Sister Size Calculator
Sister sizes are different bra sizes that hold roughly the same cup volume on a different band. The rule is simple: go down one band size and up one cup letter, or up one band size and down one cup letter. So if a 34C fits in the cups but the band feels slightly tight, 36B gives you a roomier band with a similar cup volume. Type your current size below to see both of your sister sizes instantly, in your country's sizing.
Find Your Sister Sizes
Sister Size Examples
Here is how the band-up / band-down rule works for a few common sizes. Try them in the tool above to check the math.
| Your size | Country | Sister size (band down, cup up) | Sister size (band up, cup down) |
|---|---|---|---|
| 34C | US | 32D | 36B |
| 36DD | US | 34DDD | 38D |
| 34E | UK | 32F | 36DD |
Note: sister-size conversions are approximate and brand fit may differ. A sister size keeps a similar cup volume, but a different band tension changes how the bra sits — always try on when you can.
When Sister Sizes Help
- Your size is out of stock. Check the sister sizes first before giving up on a style you like.
- The band is the problem. If the cups fit but the band digs in or rides up, a sister size usually fixes the fit without changing the cup volume.
- Between band sizes. If one band feels too tight and the next feels too loose, the sister size in either direction is worth trying.
Do not have a size yet? Use the bra size calculator to get your estimated size first, look it up on the bra size chart, or jump straight to the US bra size calculator or Bra size calculator UK. Our sister sizes guide explains the concept in more depth.
Frequently Asked Questions
What is a sister size?
A sister size is a bra size with about the same cup volume as your usual size but a different band size. Go down one band and up one cup (for example, 34C becomes 32D), or go up one band and down one cup (34C becomes 36B).
When should I try a sister size?
Try a sister size when your usual size is out of stock, or when the band is the main problem. If the band feels too tight but the cups fit, go up a band and down a cup. If the band rides up or feels loose but the cups fit, go down a band and up a cup.
Is the fit exactly the same as my usual size?
Not exactly. The cup volume is similar, but a different band tension changes how the cups sit on your body, and brand fit varies. A sister size is a good backup option, but try it on whenever you can.
Do sister sizes work the same in every country?
The idea is the same everywhere: one band down means one cup up, and one band up means one cup down. The band step is 2 for inch-based bands (US, UK, AUS, IN) and 5 for centimeter-based bands (EU, FR, JP), and cup letters follow each country's own naming — so select your country in the tool to get the right letters.
Can every bra size have sister sizes?
Most sizes have a sister size one band up and one band down. At the edges of the range — very small bands or the largest cup letters — one direction may not exist, and the tool will tell you when that happens.