If you've ever found yourself between sizes or unable to locate your exact bra size in a particular style, understanding sister sizes can be a game-changer. Sister sizes are alternative bra sizes that share the same cup volume while offering different band lengths, providing you with more options when shopping for the perfect fit.
The concept of sister sizes is one of the most valuable yet frequently misunderstood aspects of bra fitting. Once you grasp how they work, you'll unlock a whole new world of bra shopping possibilities and never feel limited by size availability again.
What Exactly Are Sister Sizes?
Sister sizes are bra sizes that have the same cup volume but different band sizes. When you move up a band size, you move down a cup size, and vice versa—this maintains the same cup capacity while adjusting the overall fit around your torso.
For example, if your calculated size using our Australian bra size calculator is 12D, your sister sizes would be 10DD (smaller band, larger cup letter) and 14C (larger band, smaller cup letter). All three of these sizes—10DD, 12D, and 14C—have cups that hold the same breast volume; only the band length differs.
This relationship exists because cup sizes aren't absolute measurements. A D cup on a size 10 band is physically smaller than a D cup on a size 16 band. The cup size represents the difference between bust and underbust measurements relative to the band size, not a fixed volume.
When to Use Sister Sizes
Sister sizes become invaluable in several common scenarios that Australian women encounter regularly. The most frequent situation is when your exact size isn't available. Perhaps you've found the perfect bra style, but the store has sold out of your size. Knowing your sister sizes means you can confidently try an alternative that might work just as well.
Another common scenario involves sizing variations between brands. Australian brands like Berlei might fit slightly differently from international brands like Freya or Wacoal. If a particular brand's version of your usual size doesn't feel quite right, trying a sister size often provides the solution.
Sister sizes are also helpful when you're between sizes. If the band on your calculated size feels slightly too snug, trying the sister size with a larger band might provide the comfort you need without sacrificing proper cup coverage.
How to Find Your Sister Sizes
Finding your sister sizes is straightforward once you understand the pattern. Starting from your primary size, follow these steps:
To find a sister size with a smaller band: decrease the band by one size and increase the cup by one letter. For example, from 14C, you would move to 12D.
To find a sister size with a larger band: increase the band by one size and decrease the cup by one letter. From 14C, you would move to 16B.
Our bra size calculator automatically displays your sister sizes alongside your primary size, making it easy to see all your options at a glance. This feature is particularly useful for Australian women shopping online or preparing for a fitting room visit.
Understanding the Limits of Sister Sizes
While sister sizes are incredibly useful, they have their limitations. Generally, it's best to stick within one or two sizes from your calculated primary size. Moving too far away—say, from a 12D to a 16B—means the proportions of the bra change significantly, and the fit may feel quite different from what you're accustomed to.
The band is responsible for approximately 80% of a bra's support, so going up in band size means accepting less support from the band. This might be fine for lighter styles like bralettes but could be problematic for larger busts that rely heavily on band support.
Additionally, when you change band sizes, the position of the straps, underwire length, and overall proportions of the bra also shift. A sister size will have the same cup volume but won't necessarily sit on your body in exactly the same way as your primary size.
Sister Sizes for Different Bra Styles
Different bra styles benefit from sister sizes in different ways. For everyday t-shirt bras and moulded cup styles, sister sizes work remarkably well because these bras are designed to be somewhat forgiving in their fit.
Balconette and plunge styles may be more sensitive to size changes since their more precise cuts are designed to create specific silhouettes. With these styles, you might prefer to stay closer to your primary size when possible.
Sports bras often work beautifully with sister sizes. Many women prefer a slightly tighter band during exercise for enhanced support, making a sister size with a smaller band and larger cup an excellent option for athletic activities.
Common Sister Size Combinations
To help you visualise the sister size relationship, here are some common Australian size groupings. All sizes within each group share the same cup volume:
Group 1: 10C – 12B – 14A
Group 2: 10D – 12C – 14B – 16A
Group 3: 10DD – 12D – 14C – 16B
Group 4: 10E – 12DD – 14D – 16C
Group 5: 12E – 14DD – 16D – 18C
Notice how the cup letter changes as you move through each group, but the cup volume remains constant. This is the fundamental principle behind sister sizing.
Tips for Trying Sister Sizes
When trying sister sizes in a fitting room, pay attention to specific fit indicators. For a sister size with a smaller band, ensure the band doesn't feel too tight—you should be able to fit two fingers under it comfortably. Check that the cups provide the same coverage as your primary size without any spillage or gaping.
For a sister size with a larger band, verify that the band sits firmly against your body without riding up at the back. You may need to use a tighter hook setting initially, as new bras should generally be worn on the loosest setting to allow for stretch over time.
Always do the "scoop and swoop" when trying on any bra, including sister sizes. Lean forward, place your breasts into the cups, and use your hand to gently sweep all breast tissue from under your arms into the cups. This ensures you're assessing the true fit.
Using Sister Sizes with Our Calculator
Our free Australian bra size calculator takes the guesswork out of finding your sister sizes. Simply enter your underbust and bust measurements, and we'll display not only your primary size but also your recommended sister sizes for easy reference.
Armed with this knowledge, you can approach bra shopping with confidence. Whether you're browsing online retailers, visiting Australian department stores, or shopping overseas, understanding sister sizes gives you the flexibility to find beautiful, comfortable bras that fit perfectly.