Bust Size Calculator
A Bust Size Calculator is a simple way to estimate your bra size using your underbust and full-bust measurements. With the right measurements, you can get an estimated band size, cup size, overall bust size, and sister sizes that may help when shopping for bras.
Our calculator also lets you select different sizing systems, including US, UK, European, French, and Australian sizing.
Keep in mind that bra sizing is not completely standardized. Different brands and bra styles can fit differently, so an online calculator should be considered a helpful starting point rather than a guarantee of fit.
What Is a Bust Size Calculator?
A bust size calculator uses measurements around your torso and bust to estimate your bra size.
The two most important measurements are:
- Underbust: The measurement directly underneath your breasts.
- Full bust: The measurement around the fullest part of your breasts.
The difference between these measurements is used to estimate the cup size, while the underbust measurement is used to estimate the band size.
The calculator also asks for chest width, which can provide an additional measurement reference, although the basic bra-size calculation primarily relies on the underbust and full-bust measurements.
How to Measure Your Bust Correctly
Accurate measurements are important if you want a useful calculator result.
Measuring Your Underbust
To measure your underbust:
- Stand naturally with your arms relaxed.
- Place a soft measuring tape directly underneath your breasts.
- Keep the tape level around your torso.
- Make the tape snug without pulling it uncomfortably tight.
- Record the measurement in inches.
This measurement is used to estimate your band size.
Measuring Your Full Bust
To measure your full bust:
- Stand upright in a relaxed position.
- Wrap the measuring tape around the fullest part of your breasts.
- Keep the tape horizontal.
- Do not compress the breasts with the tape.
- Record the measurement in inches.
The full-bust measurement is compared with the underbust measurement to estimate the cup size.
How Is Cup Size Calculated?
The calculator determines the difference between your full-bust and underbust measurements.
For example:
Full bust − Underbust = Bust difference
If your underbust measurement is 32 inches and your full-bust measurement is 35 inches:
35 − 32 = 3 inches
The calculator then uses this difference to select an estimated cup size from the appropriate sizing chart.
Because cup-size conventions differ between countries and brands, the same measurements may not produce exactly the same labeled size everywhere.
How Is Band Size Calculated?
Band size represents the approximate size of the bra band around your rib cage.
The calculator uses your underbust measurement and applies a sizing formula based on the selected system.
For US, UK, and Australian calculations, the calculator adjusts the underbust measurement to produce an even-numbered band size.
European and French systems use different numerical conventions, so the resulting band number may look quite different even when the underlying body measurement is the same.
Understanding the Different Sizing Systems
The calculator supports several sizing systems.
US Bra Sizes
US sizing commonly uses numerical band sizes such as 30, 32, 34, 36, and 38, combined with cup letters.
UK Bra Sizes
UK sizing also uses numerical band sizes but has a different cup progression, particularly at larger cup sizes. For example, UK sizing commonly uses sequences involving FF, GG, and HH.
European Sizes
European bra sizes generally use larger numerical band numbers, such as 65, 70, 75, 80, and 85.
French Sizes
French sizing uses its own numerical system and generally has larger band numbers than US or UK labels for the same body measurement.
Australian Sizes
Australian sizing can use numerical bands alongside cup-size conventions that differ from US and UK systems.
Because international conversion can be complicated, always check the manufacturer’s own size chart before buying.
What Is Your Bust Size?
The calculator displays a Your Bust Size result by combining the estimated band size and cup size.
For example, a result might look like:
34C
The number represents the band size, while the letter represents the cup size.
However, the exact fit depends on factors such as breast shape, bra construction, brand sizing, and personal preference.
What Is the Bust-to-Band Ratio?
The calculator also displays a Bust to Band Ratio.
It calculates the percentage difference between the full-bust and underbust measurements:
(Full Bust − Underbust) ÷ Underbust × 100
For example, if the underbust is 32 inches and the full bust is 36 inches:
(36 − 32) ÷ 32 × 100 = 12.5%
This provides a numerical way to compare the difference between your bust and underbust measurements.
It should not be interpreted as a medical measurement or a universal definition of body shape.
What Is the Size Classification?
The calculator provides a general classification based on the estimated cup-size index. Depending on the result, it may display categories such as:
- Petite
- Small
- Medium
- Full
- Large
- Extra Large
These classifications are simplified calculator categories rather than standardized medical or anatomical classifications.
People with the same bra size can have noticeably different breast shapes and body proportions.
What Are Sister Sizes?
Sister sizes are bra sizes that have approximately similar cup volume while using different band sizes.
For example, moving to a smaller band generally requires moving to a larger cup letter to maintain a similar approximate cup volume.
Similarly, moving to a larger band generally requires moving to a smaller cup letter.
Sister sizing can be useful when your calculated size is close but the band feels too tight or too loose.
Smaller-Band Sister Size
A smaller band size is paired with a larger cup letter.
This can be useful if the original band feels too loose but the cup volume seems appropriate.
Larger-Band Sister Size
A larger band size is paired with a smaller cup letter.
This may be useful if the original band feels too tight while the cup volume seems appropriate.
Sister sizes are not guaranteed to fit identically because bra construction and cup shape also affect fit.
Why Your Calculator Result May Not Fit Perfectly
An online bust calculator provides an estimate, but several factors can affect real-world bra fit.
These include:
- Different brand sizing
- Different cup shapes
- Underwire construction
- Stretch in the fabric
- Breast shape
- Torso shape
- Bra style
- Manufacturing differences
- Personal fit preferences
For example, a molded T-shirt bra and an unlined bra may fit differently even when they carry the same size label.
Bust Measurement vs. Chest Width
The calculator asks for chest width as an additional measurement.
Chest width is different from the circumference measurements used for conventional bra sizing. It can provide useful information about the front width of the torso, but it does not directly determine cup size in the calculator’s formula.
The primary measurements for the displayed bra-size estimate are the underbust and full-bust measurements.
Tips for Getting a Better Measurement
For more consistent results:
- Use a flexible measuring tape.
- Measure against bare skin or thin clothing.
- Keep the tape horizontal.
- Avoid pulling the tape excessively tight.
- Stand naturally.
- Take each measurement twice.
- Measure to the nearest half inch when possible.
- Check the bra manufacturer’s size chart before purchasing.
If your measurements fall between sizes, trying nearby sizes can help you determine which combination of band and cup provides the most comfortable fit.
Frequently Asked Questions
Is a bust calculator accurate?
It can provide a useful estimate, but it cannot guarantee the correct bra size. Brand-specific sizing and individual breast shape can affect the actual fit.
Should the full bust be larger than the underbust?
Normally, yes. The full-bust measurement is taken around the fullest part of the breasts, while the underbust is measured underneath them.
Can I use centimeters instead of inches?
This particular calculator is designed for measurements entered in inches. If you measure in centimeters, convert the measurements to inches before entering them.
Why do different calculators give different bra sizes?
Different calculators may use different sizing formulas and cup-size charts. International sizing systems also use different conventions.
What if my calculated size feels uncomfortable?
Try checking your measurements first. You can then consider a nearby sister size or compare your measurements with the specific brand’s sizing guide.
Does cup size mean the same thing for every band size?
No. Cup letters are relative to the band size. A C cup on one band size does not necessarily have the same volume as a C cup on another band size.
Final Thoughts
A Bust Size Calculator can make it easier to estimate your band size, cup size, and overall bra size from measurements you can take at home. Measuring your underbust and full bust carefully is the most important step.
Because bra sizing varies between brands and countries, use the calculated result as a starting point. Check the manufacturer’s sizing chart and pay attention to how the bra feels and fits rather than relying solely on the number or letter printed on the label.