Estimate a starting bra size from your snug underbust and fullest-bust measurements, see nearby sister sizes, and learn how to check band and cup fit. Bra sizing varies by brand, so use the result as a fitting starting point.
Bra Size Calculator
Choose a mode, enter your measurements, and press Calculate for a starting estimate.
in
Measure around the torso directly under the bust. Keep the tape level, snug but not painfully tight, and parallel to the floor where practical. Do not measure over a padded bra. Measure twice if the number seems unexpected.
in
Measure around the fullest part of the bust. Keep the tape level and comfortably touching the body without compressing breast tissue. The measurement should be repeatable.
in
Measure around the torso directly under the bust, level and snug but not painfully tight, without a padded bra.
in
Measure around the fullest part of the bust, level and not compressing breast tissue.
Want to know how to check whether this size actually fits? See How to Check the Fit in the article below.
A bra size normally combines two pieces of information: a band size and a cup designation. The band is related to the circumference directly beneath the bust. The cup designation is related to the difference between the bust and underbust measurements. That sounds simple, but bra sizing is not perfectly standardized. Different brands can use different band tension, different cup sequences, different grading systems, different shapes, and different fabrics. That is why CalculateThisWay describes the answer as a starting size estimate rather than a guaranteed correct bra size.
How to Measure Your Underbust
Wrap a flexible measuring tape directly under the bust. Keep the tape level, snug, and comfortably close to the body. Do not compress the torso as tightly as possible. The goal is to obtain a repeatable body measurement. ISO 8559-1 provides standardized terminology for anthropometric clothing measurements, including under-bust measurement concepts, though it does not create one universal retail bra-size formula.
Visual: How to Measure Underbust
Simplified, non-sexualized torso outline showing the underbust measurement line, taken directly beneath the bust.
Underbust measurement landmark
Landmark
Instruction
Underbust
Measure around the torso directly under the bust, tape level and snug but not painfully tight
If you measure 31 inches, the calculator's even-band starting reference may be 32. Nearby band sizes may still be worth trying depending on the bra.
How to Measure Your Full Bust
Measure around the fullest part of the bust. The tape should stay level and should not compress breast tissue.
Visual: Full Bust Measurement
Simplified, non-sexualized torso outline showing the full-bust measurement line, taken around the fullest part of the bust.
Full bust measurement landmark
Landmark
Instruction
Full bust
Measure around the fullest part of the bust, tape level, not compressing breast tissue
Suppose underbust = 32 inches and full bust = 37 inches. Difference: 37 − 32 = 5 inches. Under one common U.S. reference sequence, a difference near five inches corresponds approximately to a DD / E cup step. That provides a starting reference such as 32DD. It does not guarantee that every 32DD bra will fit the same way.
How Is Band Size Estimated?
CalculateThisWay uses the snug underbust as the main measurement anchor. For common U.S. and U.K. even-number band sizing, the calculator rounds to the nearest even band number, rounding upward when a measurement falls exactly halfway between two even sizes. This is intended as a fitting starting point. A brand's stretch, materials, and sizing system can make the preferred band different.
Starting Band = nearest even whole inch of snug underbust
Possible Nearby Bands = Starting Band − 2 and Starting Band + 2
The calculator compares the full-bust measurement with the snug underbust measurement, using the unrounded difference, then rounds only at the final reference step.
Cup Difference (unrounded) = Full Bust − Snug Underbust
Cup Difference Reference = Cup Difference rounded to the nearest whole inch
Example
Bust: 37 in. Underbust: 32 in. Difference: 5 in. A common U.S. reference sequence may interpret this as approximately DD / E. A common U.K. sequence may label cup steps differently once double letters enter the sequence. The result always displays which sizing reference was used.
Visual: Band + Cup Anatomy
In a size such as 32DD, the 32 is the band reference and the DD is the cup reference. Neither number means much without the other.
Parts of a bra size label
Part
Meaning
32
Band reference, from the rounded underbust measurement
DD
Cup reference, from the bust-to-underbust difference
Why Do Some Bra Calculators Add 4 Inches?
Older or traditional fitting systems may add four or five inches to the underbust measurement before assigning a band. That can produce dramatically different results. For example, underbust 30 inches: a direct-measurement method may suggest a 30 band, while a traditional plus-four method may suggest 34. Changing the band also changes the cup calculation. Academic research examining traditional bra-size calculation has questioned the validity and reliability of methods based on arbitrary additions to underbust measurements. CalculateThisWay therefore does not use a plus-four rule as its default.
Illustrative Comparison
Measured underbust: 30 in. Direct reference band: 30. Traditional plus-four example: 34. The resulting cup letter changes because the band used in the subtraction changes, even though the two underlying measurements never changed.
Why Do Bra Calculators Give Different Sizes?
Common reasons include different band formulas, different rounding, different cup sequences, different measurement instructions, respiratory state during measurement, and brand-specific sizing. Research published in the British Journal of Sports Medicine has shown that measurement method and respiratory state can affect calculated sizes. That is one reason an online calculator should be treated as a starting point rather than a final answer.
What Are Sister Sizes?
Sister sizes are nearby bra sizes designed to have roughly similar cup-volume references while changing the band. For example, 34C has common sister-size references of 32D and 36B. Moving to a smaller band generally means moving up one cup step. Moving to a larger band generally means moving down one cup step. This can help when the cups seem appropriate but the band feels too loose or too snug.
Visual: Sister Size Ladder
Band up means cup step down. Band down means cup step up. Cup letters alone do not represent a fixed breast volume.
Sister size ladder example
Size
Relationship
32D
Tighter-band sister (smaller band, cup step up)
34C
Starting size
36B
Looser-band sister (larger band, cup step down)
Is a C Cup Always the Same Size?
No. Cup letters are related to the band. A 32C, a 34C, and a 36C do not represent one identical physical cup volume. This is why it is misleading to discuss cup letter alone without the band number.
Visual: Why Cup Letter Alone Is Not Volume
Conceptual illustration only, not an anatomical or photographic representation. The same cup letter corresponds to increasingly larger conceptual volume as the band size increases.
Same cup letter, different conceptual volume by band
Size
Conceptual Relative Volume
32C
Smallest of the three
34C
Middle of the three
36C
Largest of the three
How to Check the Fit
A useful band fit is generally secure, level around the torso, and not painfully restrictive. If the band rides significantly upward in the back, the band may be too loose. If it creates painful pressure or makes normal breathing uncomfortable, a larger band or different bra construction may be worth trying. Fit should be judged on the actual bra, not just the number on the label.
Possible signs that a cup may be too small include overflow, breast tissue being cut into, and underwire resting on breast tissue. Possible signs that a cup may be too large or the wrong shape include significant empty space, wrinkling, and gaping. Shape mismatch can create gaping even when cup volume is not actually too large, which is why calculator numbers cannot evaluate bra shape. Not every bra style has a center gore or underwire, so check these details against the actual garment.
Straps should not be doing all of the support work; most of the lift should come from the band and cup rather than the straps alone.
What If One Breast Is Larger?
Natural asymmetry is common. For fitting, many people size for the larger breast and adjust the smaller side through strap adjustment, removable inserts, or bra style. CalculateThisWay does not attempt to diagnose asymmetry.
US vs UK Bra Cup Sizes
U.S. and U.K. cup lettering diverges after some early cup sizes. For example, double letters can be used differently. A U.K. sequence commonly includes D, DD, E, F, FF, G, GG, while U.S. brands may use sequences such as D, DD, DDD, G, H, depending on the brand. This means a converter should always specify U.S. reference or U.K. reference. Do not assume the same letter means the same thing internationally.
Can I Convert Bra Sizes Internationally?
Band conversions can provide a rough reference. However, cup lettering and brand construction vary. For international shopping, the best approach is to start with measurements, check the specific brand's chart, and review the retailer's fit guidance. A generic international conversion is helpful for orientation but should not override the manufacturer's own chart.
What Is the Most Accurate Way to Find My Bra Size?
An online calculator can narrow the starting point. Actual fitting requires trying the garment. A strong approach is to measure consistently, start with the calculator estimate, try nearby sister sizes, check band behavior, check cup containment, check comfort, and compare different bra shapes if needed. The number on the tag is a tool for finding a fit, not a judgment about the body.
Why Bra Shape Matters
Two bras labeled with the same size can fit differently. Differences can include cup depth, cup width, wire width, cup height, strap placement, stretch, padding, and seam shape. This explains why one 34D may fit while another 34D does not.
Should I Measure With a Bra On?
For the underbust measurement, measure directly around the torso beneath the bust. For the full bust, avoid a heavily padded or compressive bra because it can change the measurement. If support is needed to make measurement easier, use a thin non-padded bra that does not substantially alter the breast shape.
How Often Should I Recheck My Bra Size?
Re-measure whenever fit changes noticeably. Body measurements can change over time. A previous size does not need to remain the same forever. The goal is simply to find a comfortable and supportive current fit.
Bra Size Calculator vs Clothing Size
Bra sizing is its own sizing system. Do not infer dress size, shirt size, body weight, or health status from bra size. Those measurements are not interchangeable.
The Bottom Line
A bra calculator can provide a useful starting point from underbust and full-bust measurement. But there is no universal bra-size formula that guarantees the same fit across every brand. CalculateThisWay gives you a starting size, nearby sister sizes, the measurement math, and practical fit checks. The final decision should come from how the actual bra fits.
Worked Examples (Illustrative Starting Estimates)
The examples below use the exact same calculation engine as the calculator above. Values are computed live in JavaScript rather than typed in as fixed text, so they will always match what the calculator itself would return for these inputs.
Example 1: Direct Inches
Underbust 32.0 in. Full bust 37.0 in.
Example 2: With Sister References
Underbust 34.0 in. Full bust 38.0 in.
Example 3: Sister Sizes of 34C
Current size 34C (U.S. reference).
Example 4: Metric Input
Underbust 81.28 cm. Full bust 93.98 cm.
Try It Yourself
Underbust 30 in. Full bust 34 in. U.S. reference sequence.
What starting band and cup difference reference does this suggest?
Starting band: 30 (underbust is already even). Difference: 34 − 30 = 4 in, which maps to D under the common U.S. reference sequence. Starting size: 30D.
Questions People Ask
How do I calculate my bra size?
Enter your snug underbust and full bust measurement into the calculator above. It rounds your underbust to a starting band reference and compares the difference between your full bust and underbust to a common cup reference sequence.
How do I measure bra size at home?
Use a flexible tape measure. Measure snugly around your torso directly under your bust for the band reference, and around the fullest part of your bust for the full-bust measurement, keeping the tape level.
Where do I measure my band?
Wrap the tape directly under the bust, keeping it level and snug but not painfully tight, without measuring over a padded bra.
Where do I measure my bust?
Measure around the fullest part of the bust with the tape level and not compressing breast tissue.
What is underbust measurement?
The underbust measurement is the circumference of the torso directly beneath the bust, which this calculator uses as the anchor for the starting band reference.
How tight should the measuring tape be?
The tape should be snug enough to stay in place without slipping, but not pulled tight enough to dig into the skin or compress breast tissue.
How do I calculate cup size?
Subtract your snug underbust measurement from your full bust measurement. The unrounded difference is rounded to the nearest whole inch and matched to a cup reference sequence.
What does the difference between bust and band mean?
It is used as a reference for cup volume. A larger difference generally corresponds to a larger cup reference letter in the sequence used by this calculator.
What bra size is a 5-inch difference?
Under the common U.S. reference sequence used here, a difference near five inches corresponds to approximately a DD/E cup step, though this is a starting reference, not a universal rule.
Why do bra calculators give different answers?
Calculators can use different band formulas, rounding rules, cup sequences, and measurement instructions, and even respiratory state during measurement can affect the result.
What is the plus-4 bra sizing method?
It is a traditional method that adds four or five inches to the underbust measurement before assigning a band, which can produce a noticeably different band and cup than a direct measurement approach.
Does CalculateThisWay add four inches?
No. This calculator uses your snug underbust measurement directly as the starting band reference rather than adding four or five inches by default.
What are sister sizes?
Sister sizes are nearby band and cup combinations that reference a similar cup volume. Moving one band size down generally corresponds to moving one cup step up, and moving one band size up generally corresponds to moving one cup step down.
What is the sister size of 34C?
Using the common U.S. reference sequence, 34C has a tighter-band sister reference of 32D and a looser-band sister reference of 36B.
What is the sister size of 36C?
Using the common U.S. reference sequence, 36C has a tighter-band sister reference of 34D and a looser-band sister reference of 38B.
What is the sister size of 32DD?
Using the common U.S. reference sequence, 32DD/E has a tighter-band sister reference of 30DDD/F and a looser-band sister reference of 34D.
Is a 32C the same cup size as 36C?
No. Cup letters are relative to the band number, so a 32C and a 36C do not represent the same physical cup volume.
Does cup letter mean breast size by itself?
No. A cup letter only has meaning alongside its band number, since the same letter represents different volumes at different band sizes.
How should a bra band fit?
A band should generally feel secure and level around the torso without painfully digging in, and it should not ride up noticeably at the back.
Why does my bra band ride up?
A band that rides up in the back is commonly a sign that the band is too loose, though bra shape and strap adjustment can also play a role.
Why do my bra cups gap?
Gapping can mean the cup is larger than needed, or that the cup shape does not match your breast shape, even when the volume is roughly correct.
Why do I spill out of my bra cups?
Overflow or spilling at the top or sides of the cup commonly suggests the cup is too small, though band fit can also affect how the cup sits.
Why does underwire sit on breast tissue?
This commonly happens when the cup is too small or the wrong shape, so the underwire cannot fully encircle the breast tissue and rests on it instead.
Can two bras in the same size fit differently?
Yes. Cup depth, cup width, wire width, fabric stretch, and overall construction can all vary between styles and brands even at the same labeled size.
Does bra shape matter?
Yes. Two bras labeled with the same size can fit differently because of differences in cup shape, depth, width, and material, which is why fit should be judged on the actual garment.
Are U.S. and U.K. bra sizes the same?
Band numbers are generally similar, but cup lettering sequences diverge, especially once double letters such as DD, E, F, and FF appear.
Why does UK sizing use FF and GG?
Common U.K. reference sequences continue cup lettering with doubled letters such as FF and GG after single letters are used up, while some U.S. sequences use different conventions such as DDD.
Can I convert US bra size to UK size?
This calculator provides a reference conversion using common sequences for each system, but manufacturer charts can vary, so treat any conversion as a starting point.
Are international bra conversions exact?
No. International conversions are rough references. Brand-specific charts and an in-person or video fitting remain the most reliable way to confirm fit for a specific product.
Should I measure wearing a bra?
For the underbust measurement, measure directly against the torso. For the full bust, avoid a heavily padded or compressive bra, since it can distort the measurement.
What if my breasts are different sizes?
Natural asymmetry is common. Many people size for the larger side and use strap adjustment, removable inserts, or bra style to accommodate the smaller side.
Is the calculator a professional fitting?
No. This calculator provides a starting size estimate from two measurements. An in-person or video fitting can account for garment-specific details that a formula cannot.
Is bra size a health measurement?
No. Bra size reflects clothing fit, not a measurement of health, body weight, or body fat.
Sources & Further Learning
ISO 8559-1:2017, Size designation of clothes, Part 1: Anthropometric definitions for body measurement:iso.org/standard/61686.html. This standard defines consistent anthropometric body-measurement terminology; it does not define one universal retail bra-size formula.
British Journal of Sports Medicine, How do respiratory state and measurement method affect bra size calculations?:bjsm.bmj.com/content/40/12/970 (also available via PubMed Central). This research found that respiratory state and measurement method affected calculated bra size and concluded that traditional sizing methods have limitations.
Methodology & Limitations
The starting band reference rounds the snug underbust measurement to the nearest even whole inch, rounding upward at an exact halfway point, using the snug underbust directly rather than a plus-four or plus-five addition.
The cup difference reference subtracts the snug underbust from the full bust using full, unrounded precision, then rounds only at the final reference step.
Cup letters are shown as a labeled U.S. or U.K. reference sequence. Values outside the supported range are shown as falling beyond the simplified reference rather than inventing a letter.
Sister sizes move the band two inches in one direction and the cup reference one step in the opposite direction; this is a reference relationship, not a claim of identical physical volume.
This calculator does not diagnose asymmetry, body fat, body weight, or any health condition, and does not claim ISO 8559-1 or any single study created its exact sizing formula.
All results are displayed as a starting estimate, not a guarantee, because bra sizing is not standardized across brands, countries, styles, or construction methods.
This calculator provides a starting bra-size estimate from two tape measurements using common U.S. and U.K. reference sequences. It does not guarantee an exact bra size and cannot account for every brand's band tension, cup construction, materials, or individual body shape. Bra sizing is not standardized across brands, countries, or styles. Use the result as a fitting starting point, try on the actual garment, and check band and cup fit directly using the guidance above. This page does not provide medical advice and does not assess body fat, body weight, or health status.