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42DD Equivalent: Sister Size and Fit Guide

1 day ago
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Written by Carmen Rodriguez

Finding the right bra size isn’t just about knowing your number, it’s about understanding how band size and cup volume work together. If you wear a 42DD, you may discover that a slightly different size fits better depending on brand, fabric, or body changes. Finding your perfect bra size depends on your unique shape and how much breast tissue needs to be supported.

This guide explains what a 42DD equivalent really means, how to calculate sister sizes, and how to use them to achieve a more comfortable, supportive fit.

What Does 42DD Equivalent Mean?

A 42DD equivalent refers to bra sister sizes that share the same cup volume as a 42DD but have a different band size. The term 'bra size equivalent' is used to describe how different sizing systems and brands label bras with the same cup volume, helping you find the right fit across various options. These sizes are interchangeable in volume, even though the label looks different.

Purpose of This Guide

This guide helps you:

Band Size and Cup Size Relationship

Bra sizing is proportional:

  • Band and cup size measure your ribcage and bust difference.
  • Cup size represents the difference between bust and band

Your current band size and current cup size are the starting point for finding a better fit. The band's width affects how much breast tissue the cup can hold.

When the band changes, the cup letter must also change to maintain the same cup volume.

If you go up in band size, you may need to go down in cup size, and vice versa, because sister sizes usually run in opposite directions with one-size shifts.

This is why a DD cup is not the same size across all bands.

What Is a 42DD Equivalent and Bra Sister Size?

What Is a Bra Sister Size?

A bra sister size (also known as a sister size bra) is a size with:

  • The same cup volume
  • A different band and cup letter combination

Cup Volume vs. Band Size

Cup letters do not stand alone. A DD cup on a 42 band is larger than a DD cup on a 36 band. Sister sizes have the same cup size (volume) even if the band and cup letter change, meaning the cup fit remains consistent across these sizes.

US and UK band sizes use inches and are generally the same (42). Sister sizing works best when adjusting just one band size up or down. A US 42 band is an AU 20 band. US sizing usually skips to DDD or F after DD, while UK brands often use DD, E, F, FF. Australian (AU) band sizes correlate to dress sizes. US and UK sizing typically use DD as the fifth cup size. US Sizing generally progresses as A, B, C, D, DD, DDD/F, G, H, I, J, etc. A US 42DD bra size is generally equivalent to a 95E in Europe and a 20DD in Australia. A US 42DD bra size is generally equivalent to a 95E in Europe and a 20DD in Australia. European (EU) and French (FR) systems use centimeters and are different from each other (EU 95 vs FR 110). Many EU and FR brands use a single letter progression (D, E, F) whereas an E cup is the equivalent of a US/UK DD cup. UK sizing uses double letters (FF, GG, etc.) beyond DD/E, whereas US sizing uses DDD/F, G, H, etc.

Common fit issues include gapping cups, spilling over the cups, and bands riding up. If you experience spillage over the cups, consider going up a cup size or trying a full-coverage style.

Why You May Need a 42DD Sister Size

You may need a sister size if:

  • The band feels too tight or too loose. If your band feels loose, try a smaller band for a bit more support.
  • The bra style runs small or large
  • Your weight or ribcage size has changed
  • You’re between sizes

Adjusting to a sister size can provide a more comfortable fit and extra comfort, as even a bit of difference in band size can make your bra feel just right.

How to Calculate Bra Sister Sizes From Your Current Bra Size

Step 1: Note Your Current Bra Size

Start with 42DD.

Step 2: Go Up One Band and Down One Cup

  • 42DD → 44D

This is called going up one band size and down one cup size, which keeps the cup volume the same while increasing band comfort. If you go up in band size, you may need to go down in cup size.

Step 3: Go Down One Band and Up One Cup

  • 42DD → 40DDD (or 40E)

This gives a firmer band with the same cup volume.

42DD Sister Size Chart and Bra Size Chart

42DD Sister Size Table

Band Change

Sister Size

Cup Volume

Down 1

40DDD / 40E

Same

Same

42DD

Same

Up 1

44D

Same

Note: Fit can vary by brand, fabric stretch, wire shape, and cup depth. Always try more than one size.

Sister Bra Sizes for 42 Band Size

If you prefer a 42 band, here are related sizes:

  • 42D (slightly smaller cup)

  • 42DD (true reference size)

  • 42DDD / 42E (larger cup size)

Best practice: Try 42DD, 42DDD, and 40DDD together to compare fit. Trying a new style of bra may also help you find a better fit.

When Your Cups Fit but Band Doesn’t: Use Sister Size

  • Band feels too tight?→ Try 44D

  • Band feels too loose?→ Try 40DDD / 40E

Sister sizes run in opposite directions, when you go up in band size, you go down in cup size, and vice versa. This is how bra sister sizes work: they maintain the same cup volume while allowing you to adjust the band for better comfort and support.

When Your Band Fits but Cups Don’t: Adjust Cup Size

  • Spillage or overflow?→ If the cups feel small, go up one cup size (42DDD). If the cups feel small, you should increase your cup size for a better fit.

  • Gaping or wrinkling?→ If the cups feel big, go down a cup size (42D) or try a different cup shape.

If neither your cup nor your band fits, you may need to adjust only the band size while keeping the cup size the same.

Cup fit issues are often caused by cup shape, not just size.

Check Cup Fit and Achieve the Perfect Fit

Signs Cups Fit Correctly

  • No spillage or bulging of the breasts
  • No gaping at the top
  • Underwire fully encases breast tissue
  • Center gore lies flat

If you prefer to go braless, nipple covers are another discreet and comfortable solution.

Signs of a Perfect Band Fit

  • Band sits level across your back
  • Snug but comfortable
  • Does not ride up

The band should provide most of the support, not the shoulders or straps. Well-designed straps help distribute weight evenly and prevent shoulder discomfort.

Hook Fit Tip

Always try a new bra on the loosest hook. This allows room for natural stretch over time. If your bra only feels comfortable when fastened on the tightest hook, you may need a smaller band size.

Common Fit Problems With 42DD and Solutions

Spillage

  • Solution: Try 42DDD or 40DDD. If you experience spillage, consider trying a larger cup size, as increasing the cup size can help resolve this issue and provide a better fit.

  • Check for deeper cup styles

Gaping Cups

  • Solution: Try 42D or a different cup shape (balconette or plunge)

Band Riding Up

  • Solution: Try 40DDD for firmer support

  • Avoid overly stretchy bands

International Bra Sizes: 42DD Equivalent Across Regions

UK Size Equivalent

  • 42DD (US) = 42DD (UK)
    (Some brands may label the next size as E)

EU Size Equivalent

  • 42DD (US) ≈ 95E (EU)

Label Differences to Know

  • US: DD, DDD
  • UK: DD, E, F
  • EU: E, F
  • Always check the brand’s size chart.

Shopping Tips to Find Your Sister Bra Sizes

  • Try three sizes: your usual size plus two sister sizes
  • If your preferred size is out of stock, consider trying a new size or sister size to help you find your perfect bra.
  • Raise your arms when trying bras to test cup security
  • Recheck your size every 6–12 months
  • Different bra styles may require different sizes

FAQs: Bra Sizes, Sister Bra Size Chart, Cup Size Questions

Do Sister Sizes Actually Work?

Yes. Sister sizes maintain the same cup volume and are widely used by professional fitters.

Why Is DD Sometimes Called E or F?

Cup labeling varies by country and brand. The volume is what matters, not the letter.

When Should I Get a Professional Fitting?

If you experience ongoing discomfort, pain, or inconsistent sizing across brands, a professional fitting can be helpful.

Final Thoughts

Understanding your 42DD equivalent empowers you to shop smarter, fit better, and feel more comfortable. Sister sizing isn’t a workaround, it’s a professional fitting tool that helps you adapt to real bodies and real bras.

At Olivia Paisley Intimates, we believe the best bra is the one that fits you, not just the label.

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