Last updated: September 12, 2025
Use our Bra Size Calculator to find your band and cup size from two measurements: snug underbust and fullest bust. Enter your numbers in cm or inches to get sizes across UK/US, EU/FR/IT, AU/NZ, JP, and IN systems—plus sister sizes and expert fit tips to fine-tune comfort and support.
To measure your bra size at home, wrap a tape measure snugly around your ribcage (underbust) and lightly around your fullest bust. The calculator derives the band size and maps the bust–underbust difference to a cup letter for your region. Always try sister sizes and follow a comprehensive fit-check to adjust for brand and style differences.
Measure Bust & Band Precisely – Your Bra Size Calculator
How the Bra Size Calculator Works: Modern vs. Legacy Sizing
The core function of any bra size calculator is to translate two key body measurements—underbust and bust—into a standard band number and cup letter. However, the exact formula used is critical. Our tool is based on the modern fitting method, which has largely replaced the confusing and inaccurate legacy (+4) method.
The Modern Fitting Baseline (No +4)
The modern approach, favored by professional fitters, acknowledges that 80% of breast support comes from the band, and modern bra materials offer adequate stretch and recovery.
| Measurement | How It’s Used | Band Size Calculator Logic |
| Underbust (Snug) | Measured snugly, directly beneath the breast tissue. | Band Size: The number is derived directly from your underbust measurement, usually rounded to the nearest even whole number (e.g., a 31.5-inch underbust rounds to a 32 band). This ensures the band is firm enough to anchor the bra and provide support. |
| Bust (Fullest) | Measured around the fullest point of the bust. | Cup Size: Calculated by finding the difference between your fullest bust and your underbust. This difference, measured in inches or centimeters, is mapped to a regional cup letter progression (e.g., 1-inch difference = A cup, 2-inch difference = B cup, etc.). |
Understanding the Legacy (+4) Method
For decades, many brands—most notably in the US market—instructed users to add four or five inches to their underbust measurement to determine their band size.
Example (Legacy): Underbust 30″ Band 34 (30 + 4).
Example (Modern): Underbust 30″ Band 30.
The legacy method often led to bands that were too loose and cups that were too small, resulting in the wearer relying on shoulder straps for support (leading to pain). While we recommend the modern method, if you have only ever worn a size derived from the rule (e.g., a 36C), you will find the modern equivalent is a smaller band and larger cup (its sister size, such as a 32E/DD).
Measuring Bra Size at Home: A Step-by-Step Guide
The accuracy of your calculated size depends entirely on the precision of your measurements. Use a soft, non-stretchy tape measure and follow these instructions carefully.
Tools and Preparation for Measurement
Tools: A flexible, tailor’s measuring tape. If you don’t have one, use a piece of non-elastic string, mark the measurements, and then lay it next to a rigid ruler.
Clothing: Measure while wearing a lightweight, non-padded bra, or ideally, braless. Padded bras will distort the cup volume measurement.
Posture: Stand up straight, relaxed, with your arms at your sides.
Step 1: Underbust (Band) Measurement
This measurement is the most critical as it determines your band size calculator output.
Wrap the measuring tape directly underneath your breast tissue and around your ribcage.
Ensure the tape is level all the way around, running parallel to the floor.
Breathe out: Exhale fully and pull the tape measure snugly—tight enough that you feel the resistance of your ribcage, but not so tight that it causes pain or leaves a major indentation. You want a firm, secure measurement.
Record this number in your chosen unit (inches or cm).
Step 2: Bust (Fullest) Measurement
This determines your cup volume relative to the band.
Wrap the tape measure around the fullest part of your bust, typically across the nipple line.
The tape should be lightly taut but not compress the breast tissue. It should just skim the skin.
For Projected Shapes: If you have full, projected breasts (they stick out more than they spread across your chest), you may want to take a Leaning Bust measurement. Bend over 90 degrees and let your breasts hang naturally, then measure. Use the larger of the two bust measurements (Standing or Leaning) for the calculator.
Record this number.
Handling Asymmetry
It’s very common for one breast to be larger than the other. Always measure and size based on the larger side. Sizing for the smaller side will lead to spillage on the larger side. You can use a removable breast pad in the smaller cup to even out the appearance.
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International Bra Size Conversions and Cup Progression
One of the biggest obstacles in bra shopping is the lack of standardized sizing. Your UK bra size will differ from your US bra size, and both will differ from the numeric bands of the EU bra size system.
Key Regional Sizing Systems
| Region | Band Sizing | Cup Progression | Letter Sequence Divergence |
| UK (United Kingdom) | Inches (28, 30, 32, 34…) | 1 inch | Single and Double letters: D, DD, E, F, FF, G, GG… |
| US (United States) | Inches (28, 30, 32, 34…) | 1 inch | Often uses Triple/Single letters: D, DD or E, DDD or F, G, H, I, J… |
| EU/FR/ES (European Union/Spain) | Centimeters (65, 70, 75, 80…) | 2 cm | No double letters: D, E, F, G, H, I… |
| AU/NZ (Australia/New Zealand) | Same band numbers as UK/US. | 1 inch | Follows US or simplified UK progression: D, DD, E, F, G… |
| JP (Japan) | Centimeters (65, 70, 75, 80…) | 2.5 cm | Uses letters with a metric band: D, E, F, G… |
| IN (India) | Often follows US or UK standards but can be inconsistent. | Varies | Consult brand-specific charts. |
Conversion Caveat: The biggest confusion is in the large cup sizes. For example, a UK DD cup is equivalent to a US DD or E cup. A UK FF cup is equivalent to a US H cup in many brands. Always reference the brand’s specific chart when comparing.
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Sister Size Calculator: Finding Your Alternative Fits
Once you have your primary calculated size, you can explore sister sizes. This is crucial for fit tuning because band and cup size are not independent; they are proportional.
Sister Sizes Explained
A sister size is a bra size that maintains the exact same cup volume while changing the band size. This works because cup letters are not absolute volumes; they represent the proportion of breast tissue relative to the ribcage.
A 36C has the same cup volume as a 34D and a 38B.
| Your Calculated Size (UK) | Band Down, Cup Up (Snug Fit) | Band Up, Cup Down (Roomy Fit) |
| 34D | 32DD | 36C |
| 36FF | 34G | 38F |
| 32H | 30HH | 34GG |
When to Try a Sister Size
The band feels too loose, but the cup volume is perfect: Go down a band and up a cup (e.g., 36D 34DD). This achieves a firmer band for better support without losing breast volume.
The band feels too tight, but the cup volume is perfect: Go up a band and down a cup (e.g., 32DD 34D). This gives you more breathing room in the band while maintaining the necessary cup volume.
Brand Variations: Some brands run tight or loose. A sister size can help you adapt to these differences without sacrificing cup volume.
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Bra Fit-Check & Troubleshooting: The Key to Comfort
A perfect fit is only confirmed by how the bra sits on your body. Follow this comprehensive fit-check before keeping any bra.
| Fit Issue | Symptom | Cause & Solution |
| Gore Tacking | The center piece of fabric/wire between the cups (the gore) lifts away from your chest. | Cause: The cup is too small or too shallow for your breast root. Solution: Go up a cup size, or try a bra designed for projected shapes (e.g., Plunge or Full Cup). |
| Cup Gaping | Wrinkling or loose fabric at the top of the cups, especially when you move your arms. | Cause: The cup is too large or your shape is shallow, but the cup is too projected. Solution: Go down a cup size, or try a bra designed for shallow shapes (e.g., Balconette or Half Cup). |
| Quad-Boob | Breast tissue spills over the top, sides, or bottom of the cups. | Cause: The cup is definitely too small. Solution: Go up one or two cup sizes immediately. |
| Band Riding Up | The back band sits higher than the front, curving up your back. | Cause: The band is too loose. Solution: Go down a band size (and up a cup—a sister size). The band should be level and horizontal. |
| Wire Width | The underwire digs into breast tissue or rides up into the armpit. | Cause: The wires are too narrow for your breast root. Solution: You need a wider wire bra; try brands known for wide, shallow cups. |
Understanding Breast Shape: Root Width and Projection
Projected: Your breast tissue extends outward from the chest wall significantly. You need deep cups and a narrow or average wire.
Shallow: Your breast tissue is spread widely across your chest wall with less depth. You need a wider wire and a shallower cup volume.
A bra that is the “correct size” but the “wrong shape” will never fit well.
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Special Cases: Sports, Nursing, and Plus Size Bra Sizes
Sizing changes dramatically depending on the stage of life or activity.
Sports Bra Size
Sizing a sports bra requires balancing compression and comfort. For high-impact activities (running, HIIT), support is everything.
High-Impact Encapsulation: If the bra uses band/cup sizing, stick to your calculated size but ensure the band is firm on the loosest hook. This allows you to tighten it as the material stretches over time. The straps should feel secure but not painful.
Compression Bras: If the bra is sized S/M/L, measure both your underbust and fullest bust and use the brand’s specific size chart. You want it snug enough to restrict movement but loose enough for deep breathing.
Nursing Bra Sizing During Pregnancy and Postpartum
During pregnancy, your ribcage can expand, and after delivery, your cup size will fluctuate significantly due to milk production.
Pregnancy Sizing: Measure your bra size around the third trimester. Add 1 to 2 inches to your measured underbust for your band size (e.g., if you measure 34, try a 36 band).
Postpartum Sizing: Measure your bust at the fullest point when your breasts are heaviest (usually before a feeding). The cup should accommodate both your breast tissue and a nursing pad without compression, which can lead to clogged ducts.
Plus Size Bra Sizes (DD/E/F/G/H/J/K Cups)
Plus-size fitting often involves larger cup letters (DD, E, F, G, H, I, J, K) and band sizes (40+).
Support Priority: Due to the weight of the breast tissue, a firm band is even more critical. Do not go up a band size just for comfort; use an extender initially if needed.
Wide Wires: Look for bras with wider wires and deep side panels for better containment and shaping.
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Step-by-Step Usage of the Calculator
Here is your quick guide to using our tool and getting your perfect fit:
Measure: Use the How to measure bra size at home guide above to get your Snug Underbust and Fullest Bust measurements in inches or centimeters.
Input: Enter the two numbers into the calculator fields.
Choose Region: Select your preferred region (e.g., United Kingdom or United States).
Calculate: Click “Calculate” to view your primary size.
Check Conversions: Review the international conversion table in your results to see your equivalent size in EU, AU, and JP systems.
Try Sister Sizes: The results will display snug and roomy sister sizes. Consider these if you know you prefer a tighter or looser band.
Shop & Fit-Check: Purchase a bra in your recommended size and perform the comprehensive Fit-Check before removing the tags.
Frequently Asked Questions (FAQs)
The average bra size has trended upward, often cited as being around 34D to 34DD. This increase is largely attributed to better fitting techniques (adopting the modern, no-+4 method) and more inclusive sizing offered by brands, leading people out of ill-fitting, too-loose bands and too-small cups.
A bra size is always written as a number followed by a letter (e.g., 34D). The number is the band size (your underbust measurement), and the letter is the cup size, representing the difference between the bust and underbust measurements.
UK and US bra sizes use the same band numbers, but cup letters diverge after D. A UK DD is often a US E. A UK F is often a US G. Because US sizing is inconsistent, always check the brand's specific chart, or use the UK size as a more standardized baseline.
The smallest standard band size calculator result is usually a 28 band. The smallest cup size available is typically an AA or AAA, which corresponds to a bust-underbust difference of less than one inch.
A C cup size signifies that the difference between your fullest bust measurement and your underbust measurement is approximately 3 inches (or about 7.5 cm in the metric system). Remember that cup volume is relative: a 38C cup has a significantly larger volume than a 32C cup.
For high-impact sports, use your measured size but look for encapsulation (individual cups) rather than compression (uniboob). The band should be extremely firm, and the material should offer minimal bounce, even when jumping.
Yes. Bra sizes are not standardized across brands. Materials, styles, and quality vary. Your calculated size is a starting point, but you should always measure, calculate, and then reference the brand’s size chart or read reviews before purchasing.
There is no "biggest bra size" limit, as extended sizing brands now offer bands up to 50 or 60 and cup sizes up to a K, L, M cup or higher (based on the UK sizing system's consistent letter progression).
A properly fitting bra should have a band that is snug and level across your back, providing about 80% of the support. The center wire (gore) should tack flat against your sternum. The cups must fully contain all breast tissue without gaping or causing spillage (quad-boob) at the top or sides.
A, G, H, and J are standard cup sizes in various systems. A is a small cup (1-inch difference). G and H are large cup sizes found in US and UK sizing. J is typically a very large cup size.
These letters represent cup size: B is a 2-inch difference, C is 3 inches, and D is 4 inches. G, I, and J are larger cup sizes (e.g., G is often 7 inches) used in US or UK systems. The volume of the breast tissue they hold is relative to the band size (e.g., a 32D is smaller than a 38D).
The European (EU) system and the American (US) system use the same letters up to D. After D, the US system often uses triple letters like DDD/F, while the EU system uses the next single letter (E, F, G...). The EU band size is numeric (e.g., 70, 75, 80) and measured in centimeters, unlike the US/UK band size (e.g., 32, 34, 36) in inches.
32C and 34C both represent a C cup, meaning a 3-inch difference between the bust and underbust. The difference is the band size: 32C is for a snug underbust of approximately 32 inches, and 34C is for a snug underbust of approximately 34 inches. The 34C has a larger cup volume than the 32C.
AA is one of the smallest cup sizes, signifying less than a 1-inch difference between the bust and underbust. DDD is an expanded American cup size used for larger busts, typically equivalent to the UK F cup size, or occasionally the US F. It means a 6-inch difference between the bust and underbust.
Privacy and Disclaimer
Privacy Notice: This Bra Size Calculator runs entirely in-browser. Your measurements are used instantaneously for calculation and are never stored, transmitted, or collected by our servers. We do not use cookies or local storage unless you explicitly choose to save your measurements for future convenience.
Disclaimer: Bra size calculations are estimates. Fit varies widely by brand, style, material, stretch, and personal preference. For post-surgery, medical, or clinical sizing needs, please consult a healthcare professional or a certified specialist fitter.
