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Bra Size Calculator — Free Online Fitting Tool

Calculate your bra size using band and bust measurements. Get your size in both US and UK systems, along with fitting guidance to help you find comfortable, well-fitting bras.

Sizing System
cm
cm

Your Bra Size

30DD

US Sizing

Band Size30
Cup SizeDD

Both Systems

US Size

30DD

UK Size

30DD

Summary: Based on your measurements, your estimated bra size is 30DD (US) or 30DD (UK). Bra sizing varies across brands and styles. Use this as a starting point and try on several sizes for the best fit.

How to Use the Bra Size Calculator

  1. Select your unit system: Choose Metric (centimeters) or Imperial (inches). If your tape measure shows centimeters, select Metric. If it shows inches, select Imperial.
  2. Choose your sizing system: Select US or UK sizing. US sizing is standard in North America. UK sizing is used by many international brands. The calculator shows both regardless of your selection.
  3. Measure and enter your band (underbust): Standing upright, wrap the tape measure snugly around your ribcage directly under your bust. Keep the tape level and firm but not tight. Enter this measurement in the band field.
  4. Measure and enter your bust: Wrap the tape measure around the fullest part of your bust. The tape should be level and gently touching the skin without compressing. Stand straight with arms at your sides. Enter this measurement in the bust field.
  5. Review your results: The calculator displays your recommended bra size in both US and UK systems. Both band size and cup letter are shown. Use this as a starting point and try adjacent sizes for the best fit.

Tip: For the most accurate measurement, have someone else help you measure, or measure in front of a mirror to ensure the tape is level. Wear an unlined bra or no bra during measurement.

Bra Sizing Formula and Method

Band Size = Round underbust (inches) to nearest even number
Cup Size = Bust (inches) - Band Size

Cup Size Conversion

  • 0" difference: AA cup
  • 1" difference: A cup
  • 2" difference: B cup
  • 3" difference: C cup
  • 4" difference: D cup
  • 5" difference: DD (US) / DD (UK)
  • 6" difference: DDD (US) / E (UK)
  • 7" difference: G (US) / F (UK)

Step-by-Step Calculation Example

For someone with underbust measurement of 76 cm and bust measurement of 91 cm:

  1. Convert band to inches: 76 / 2.54 = 29.9 inches
  2. Round to nearest even number: 30
  3. Convert bust to inches: 91 / 2.54 = 35.8 inches
  4. Round bust: 36
  5. Difference: 36 - 30 = 6 inches
  6. Cup size: 6" difference = DDD (US) / E (UK)
  7. Final size: 30DDD (US) / 30E (UK)

Practical Examples

Example 1: Emma's First Fitting

Emma has been wearing a 34B but suspects the fit is wrong. She measures her underbust at 72 cm and bust at 87 cm:

  • Band: 72 cm / 2.54 = 28.3 inches → rounds to 28
  • Bust: 87 cm / 2.54 = 34.3 inches → rounds to 34
  • Difference: 34 - 28 = 6 inches
  • Recommended size: 28DDD (US) / 28E (UK)

Emma discovers she has been wearing the wrong size. Her actual band is 28, not 34, and her cup is DDD, not B. This is common; studies suggest that up to 80% of people wear the wrong bra size. Emma's sister sizes (30DD, 32D) may be easier to find in stores.

Example 2: Li Shopping Online

Li wants to order from a UK brand and needs her UK size. Her measurements are band 33.5 inches and bust 38 inches:

  • Band: 33.5 rounds to 34
  • Difference: 38 - 34 = 4 inches
  • US size: 34D
  • UK size: 34D

For a D cup, US and UK sizing are identical. Li can confidently order a 34D from the UK brand. She also notes her sister sizes (32DD, 36C) in case the brand runs differently than expected.

Example 3: Nicole Post-Pregnancy

Nicole is 6 months postpartum and her body has changed. Her new measurements are underbust 82 cm and bust 100 cm:

  • Band: 82 cm / 2.54 = 32.3 inches → rounds to 32
  • Bust: 100 cm / 2.54 = 39.4 inches → rounds to 39
  • Difference: 39 - 32 = 7 inches
  • US size: 32G / UK size: 32F

Nicole's size has changed significantly from her pre-pregnancy 34C. She plans to remeasure every few months as her body continues to adjust postpartum, especially once breastfeeding ends.

Bra Size Conversion Reference Table

Difference (in) US Cup UK Cup Example (Band 34)
0AAAA34AA
1AA34A
2BB34B
3CC34C
4DD34D
5DDDD34DD
6DDDE34DDD / 34E
7GF34G / 34F

Tips and Complete Guide to Bra Fitting

Signs of a Good Fit

A well-fitting bra should have the band sitting level and snug on the loosest hook (new bras stretch over time, and you tighten by moving to inner hooks). You should be able to slide two fingers under the band comfortably. The center gore (the piece between the cups) should lie flat against your sternum. Cups should fully contain breast tissue without gaping at the top or sides, and without tissue spilling over the edges. Straps should sit comfortably without digging in, and should not do most of the supporting work. The band provides 80% of support, while straps provide only 20%. If straps slip frequently, the band may be too loose.

Understanding Sister Sizes

Sister sizes are your most practical tool for finding a comfortable fit across different brands. Your sister sizes are the sizes one band up with one cup down, and one band down with one cup up. For example, if your calculated size is 34C, your sister sizes are 32D (tighter band, same cup volume) and 36B (looser band, same cup volume). When shopping a new brand, bring your calculated size plus both sister sizes to the fitting room. The band should be the snuggest comfortable fit on the loosest hook, as it will stretch over the garment's lifetime.

When to Remeasure

Several life events and changes warrant remeasurement: weight gain or loss of 5 pounds or more, pregnancy and postpartum changes, starting or stopping breastfeeding, hormonal changes (menopause, starting or changing birth control), significant changes in exercise routine (especially if building or losing chest muscle), and if your current bras have become uncomfortable. Bras themselves also change over time due to elastic degradation, so a bra that fit perfectly 6 months ago may no longer provide proper support.

Common Mistakes to Avoid

  • Adding inches to your band measurement: The outdated practice of adding 4-5 inches to the underbust measurement was necessary when bras had non-stretch bands. Modern bras use elastic fabrics and should be sized based on the actual underbust measurement rounded to the nearest even number.
  • Wearing the same size for years without remeasuring: Body changes are normal and gradual, and you may not notice when your bra no longer fits properly. Annual measurement is recommended.
  • Choosing a larger band for comfort: A too-large band rides up in the back and shifts during movement, reducing support. If the band feels tight, try a sister size (one band up, one cup down) rather than simply going up a band size.
  • Ignoring cup fit in favor of band comfort: Both band and cup must fit correctly. Cups that are too small push tissue out, creating the appearance of back fat, while cups too large gap and do not provide proper shaping.
  • Buying based on letter alone: A 32D and a 38D are very different cup volumes. Cup volume is relative to band size, not absolute. Always consider band and cup together.

Frequently Asked Questions

To measure your band size, stand upright and wrap a soft measuring tape snugly around your ribcage, directly beneath your bust. The tape should be level all the way around and firm against your skin without being uncomfortably tight. You should be able to slide one finger under the tape. Breathe normally and take the measurement at the end of an exhalation. Round the measurement to the nearest even number for US sizing, and to the nearest whole number for UK sizing. If you get an odd number, round up to the next even number. For the most accurate results, wear an unlined bra or no bra while measuring.

To measure your bust, wrap the tape measure around the fullest part of your bust while wearing a well-fitting, unlined bra or no bra. The tape should be level around your body and gently touching your skin without compressing breast tissue. Stand straight with your arms at your sides and breathe normally. Do not pull the tape tight or let it hang loose. The difference between your bust measurement and band measurement determines your cup size. Each inch of difference corresponds to one cup size. For example, a 1-inch difference is an A cup, 2 inches is B, 3 inches is C, and so on.

US and UK bra sizing share band sizes (which are measured in inches in both systems) but differ in cup size naming starting at DD. In the US system, the progression after D is DD, DDD (or E), G, H, I, J. In the UK system, it is DD, E, F, FF, G, GG, H, and so on. This means a US DDD is equivalent to a UK E, and a US G is roughly equivalent to a UK F. The UK system uses double letters (FF, GG, HH) to indicate half-step increments, which can provide more precise sizing for larger cup sizes. European sizing (EU) uses different band numbers based on centimeters and a separate cup letter system. Always check which sizing system a brand uses before ordering.

Bra sizing varies between brands because there is no universal manufacturing standard. Factors that cause variation include different measurement methods used by each brand, variations in fabric stretch and construction, differences in cup shape (full cup, demi cup, plunge), and whether the brand designs for specific body types. Some brands run true to size while others are known to run small or large in the band or cup. Additionally, the same size from the same brand may fit differently across different styles. This is why trying on bras is recommended, and why knowing your sister sizes (adjacent sizes that share the same cup volume) is helpful for finding the best fit across brands.

Sister sizes are bra sizes that share the same cup volume but have different band and cup combinations. For every band size you go up, you go down one cup size to maintain the same cup volume, and vice versa. For example, the sister sizes of 34C are 32D (smaller band, larger cup letter) and 36B (larger band, smaller cup letter). This concept is useful when a bra fits well in the cup but the band is too tight or too loose. If your 34C band feels tight, try a 36B. If the band feels loose, try a 32D. The cup volume remains approximately the same in all three sizes. Sister sizes are a practical tool for finding comfortable bras across different brands that may size their bands slightly differently.

Bra size should be reassessed at least once a year, and more frequently if you experience weight changes of 5 pounds or more, hormonal changes (pregnancy, breastfeeding, menopause, starting or stopping birth control), significant changes in physical activity level, or if your current bras feel uncomfortable. Body composition changes naturally over time, and wearing the wrong size can cause discomfort, poor posture, shoulder pain, and inadequate support. Signs that you may need a different size include the band riding up in the back, straps digging into shoulders, cups gaping or overflowing, the center gore not lying flat against your sternum, or red marks and skin irritation after wearing.

This calculator provides a reliable starting point based on your measurements, but it cannot guarantee a perfect fit because bra sizing depends on more than just band and bust circumference. Factors that affect fit include breast shape (full on top vs. full on bottom, wide-set vs. close-set), tissue density, root width, and personal comfort preferences. Different bra styles (balconette, full coverage, sports, strapless) fit the same measurements differently. Manufacturing variations between brands add another layer of complexity. Use your calculated size as a starting point, then try on several sizes around that range to find your best fit. Many fitters recommend trying your calculated size plus one cup up, one cup down, and the sister sizes.

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Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for medical guidance.

Last updated: February 23, 2026

Sources