Calculator · Sizing
Bra Size Calculator
This free bra size calculator turns two tape measurements, your underbust and your bust, into your US bra size, with the matching UK, EU and Australian sizes and your sister sizes. It works in inches or cm, and below it you’ll find how to measure, a cup size chart and what to do if a bra still doesn’t fit.
Your US bra size
Sister sizes
How to measure bra size
You need a soft tape measure and a mirror. Wear a thin, non-padded bra or no bra, and keep the tape level all the way around your back.
- Underbust (band): wrap the tape snugly around your ribcage, directly under your bust. Breathe out, then read the measurement. It should be firm but not painful.
- Bust: measure loosely around the fullest part of your bust, usually across the nipples, without squashing the breast tissue.
- Calculate: enter both numbers in the bra size calculator above, in inches or cm. It works out your band size, cup size and sister sizes for you.
Measure two or three times and use the average. If your breasts are soft or settle low, measure your bust again while leaning forward at 90 degrees and use the bigger number.
How the bra size calculator works
Bra sizes have two parts: a band size (the number) and a cup size (the letter).
- Band size: your underbust measurement in inches, rounded to the nearest whole number. Bands come in even sizes, so an odd number is rounded up: 31 in becomes a 32 band.
- Cup size: the difference between your bust and your band size. Each inch of difference is one cup size: 1 in is an A, 2 in a B, 3 in a C, 4 in a D, and so on.
For example, an underbust of 30.5 in rounds to a 32 band, and a 35 in bust is 3 in more than that, so the size is 32C. This is the modern method most fitters use. Some older guides add 4 or 5 inches to the underbust, which gives a band that’s too big and a cup that’s too small, so be careful if a brand’s own chart does this.
Bra cup size chart
This bra cup size chart shows how many inches your bust is bigger than your band, and the cup letter that matches in US and UK sizing. The two systems are the same up to DD, then go their own way.
| Bust minus band | US cup | UK cup |
|---|
Bra size chart in inches and cm
Find your band on the left, then follow the row across to your bust measurement to read your US bra size. Use the Inches / cm switch in the calculator to change the units.
| Band (underbust) | A | B | C | D | DD | DDD | G |
|---|
Bra sister sizes
Sister sizes hold the same cup volume on a different band. When you go up one band size, go down one cup, and when you go down a band, go up a cup. So 34C, 32D and 36B all have the same cup volume.
Sister sizes are useful when your size is sold out, or when the band feels too loose or too tight but the cups fit. The calculator above shows your two closest sister sizes. If your band measurement fell on an odd number and was rounded up, try the sister size with the smaller band as well.
US, UK, EU and AU bra sizes
US and UK bands use the same numbers in inches. EU bands are in centimetres, and Australian bands use dress-size numbers.
| US / UK band | EU band | AU band |
|---|
Cups match in the US and UK up to DD. After that, the US usually goes DDD, G, H, while the UK goes E, F, FF, G. EU cups run in single letters (A, B, C, D, E, F, G) and step by 2 cm, so EU sizes in the calculator are a close approximation. Australian cups follow the UK.
Signs your bra doesn’t fit
- The band rides up at the back: it’s too loose. Go down a band and up a cup (your smaller sister size).
- The band digs in or leaves marks: it’s too tight. Go up a band and down a cup.
- Breast tissue spills over the top or sides: the cup is too small. Go up a cup.
- Gaps or wrinkles in the cup: the cup is too big, or the style doesn’t suit your shape.
- The centre panel doesn’t lie flat against your chest: usually the cup is too small.
- Straps slip or dig in: adjust them first; the band should carry most of the support.
A new bra should fit on the loosest hook, so you can tighten it as the band stretches over time.
Sports, maternity and plus size bras
- Sports bras: use the same measurements. Many sports bras come in S, M and L, so check the brand’s chart against both your band and cup. They should fit firmer than everyday bras.
- Maternity and nursing bras: your size can change through pregnancy and after birth. Measure again every few months, and in late pregnancy choose a band with room to tighten.
- Plus size bras: the calculator works the same way at any size. Bigger cup sizes vary more between brands, so try your size and its sister sizes.
Want to know your body shape too? Use your bust, waist and hips in the body type calculator, or find your dress size.
Frequently asked questions
What is the most accurate bra size calculator?
No calculator is perfect, because breast shape and brand sizing vary. The most accurate ones use your snug underbust as the band, with no 4 or 5 inches added, and count one cup per inch of difference, like the calculator on this page. Treat any result as your starting size, then check the fit and try your sister sizes.
How do I measure my bra size in cm?
Take the same two measurements in centimetres. For EU sizes, round your underbust to the nearest 5 cm to get the band (for example 73 cm becomes 75), then count one cup for about every 2 cm your bust is bigger than that band. To get a US or UK size instead, switch the calculator to cm and it converts for you.
How do I measure my bra size in the UK?
The UK uses the same two measurements and the same inch-based bands as the US, so a US 34 band is a UK 34. Cups are identical up to DD; after that the UK goes E, F, FF, G, GG, H, while the US goes DDD, G, H. The calculator shows your UK size next to your US size.