Finding the right bra size starts with accurate measurements. A Bra Cup Calculator can help turn your band and bust measurements into an estimated bra size by calculating the difference between the two measurements.

This calculator asks for two primary measurements: band size and bust size. It then adjusts an odd band measurement to the next even number, calculates the bust-to-band difference, and uses that difference to determine a cup size.

The calculator also provides a full bra size and two suggested sister sizes, giving you additional sizes to consider when the calculated size does not provide the desired fit.

The tool includes US, UK, and EU sizing options, although the selected measurement system does not change the calculation used by this particular calculator. The results should therefore be treated as an estimate rather than a universal sizing standard.

What Is a Bra Cup Calculator?

A Bra Cup Calculator is a sizing tool that estimates a bra size from measurements taken around the body.

The two main measurements are:

• Band size: the measurement around the ribcage or underbust area
• Bust size: the measurement around the fullest part of the bust

The difference between these measurements is used to estimate the cup size.

For example, if the adjusted band measurement is 34 inches and the bust measurement is 36 inches, the difference is:

36 − 34 = 2 inches

The calculator then maps that difference to its built-in cup-size chart.

How the Bra Cup Calculator Works

This calculator follows several steps to produce its result.

1. It reads the band measurement.
2. It reads the bust measurement.
3. It checks that both measurements are valid.
4. It adjusts an odd band measurement upward to the next even number.
5. It calculates the difference between bust and adjusted band size.
6. It maps that difference to a cup-size sequence.
7. It combines the adjusted band and cup size.
8. It calculates two sister sizes.

The result includes five pieces of information:

• Recommended band size
• Cup size
• Full bra size
• Size difference
• Sister sizes

How to Use the Bra Cup Calculator

Step 1: Measure Your Band Size

Enter your band measurement in inches.

For example:

34 inches

The calculator accepts positive measurements.

If you enter an odd number, the calculator rounds it upward to the next even number.

For example:

33 inches → 34 inches

and:

35 inches → 36 inches

This adjusted measurement is then used in the cup-size calculation.

Step 2: Measure Your Bust Size

Enter your bust measurement in inches.

This should generally be measured around the fullest part of the bust.

For example:

36 inches

The calculator requires the bust measurement to be equal to or greater than the entered band measurement.

Step 3: Select the Measurement System

The calculator offers:

• US Sizing
• UK Sizing
• EU Sizing

However, the calculation itself uses the same band and cup-size logic regardless of which option is selected. The measurement-system selection does not alter the resulting calculation.

Therefore, the most important inputs for this calculator are the band and bust measurements.

Step 4: Calculate Your Result

After entering your measurements, select Calculate.

The calculator displays your estimated:

• Band size
• Cup size
• Full bra size
• Measurement difference
• Sister sizes

How to Calculate Your Bra Cup Difference

The calculator first adjusts the band size when necessary.

The calculation is:

Adjusted Band = Band Measurement

If the band measurement is odd:

Adjusted Band = Band Measurement + 1

The cup difference is then:

Difference = Rounded Bust Size − Adjusted Band Size

For example, suppose you enter:

• Band: 33 inches
• Bust: 37 inches

The band is odd, so it becomes:

33 → 34 inches

The difference is:

37 − 34 = 3 inches

The calculator then uses a 3-inch difference to determine the cup size.

Understanding the Calculator’s Cup-Size Chart

The calculator uses the following cup-size sequence:

Difference	Cup Size
0	AA
1	A
2	B
3	C
4	D
5	DD/E
6	DDD/F
7	G
8	H
9	I
10	J
11+	K

This is the calculator’s built-in mapping.

Different bra brands and sizing systems may use different cup-letter sequences, so the result should be considered an estimate rather than a guarantee of fit.

Worked Example: 34-Inch Band and 36-Inch Bust

Suppose your measurements are:

• Band size: 34 inches
• Bust size: 36 inches

The band is already an even number, so no adjustment is necessary.

Step 1: Adjust the Band

34 inches

Step 2: Calculate the Difference

36 − 34 = 2 inches

Step 3: Find the Cup Size

According to the calculator’s cup-size sequence, a 2-inch difference corresponds to:

B

Step 4: Combine the Measurements

The estimated full size is:

34B

The calculator also generates sister sizes based on the cup index and band adjustments.

Worked Example With an Odd Band Measurement

Consider:

• Band measurement: 33 inches
• Bust measurement: 37 inches

Because 33 is odd, the calculator adjusts it to:

34 inches

The difference becomes:

37 − 34 = 3 inches

A 3-inch difference corresponds to:

C

The calculated full size is therefore:

34C

This demonstrates why entering an accurate band measurement matters. The calculator does not simply use the original odd measurement when determining the cup size.

What Are Sister Sizes?

Sister sizes are sizes with different band measurements but related cup volume.

The concept can be useful when a calculated bra size does not fit exactly as expected.

For example, changing the band size while changing the cup letter in the opposite direction can help maintain a similar overall cup volume.

This calculator provides two sister-size results:

• One with the band reduced by 2 inches
• One with the band increased by 2 inches

The corresponding cup index is adjusted in the opposite direction.

How the Calculator Calculates Sister Sizes

Suppose the calculator produces a full size with an adjusted band of 34 and a cup index corresponding to C.

The first sister-size calculation uses:

34 − 2 = 32

and moves one step upward in the cup-size sequence.

The second uses:

34 + 2 = 36

and moves one step downward in the cup-size sequence.

The exact displayed results depend on the cup index calculated from your measurements.

Why Sister Sizes Can Be Useful

A bra can sometimes feel too tight or too loose even when the calculated cup size seems correct.

Sister sizes provide alternative combinations of band and cup measurements to consider.

For example, if the band feels too tight, moving to a larger band and a corresponding smaller cup can provide a different fit.

If the band feels too loose, moving to a smaller band with a corresponding larger cup can be another option.

The calculator provides these alternatives automatically.

Why Accurate Measurements Matter

The quality of a bra-size estimate depends heavily on the measurements entered.

A small difference in measurement can change the calculated cup size because the calculator maps the bust-to-band difference to a specific cup category.

For example:

36 − 34 = 2 inches

while:

37 − 34 = 3 inches

Those one-inch differences correspond to different cup categories in the calculator.

For this reason, measuring carefully is important.

Tips for Measuring Band Size

For a useful measurement, keep the measuring tape reasonably level around the body.

Avoid pulling the tape extremely tight or leaving it excessively loose.

The measurement should represent the area used for the band rather than the fullest part of the bust.

If possible, measure more than once and check that the results are reasonably consistent.

Tips for Measuring Bust Size

The bust measurement is taken around the fullest part of the bust.

Keep the measuring tape level around the body.

It should not be pulled so tightly that it compresses the bust significantly, because that can produce a smaller measurement than intended.

Likewise, leaving the tape too loose can produce an unnecessarily large measurement.

Band Size and Cup Size Work Together

A cup letter by itself does not completely describe bra size.

For example:

34C

and:

36C

do not represent exactly the same overall size because the band measurements are different.

This is why the calculator provides a complete size such as 34C, rather than presenting only the cup letter.

The cup calculation is tied to the adjusted band measurement.

What Happens if Bust Size Is Smaller Than Band Size?

The calculator checks whether the bust measurement is smaller than the entered band measurement.

If:

Bust < Band

the calculator displays an error and asks for valid measurements.

This is because its calculation assumes the bust measurement is equal to or greater than the band measurement.

For example, entering:

• Band: 36
• Bust: 34

will not produce a result.

What Happens if the Band Measurement Is Odd?

An odd band measurement is increased by one inch.

For example:

Entered Band	Adjusted Band
31	32
33	34
35	36
37	38
39	40

The adjusted value is what the calculator uses in the difference calculation and final bra size.

Does US, UK, or EU Sizing Change the Result?

The calculator provides three measurement-system choices:

• US
• UK
• EU

However, the selected system is not used to modify the mathematical result.

The same adjusted band and cup-size sequence is applied regardless of the selected option.

This means you should not assume that choosing UK or EU automatically converts the result into a complete country-specific bra size.

Understanding the Full Bra Size

The calculator combines the adjusted band number with the calculated cup letter.

For example:

34 + B = 34B

Another example:

36 + D = 36D

The full bra size is more informative than the cup letter alone because it includes both band and cup information.

Bra Size Can Vary Between Brands

A calculator can provide a useful starting point, but bra sizing is not completely standardized across every brand and style.

Differences can occur because of:

• Brand sizing conventions
• Cup shape
• Band construction
• Fabric stretch
• Underwire design
• Bra style
• Manufacturing differences

Therefore, a calculated size may not fit identically in every bra.

Why Fit Matters Beyond the Calculated Size

A numerical size is only one part of finding a comfortable bra.

A properly fitting bra generally needs a band that feels secure without being excessively tight, while the cups should contain the bust without excessive gaps or overflow.

The straps should also sit comfortably without carrying all of the support.

If the calculated size does not feel right, trying the calculator’s sister sizes may provide useful alternatives.

Common Bra Sizing Mistakes

Several measurement mistakes can affect a calculated result.

Measuring the Band Too Loosely

An overly loose band measurement can produce a larger calculated band.

Measuring the Bust Too Tightly

Compressing the bust during measurement can produce a smaller bust measurement.

Using an Uneven Tape Position

The measuring tape should remain reasonably level around the body.

Focusing Only on the Cup Letter

A cup letter without a band number does not provide the complete bra size.

Assuming Every Brand Fits Identically

Different brands and styles can fit differently even when the labeled size is the same.

Frequently Asked Questions

1. What is a Bra Cup Calculator?

A Bra Cup Calculator estimates a bra band size and cup size using band and bust measurements.

2. How does the calculator determine cup size?

It calculates the difference between the bust measurement and the adjusted band measurement, rounds that difference, and maps it to its built-in cup-size sequence.

3. What happens if my band measurement is odd?

The calculator increases an odd band measurement by one inch. For example, 33 becomes 34.

4. What measurements do I need?

You need a band measurement and a bust measurement, both entered in inches.

5. Can I enter a bust measurement smaller than my band measurement?

No. The calculator requires the bust measurement to be equal to or greater than the band measurement.

6. What does a 2-inch difference mean?

According to the calculator’s built-in cup chart, a 2-inch difference corresponds to a B cup.

7. What does a 3-inch difference mean?

According to the calculator’s cup sequence, a 3-inch difference corresponds to a C cup.

8. What is a full bra size?

A full bra size combines the band number and cup letter, such as 34B or 36C.

9. What are sister sizes?

Sister sizes are alternative band-and-cup combinations that use a different band measurement with a corresponding change in cup size.

10. Does the measurement-system selector convert US, UK, and EU sizes?

No. In this calculator, the measurement-system selection does not change the calculation or automatically convert the final result.

11. Why is my calculated size different from a bra label I already wear?

Bra sizing can vary between brands, styles, materials, and sizing conventions. Your measurements may also have been taken differently.

12. Can I use the calculator for different bra brands?

Yes, you can use the calculated size as a starting point, but individual brands may have their own sizing and fit characteristics.

13. How accurate is a Bra Cup Calculator?

It provides an estimate based on the measurements and cup-size chart used by the calculator. Actual fit can vary depending on the brand, style, and individual body shape.

14. Why should I check sister sizes?

Sister sizes can provide alternative band-and-cup combinations when the calculated size does not feel comfortable or fit as expected.

15. Can a bra calculator guarantee the perfect fit?

No. A calculator can estimate a starting size, but comfort and fit can vary based on body shape, bra construction, brand, and personal preference.

Final Thoughts

The Bra Cup Calculator provides a convenient way to estimate a bra size from band and bust measurements. It adjusts odd band measurements to the next even number, calculates the bust-to-band difference, and uses that difference to select a cup size from its built-in chart.

The calculator also provides a complete bra size and two sister-size options, which can be useful when exploring alternative fits.

For the best results, take your measurements carefully and remember that the calculated size is a starting point rather than a guarantee. Different brands and bra styles can fit differently, even when they carry the same labeled size.

Because the calculator’s US, UK, and EU selections do not alter its underlying calculation, the displayed result should be interpreted according to the calculator’s own sizing system. When shopping, checking the specific brand’s size chart and considering how the bra actually fits can help you find a more comfortable option.