Cup Size Calculator
Finding the right bra size can be surprisingly difficult. Different brands may use different sizing systems, and even small measurement differences can change the recommended band or cup size. A cup size calculator provides a convenient starting point by comparing your underbust and bust measurements and estimating a bra size based on the sizing system you select.
This calculator uses two essential measurements: your underbust measurement and your bust measurement. It then calculates the difference between them to estimate the cup size and determines an appropriate band size. You can also select a fit preference and compare sizing systems used in the US, UK, EU, France, and Australia.
Because bra sizing varies between manufacturers and styles, the result should be treated as a starting point rather than a guarantee of fit.
What Is a Cup Size Calculator?
A cup size calculator is a measurement tool designed to estimate your bra size using your body measurements.
The two primary measurements are:
- Underbust: The measurement around your torso directly underneath the bust.
- Bust: The measurement around the fullest part of your bust.
The difference between these measurements is used to estimate the cup size. The underbust measurement is also used to determine the band size.
This calculator additionally provides:
- Estimated bra size
- Band size
- Cup size
- Cup difference in inches
- Sister size up
- Sister size down
- Different international sizing options
How to Measure Your Bra Size
Accurate measurements are important if you want a useful calculator result.
Measure Your Underbust
Wrap a flexible measuring tape around your torso directly underneath your bust.
Keep the tape level around your body and make the measurement snug without pulling it excessively tight.
Record the measurement in inches.
For example, your underbust might measure 34 inches.
Measure Your Bust
Next, measure around the fullest part of your bust.
The measuring tape should remain relatively level and should not be pulled tightly against your body. Avoid intentionally compressing the bust while taking the measurement.
For example, your bust measurement might be 38 inches.
Enter Both Measurements
Enter your measurements into the calculator and choose your preferred sizing system.
The calculator will then estimate the band, cup, and overall bra size.
How the Cup Size Calculation Works
The calculator determines the cup difference by subtracting the underbust measurement from the bust measurement:
Cup Difference = Bust Measurement − Underbust Measurement
For example:
- Underbust = 34 inches
- Bust = 38 inches
The difference is:
38 − 34 = 4 inches
The calculator then rounds this difference to select a cup from its built-in cup-size scale.
The calculator uses the following cup sequence:
AA, A, B, C, D, DD, DDD/E, F, FF, G, GG, H, HH, I, J, K
This means the calculated difference is mapped to the corresponding position in this sequence.
How Band Size Is Calculated
The calculator begins with the underbust measurement and rounds it to the nearest whole number for a normal fit.
The fit preference can change the result:
- Normal Fit: Rounds the underbust measurement.
- Snug Band: Uses the lower whole number.
- Loose Band: Uses the higher whole number.
The calculator then adjusts the band size to an even number.
For example, if the calculated band is 33, it becomes 34.
This provides a simple numerical estimate, but actual bra sizing can vary considerably between brands.
Understanding Fit Preferences
Everyone has different preferences when it comes to how a bra feels around the body.
Normal Fit
The normal setting rounds the underbust measurement to the nearest whole number and then adjusts it to an even band size when necessary.
This is the default option.
Snug Band
The snug setting uses the floor value of the underbust measurement before the even-number adjustment.
This may produce a smaller band estimate than the normal setting.
Loose Band
The loose setting uses the ceiling value of the underbust measurement before adjusting to an even number.
This can result in a larger band estimate.
These settings are useful for exploring how small measurement changes may affect the calculated band size.
Example: Using the Cup Size Calculator
Suppose your measurements are:
- Underbust: 34 inches
- Bust: 38 inches
- Sizing system: US
- Fit preference: Normal Fit
The band measurement begins at 34 inches, so the calculated band is 34.
Next, calculate the bust difference:
38 − 34 = 4 inches
The calculator maps that difference to its cup-size sequence. A 4-inch difference corresponds to D according to the calculator’s scale.
The estimated result is therefore:
34D
The calculator will also provide sister-size options based on the selected size.
Remember that this is a mathematical estimate. Your actual best-fitting bra may be different depending on the manufacturer, bra style, fabric, construction, and your individual shape.
What Are Sister Sizes?
Sister sizes are bra sizes that have similar cup volume while using different band measurements.
They are useful when the cup volume seems appropriate but the band feels too tight or too loose.
For example, moving to a larger band generally requires moving to a smaller cup letter to maintain a similar cup volume. Moving to a smaller band generally requires moving to a larger cup letter.
This calculator provides:
- Sister Size Up
- Sister Size Down
These can be useful when shopping for bras or comparing different brands.
However, sister sizing does not mean that two bras will necessarily feel identical. Band construction, cup shape, wire design, material, and brand-specific sizing can all affect fit.
US, UK, EU, FR, and AU Bra Sizing
Bra sizes are not completely standardized worldwide. Different countries and brands may use different conventions.
This calculator lets you select:
- US sizing
- UK sizing
- EU sizing
- FR sizing
- AU sizing
The displayed band number is adjusted depending on the selected system.
Because international bra sizing is not perfectly standardized, it is important to check the individual brand’s size chart when buying a bra.
A calculator can provide a useful estimate, but a brand’s own measurement guide should generally be consulted before making a purchase.
Why Your Calculated Size May Not Fit Perfectly
A calculator can only work with the measurements you provide. It cannot account for every factor involved in bra fit.
Your actual size may vary because of:
- Different brand sizing
- Different bra styles
- Cup shape
- Breast shape
- Band elasticity
- Underwire design
- Fabric stretch
- Measurement technique
- Personal fit preference
For example, a plunge bra, sports bra, balconette bra, and wireless bra may fit differently even when they are labeled with the same size.
Therefore, use the calculator as a starting point rather than assuming the result is universally correct.
Tips for Taking More Accurate Measurements
For a better starting estimate, keep the following tips in mind.
Use a Flexible Measuring Tape
A soft measuring tape designed for body measurements works best.
Keep the Tape Level
Make sure the tape does not slope upward or downward around your back.
Don’t Pull Too Tightly
The tape should sit against your body without compressing the tissue.
Measure More Than Once
If your measurement seems unusual, take it again. A small difference can affect the calculated result.
Avoid Guessing
Entering an estimated measurement instead of actually measuring can produce an inaccurate result.
Check the Brand’s Size Chart
Once you have a calculated size, compare it with the size chart provided by the manufacturer.
Cup Size vs. Band Size
The band size describes the measurement around your torso beneath the bust, while the cup size represents the difference between the bust and underbust measurements in the calculator’s sizing method.
A cup letter by itself does not describe a complete bra size.
For example, saying that someone wears a “D cup” does not provide enough information to determine the actual bra size. A 32D and a 38D have different band measurements and different cup volumes.
This is why it is important to consider the complete size rather than focusing only on the cup letter.
Frequently Asked Questions
1. What measurements do I need for a cup size calculator?
You need your underbust measurement and bust measurement, both entered in inches.
2. How do I measure my underbust?
Measure around your torso directly underneath your bust using a flexible measuring tape. Keep the tape level and comfortably snug.
3. How do I measure my bust?
Measure around the fullest part of your bust while keeping the measuring tape level. Avoid pulling the tape tightly.
4. How is cup size calculated?
The calculator subtracts the underbust measurement from the bust measurement to determine the cup difference, then maps the rounded difference to its cup-size scale.
5. What is the cup difference?
The cup difference is the number of inches between your bust and underbust measurements.
For example, a 38-inch bust and 34-inch underbust produce a 4-inch difference.
6. What is a band size?
Band size represents the size around your torso underneath the bust. The calculator estimates it from your underbust measurement.
7. What does normal fit mean?
Normal fit uses the rounded underbust measurement and adjusts it to an even band number.
8. What does snug band mean?
The snug option uses the lower whole-number value of your underbust measurement before making the even-number adjustment.
9. What does loose band mean?
The loose option uses the higher whole-number value of your underbust measurement before adjusting to an even band size.
10. What are sister sizes?
Sister sizes are sizes with different band and cup combinations that can have similar cup volume. They can be useful when the band or cup of your calculated size does not feel right.
11. Why does the calculator show two sister sizes?
The calculator provides one size with a band two numbers larger and another with a band two numbers smaller, while adjusting the cup letter in the opposite direction.
12. Can I use this calculator for UK bra sizes?
Yes. The calculator includes a UK sizing option. However, individual UK brands may have their own size charts, so verify the result with the manufacturer’s measurements.
13. Are US and UK cup sizes the same?
Not always. US and UK sizing systems can use different cup-letter sequences, particularly at larger cup sizes. Always check the specific brand’s size chart.
14. Why can the same bra size fit differently between brands?
Bra sizing is not perfectly standardized. Differences in cup shape, materials, band elasticity, underwire construction, and design can change how a particular size fits.
15. Is a cup size calculator 100% accurate?
No. It provides an estimate based on the measurements entered. The actual best fit can vary by brand, bra style, body shape, and personal preference.
Final Thoughts
A cup size calculator can make bra shopping easier by giving you a starting estimate based on your underbust and bust measurements. It can calculate an estimated band size, cup size, complete bra size, cup difference, and sister sizes while allowing you to compare several international sizing systems.
For the most useful result, take your measurements carefully and use the calculator as a starting point. When purchasing a specific bra, always compare the calculated result with the manufacturer’s size chart and consider how the particular style is designed to fit.