Bra Sister Size Calculator

General Sister Size Questions

What exactly is a bra sister size?

A bra sister size is an alternate bra size that holds the same cup volume but has a different band size. To create a sister size, you move one band size up and one cup size down, or one band size down and one cup size up. For example, the sister sizes of a 34C are 32D (smaller band, larger cup) and 36B (larger band, smaller cup). All three sizes hold roughly the same breast volume, but they fit differently around the ribcage and across the bust. Sister sizes exist because cup volume is relative to band size—a C cup on a 34 band holds a different volume than a C cup on a 36 band, and the letter alone doesn't tell you the whole story.

Are sister sizes really interchangeable?

Not exactly—they're close, but not identical. Sister sizes hold roughly the same cup volume, but the proportions are different: the band is either tighter or looser, and the cup is either shallower or deeper. Some people find that their sister size fits better than their calculated size, while others find it noticeably less comfortable. The band fit is the biggest difference: if you go down a band size, the bra will feel snugger around the ribcage (which some people prefer for support), and if you go up a band size, it will feel looser (which some prefer for comfort). Cup shape also changes slightly—going up a cup size (with a tighter band) gives you a deeper cup, while going down a cup size (with a looser band) gives you a shallower cup. Always try sister sizes on before committing, especially if you're switching brands.

How many sister sizes does a bra size have?

In theory, a bra size can have several sister sizes—you can continue moving up or down in band size as long as the cup letter remains within the brand's available range. In practice, most people only find one or two sister sizes useful. For a 34C, for instance:

• One step tighter: 32D
• One step looser: 36B
• Two steps tighter: 30DD
• Two steps looser: 38A

The further you move from your original size, the more the fit changes—a 30DD and a 38A both hold roughly the volume of a 34C, but they'll fit very differently on the body. Our calculator shows sister sizes in both directions so you can see all your options.

Sizing and Fit Questions

How do I know if I should go up or down in the band?

The direction you choose depends on what's wrong with your current fit:

• Go down a band size (and up a cup size) if your current band feels too loose—it rides up, the straps fall off your shoulders, or you can fit more than two fingers under the band. A tighter band provides more support.
• Go up a band size (and down a cup size) if your current band feels too tight—it leaves red marks, restricts your breathing, or causes discomfort. A looser band is more comfortable for some people.
• If your band fits well but the cup is wrong: Sister sizing won't fix this—you likely need a different cup size, not a sister size. Use our Bra Cup Size Calculator to re-check your measurement.

Why does my sister size sometimes fit better than my calculated size?

This is common, and there are several reasons:

• Brand sizing variation: Some brands run small or large in the band or cup, and a sister size may correct for this.
• Ribcage shape: Some people have a ribcage that sits between standard band sizes—a sister size may bridge the gap.
• Breast shape: Shallow, projected, wide-set, or close-set breasts may fit better in one sister size than another.
• Personal preference: Some people simply prefer a snugger or looser band than their calculated size provides.

If a sister size fits better, there's nothing wrong with wearing it—the "correct" size is the one that fits your body comfortably.

Can I use sister sizes across different brands?

Yes, but with caution. Sister sizes are based on standard sizing relationships, but brands interpret those relationships differently. A 34C in one brand might feel like a 36B in another, or a 32D in a third. When shopping across brands:

• Check the brand's specific size chart—some brands publish detailed fit notes and sizing guidance.
• Read customer reviews for fit information—reviewers often note whether a bra runs small or large.
• Try your calculated size and one sister size if you're unsure.
• Take advantage of generous return policies—many online bra retailers allow free exchanges.

Do sister sizes work for all bra types?

Sister sizing works best for structured bras with band-and-cup sizing—everyday bras, T-shirt bras, full-coverage bras, push-up bras, and similar styles. It's less reliable for:

• Sports bras: These often use S/M/L sizing or have different fit requirements—see our Sports Bra Size Calculator.
• Strapless bras: These need a snugger band than standard bras—see our Strapless Bra Size Calculator.
• Nursing bras: These accommodate size fluctuations and fit differently—see our Nursing Bra Size Calculator.
• Bralettes and seamless bras: These often use S/M/L sizing and aren't subject to standard sister-size rules.

Technical Questions

What is the difference between US and UK sister sizes?

US and UK cup size progressions diverge after DD, which means sister sizes calculated in one system don't directly translate to the other:

• US progression: AA, A, B, C, D, DD, DDD, G, H, I, J, K
• UK progression: AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J

For example, a US 34DDD is roughly equivalent to a UK 34E. Its US sister sizes would be 32G and 36DD, while its UK sister sizes would be 32F and 36E. If you're shopping across brands with different sizing systems, convert your size first—and always check the brand's specific size chart.

How do I convert my sister size if I'm shopping internationally?

To convert sister sizes across sizing systems:

• First, convert your current size to the target country's system using a size conversion chart (US to UK, US to EU, etc.).
• Then, apply the sister size rule in the target system—band size changes by 2 inches (or the local equivalent), and cup letter moves one step.
• Be aware that EU sizing uses centimeters for band size, and cup letter progressions vary by country.
• Always check the specific brand's size chart—many international brands publish conversions for US, UK, EU, and other systems.

Why does the cup letter change when I go up or down a band size?

The cup letter is not an absolute volume—it's a ratio relative to the band size. A C cup, for example, represents a specific difference between bust and underbust measurements on a particular band size. When you go up a band size, the same cup letter represents a larger volume, so to maintain the same volume, you go down a cup letter. Conversely, when you go down a band size, the same cup letter represents a smaller volume, so you go up a cup letter to compensate. This is why sister sizes exist—they preserve cup volume while changing the band fit.

Do all brands follow the same sister size rules?

Most brands follow the standard sister size rule (band ±2, cup ∓1), but there are exceptions:

• Extended sizes: Some brands skip certain cup letters or use different progressions past DD (e.g., DDD vs. E vs. F).
• Vanity sizing: Some brands adjust their sizing to flatter customers, which can throw off standard sister size calculations.
• Style-specific sizing: Push-up bras, balconettes, and other styles may fit differently than standard bras, so sister sizing may not work as expected.
• Custom sizing: Some boutique brands use non-standard band increments or cup progressions.

Always check the brand's size chart and fit notes when trying a sister size.

Usage and Troubleshooting Questions

My size is out of stock—should I buy a sister size?

Yes, sister sizing is a practical solution when your exact size is unavailable. Here's how to choose:

• If you prefer a snugger band for support, try the sister size with a smaller band and larger cup (e.g., 34C → 32D).
• If you prefer a looser band for comfort, try the sister size with a larger band and smaller cup (e.g., 34C → 36B).
• Check the brand's return policy—if sister sizing doesn't work, you want the option to return or exchange.
• Read customer reviews to see whether the brand runs small or large in the band or cup.

What if neither my size nor my sister sizes fit?

If neither your calculated size nor your sister sizes fit comfortably, the issue may be more fundamental than a sizing adjustment. Consider:

• Re-measuring: Use our Bra Cup Size Calculator to verify your measurements. Small measurement errors can lead to incorrect size calculations.
• Evaluating the fit: Check whether the band, cups, or straps are the problem—each has a different solution.
• Trying different styles: The same size can fit differently across styles (push-up, full-coverage, demi, balconette).
• Visiting a professional fitter: A trained bra fitter can identify fit issues that aren't obvious from measurements alone.
• Checking your body's changes: Weight changes, hormonal shifts, pregnancy, and aging can all affect your bra size.

How often should I re-evaluate my sister sizes?

It's a good idea to re-evaluate your bra size and sister sizes every 6–12 months, or after any significant body change (weight loss/gain, pregnancy, hormonal shifts, or changes in exercise habits). If your current bra no longer feels comfortable, re-measure first to confirm your size, then use our sister size calculator to explore alternates. Sister sizes are a tool for refinement—if your body has changed significantly, you may need a completely different size rather than a sister size.

Can sister sizing help if my bras are uncomfortable?

Sometimes—but it depends on the source of the discomfort:

• Band discomfort: Sister sizing can help. If your band is too tight, go up a band size and down a cup size. If it's too loose, go down a band size and up a cup size.
• Cup discomfort (spilling, gaping, or digging): Sister sizing may help if the issue is the cup shape, but if the issue is cup size, you need a different cup letter—not a sister size.
• Strap discomfort: Sister sizing can help if the band is too loose (causing straps to dig), but otherwise you may need a bra with wider, padded, or adjustable straps.
• Wire discomfort: Sister sizing can help if the wire is sitting on breast tissue due to an ill-fitting band, but persistent wire discomfort may mean the bra style isn't right for your shape.

Can I use this calculator for sports bras, strapless bras, or nursing bras?

Sister sizing is most reliable for standard everyday bras. Sports bras, strapless bras, and nursing bras have unique fit requirements that make sister sizing less straightforward:

• Sports bras: Often need a snugger band for support—see our Sports Bra Size Calculator.
• Strapless bras: Require a significantly snugger band to stay up—see our Strapless Bra Size Calculator.
• Nursing bras: Accommodate size fluctuations and prioritize comfort—see our Nursing Bra Size Calculator.

While sister sizes can be a useful reference for these specialty bras, they're not a substitute for the specific sizing guidance those bra types require.