on the EN 13402 standard.

Italian band numbering and Australian numbering can vary slightly between brands, so check the brand’s own chart when you can.

Cup size conversion

Cups depend on the difference between your bust and your band, so the table is arranged by that difference. It uses the Modern fit method, which takes one inch for each step.

UK and Australian cups share the same letters. Some Australian brands skip the double letters above F, and some European brands skip I.

Note where the systems part ways. UK and US cups match through D. After that the UK uses DD, E, F, FF, G, while the US uses DD, DDD, then G, with DDD often sold as E or F. EU and French cups use plain, continuous letters instead of doubles. Memorise that and most overseas size charts stop being confusing.

UK, US, EU, Japan, Australia and China

An international bra size calculator has to cope with several different habits at once. Here is what differs, in plain terms.

• UK and US use inches for the band, always an even number, with cups that run by the one-inch difference. This is the one place where “UK size” and “US size” split in the cups. There is no official standard behind them, only brand habit.
• EU labels follow EN 13402. The band is in centimetres in steps of five (65, 70, 75), and cups step every 2 cm, so a 2 cm difference is the gap between a B and a C.
• France, Spain and Belgium add 15 to the EU band. A European 75 is therefore a 90 in a French shop.
• Italy uses small counting numbers for the band (0, 1, 2, 3), which is why an Italian 3 can look strange at first glance.
• Australia and New Zealand take the UK band and subtract 22, so a UK 34 is an AU 12. Cup letters match the UK.
• Japan writes the cup first and the band second (B75), and its standard, JIS L 4006, steps cups by 2.5 cm instead of 2 cm. China writes the band first (75B). The bra size calculator for Japan and China above uses the same 2.5 cm cup steps, so the result lines up with how those stores label their bras.

The cup step difference matters more than it sounds. The table lists the nominal differences for both standards, and the chart afterwards shows how the two diverge as the cup gets bigger.

You will notice that for a given difference the European and Japanese letter can disagree by one cup. Neither is a mistake. They are different yardsticks, which is exactly why the calculator shows each region separately instead of trying to convert letter to letter.

Sister sizes explained

A sister size calculator for bras sounds technical, but the idea is simple. Your cup letter is tied to your band, so changing the band changes how large the cup really is. To keep the same cup volume you move the two in opposite directions: one band down and one cup up, or one band up and one cup down. Those two options are your sister sizes.

Use the tighter-band size if the band feels loose after a few hours but the cups fit well. Use the looser-band size if the band digs in or leaves marks. The calculator’s sister box lists both for you, so you never need to work it out manually.

Check the fit once you have your size

A bra fitting calculator can only be as good as the numbers you give it, and bodies are not perfectly regular. Always judge a bra on the body. This is what to look for, and the Fit check pop-up inside the tool shows the same list.

The band does most of the work: most of the support is meant to come from it, which is why a band that rides up is the first thing to fix. The straps are there to stop the cups from slipping, not to hold everything up.

Common mistakes and when to re-measure

• Measuring over thick clothing. A jumper can easily add an inch. Measure over thin fabric or none at all.
• Tape too loose at the underbust. The band measurement is meant to be snug. A loose tape makes the band too large and the cup too small.
• Letting the tape slope. A tape that rides up at the back reads too small. Check in a mirror.
• Treating one size as final. A calculator gives you a strong starting point. Fit differs by brand and by style, so try the sister sizes.
• Assuming weight decides it. BMI or dress size says very little about a bra size, since the same BMI can sit on very different frames.
• Never re-measuring. Bodies change. Try again every six months, and after changes in weight, pregnancy or exercise.

On the weight point: if you are following a plan to lose weight, the band size is usually the first thing to change. If you are working with a calorie deficit, it is worth taking your measurements again every few kilograms, because a size that fitted last month may not fit now.

Frequently asked questions

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