Cup Bra Size Explained: How to Get the Right Fit

Bra cup size is one of the most misunderstood aspects of lingerie fit. Many people assume cup letters measure breast size on their own but cup size only makes sense when paired with the correct band. At Olivia Paisley Intimates, we believe that understanding cup bra size is the foundation of comfort, support, and confidence.

This guide explains what cup size means, how it’s calculated, and why proper cup fit matters.

What Is a Bra Cup Size?

A bra cup size represents the difference between your bust measurement and your band (underbust) measurement. Band and cup sizes together determine bra size, but there is significant variation in how these are labeled and measured by different manufacturers and in different countries. It does not describe breast size independently.

Because there is no industry-wide standardization, bra size is only an approximate fit, and sizing can vary significantly between brands and regions.

Manufacturers' bra size labelling systems vary by country because no comprehensive international standards exist. Even within a country, one study found that the bra size label was consistently different from the measured size. The current popular system of determining bra size is inaccurate so often as to be useless.

Standard Cup Size Calculation

If your measurement falls between whole numbers, round up to the next whole number. To measure your band size, wrap a measuring tape tightly around your torso just beneath your breasts and round to the nearest even number. Measure your bust size by wrapping the tape around the fullest part of your breasts while standing straight with your arms to the side.

The difference between your bust and band measurements determines your cup size. For every one inch of difference, the cup size increases by one letter. See the table below for reference:

Difference (inches)	Cup Size
1	A cup
2	B cup
3	C cup
4	D cup
5	DD / E cup
6	DDD / F cup

The actual volume of each cup increases with band size, which is why a 38C is larger than a 32C, even though the cup letter is the same.

Why Cup Size Is Relative

Cup size is always relative to band size. For example:

• 32D and 38D are not the same volume
• 34DD has more volume than 32DD
• Cup letters scale with band measurements

There is significant variation in sizing systems, and bra sizing systems differ by country, which can complicate the shopping experience for consumers.

This is why judging size by the letter alone often leads to poor fit.

What Cup Size Looks Like on Different Bodies

The same cup size can look very different depending on:

• Ribcage width
• Torso length
• Breast shape (shallow, projected, full on top, full on bottom)
• Tissue density
• Breast position

Breast position, how high, low, or wide the breasts sit on the chest, can affect both the accuracy of cup size measurements and how the cup size appears visually.

A C cup on a petite frame may look full, while a D cup on a broader frame may appear moderate.

Common Cup Fit Problems and What They Mean

Spillage or Overflow

• Cup is too small
• Try going up one cup size or choosing a deeper cup shape

Gaping or Wrinkling

• Cup is too large or too shallow
• Try a smaller cup or a different style

Underwire Sitting on Breast Tissue

• Cup is too narrow
• Look for wider wires or a larger cup size

Cup Size vs. Breast Shape

Cup size measures volume, not shape. Two people with the same cup size may need very different bras. Shape affects:

• Cup depth
• Wire width
• Coverage level

Finding the right bra often requires a combination of measurements, such as bust, under-bust, and volume, to determine the most accurate cup bra size. Women are encouraged to try on multiple styles and sizes to find the best fit for their body shape.

Choosing bras designed for your breast shape is just as important as choosing the correct size.

Cup Volume and Breast Weight

When it comes to finding the perfect bra size, understanding cup volume and breast weight is just as important as knowing your band and cup size. The average breast weighs about 0.5 kg (1.1 lb), making up roughly 4–5% of a woman’s total body fat. Since breast tissue density is around 0.9 g/cm³, the actual volume of your breasts can vary significantly from person to person, even if two women wear the same cup size.

Cup volume is a key factor in bra fit. To estimate cup volume, you can imagine the cup as a hemisphere or a hemi-ellipsoid, using simple math formulas. For example, if you model the cup as a hemisphere, the volume is calculated as V = (2/3)πr³, where r is the cup’s radius. For a more accurate fit, some use the hemi-ellipsoid formula: volume = (2/3)πabc, with a, b, and c representing the cup’s width, depth, and wire length. While you don’t need to do these calculations yourself, it’s helpful to know that cup volume is what truly determines how much space a bra cup provides for your bust.

Breast weight and volume can change over time due to factors like pregnancy, weight fluctuations, and hormonal changes. For example, during pregnancy, many women notice a significant increase in bust size, which means their usual bra size may no longer provide a properly fitted bra. That’s why it’s important to re measure your bust and band size regularly, especially if you notice changes in your body or experience discomfort.

To get the most accurate bra size, use a tape measure while standing straight. Measure around the fullest part of your breasts for your bust measurement, and snugly around your torso just under the bust for your band size. The difference between these two measurements determines your cup size for example, a 2-inch difference usually means a B cup. However, keep in mind that cup sizes and band sizes can vary between manufacturers, styles, and even different countries, so the same person might wear different sizes depending on the brand or style of bra.

A properly fitted bra should have a center panel that lies flat against your chest, straps that don’t dig into your shoulders, and a band that fits snugly without being too tight. If your bra leaves red marks, causes neck pain, or feels uncomfortable, it may be a sign of an incorrectly fitted bra. Women wear bras in a wide range of sizes, and it’s common to need a different size when trying a new style or shopping from a different manufacturer.

For the best results, consider using a bra size calculator or consulting a professional fitter, especially if you’re shopping in different countries or trying a new style. Remember, the goal is to find a bra that supports your body comfortably, taking into account your unique cup volume, breast weight, and band size. By paying attention to these details, you can avoid the most common mistake in bra shopping, choosing a size based only on the cup letter and enjoy the comfort and confidence that comes with a truly well fitted bra.

Understanding Sister Sizes

Sister sizes allow you to change band size while keeping the same cup volume:

Primary Size	Sister Sizes
34C	32D, 36B
36D	34DD, 38C
38DD	36DDD, 40D

Sister sizing is helpful when adjusting band comfort without changing cup capacity.

International Cup Size Differences

Cup labels vary by region and can range from AA (the smallest standard cup size) up to DD, DDD, and even larger, with cup sizes generally ranging from AA to over DDD/F. Each letter typically represents a 1-inch increase in the difference between the bust and band measurement.

• US: AA, A, B, C, D, DD, DDD
• UK: AA, A, B, C, D, DD, E, F
• EU: AA, A, B, C, D, E, F
• Australia/New Zealand: AA, A, B, C, D, DD (follows UK system up to DD, then switches to a metric system for larger sizes)
• France/Spain: Based on the Continental European system, but band sizes are 15 cm larger than the EU standard.

Bra sizing systems vary significantly between countries and manufacturers. The U.S. and U.K. use inch based systems for band sizes, while most European countries use centimeter based systems. In the U.S., cup sizes increase by 1 inch, whereas in many other countries, they increase by 2 centimeters.

Some brands use grouped sizing systems such as XS (extra small), Medium, and so on, which refer to band width or overall grouped size categories rather than specific cup volumes.

Region/Country	Band Size System	Cup Size Progression	Example Cup Labels	Grouped Sizing Options
US	Inches	+1 inch per cup	AA, A, B, C, D, DD, DDD	XS, S, M, L
UK	Inches	+1 inch per cup	AA, A, B, C, D, DD, E, F	XS, S, M, L
EU (Continental)	Centimeters	+2 cm per cup	AA, A, B, C, D, E, F	XS, S, M, L
Australia/New Zealand	Inches/Centimeters	UK up to DD, then metric	AA, A, B, C, D, DD	XS, S, M, L
France/Spain	Centimeters (+15 cm band)	+2 cm per cup	AA, A, B, C, D, E, F	XS, S, M, L

Always check the brand’s sizing system before purchasing.

How Often Should You Recheck Cup Size?

Experts recommend measuring every 6–12 months, or after:

• Weight changes

• Pregnancy or breastfeeding

• Hormonal changes

• Noticeable fit discomfort

Breast size naturally changes over time.

Final Thoughts

Bra cup size is not a fixed label, it’s a dynamic measurement influenced by band size, shape, and life changes. Understanding how cup size works empowers you to find bras that truly support your body.

At Olivia Paisley Intimates, we design bras that respect both cup volume and shape, ensuring every size feels comfortable, supportive, and beautifully made.