What is a bra size?
A bra size generally combines a band number and a cup letter. The band refers to the approximate size around the torso, while the cup describes a relative volume relationship between the bust and the band.
For example, a size such as 32DD contains a band number of 32 and a cup designation of DD. The cup letter is not an independent fixed volume: it changes with the band size.
Bra sizing is not completely standardised across manufacturers or countries. A calculator can suggest a starting size, but the final fit depends on the garment's construction, brand and individual comfort.
A snug measurement around the rib cage below the bust.
A measurement around the fullest part of the bust.
Check band support, cup coverage, straps and overall comfort.
How to measure bra size correctly
Use a soft, flexible measuring tape and measure in centimetres. A mirror can help you keep the tape level around your body.
Step 1: Measure your underbust
- Wrap the tape around your torso directly beneath your bust.
- Keep the tape horizontal and comfortably snug.
- Breathe normally and avoid pulling the tape excessively tight.
- Record the measurement in centimetres.
Step 2: Measure your full bust
- Wrap the tape around the fullest part of your bust.
- Keep the tape level without pressing into the tissue.
- Stand naturally and record the measurement in centimetres.
Step 3: Enter both measurements
Enter the underbust and bust values into the calculator above. Select Calculate to see the estimated band size and illustrative UK cup.
Bra size calculator formula
This calculator follows the illustrative method described in the original tool: convert the underbust measurement into inches, round it to an even-numbered band size, then compare the bust measurement with that rounded band.
Underbust (inches) = Underbust (cm) ÷ 2.54
Approximate UK band = Underbust in inches rounded to the nearest even band number.
Cup difference (inches) = Bust (cm) ÷ 2.54 − rounded band.
The difference is rounded to an illustrative cup step and mapped to a UK-style cup label.
This is a simplified sizing convention, not a universal UK fitting standard. Some brands use different band calculations, cup progressions or fit recommendations.
Worked example: 80 cm underbust and 95 cm bust
Example 1: Original calculator values
- Underbust: 80 cm
- Bust: 95 cm
- Underbust in inches: approximately 31.50
- Rounded band: 32
- Bust in inches: approximately 37.40
- Difference: approximately 5.40 inches
Under the calculator's illustrative cup mapping, these measurements produce an estimated 32DD.
Example 2: 75 cm underbust and 90 cm bust
Underbust: 75 ÷ 2.54 ≈ 29.53 inches.
Rounded band: 30. Bust: 90 ÷ 2.54 ≈ 35.43 inches.
Difference: approximately 5.43 inches. Under the same illustrative mapping, the starting estimate is 30DD.
Example 3: 85 cm underbust and 100 cm bust
Underbust: 85 ÷ 2.54 ≈ 33.46 inches.
Rounded band: 34. Bust: 100 ÷ 2.54 ≈ 39.37 inches.
Difference: approximately 5.37 inches. Under the same illustrative mapping, the starting estimate is 34DD.
Illustrative UK cup size chart
The following table explains a common illustrative progression of cup labels using the difference between full bust and band measurements. It is not a universal brand size chart.
| Rounded difference | Illustrative UK cup |
|---|---|
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD |
| 6 inches | E |
| 7 inches | F |
| 8 inches | FF |
| 9 inches | G |
| 10 inches | GG |
Cup labels can differ between UK, US and European systems. In particular, some US brands use a different sequence after D, and European labels are not always directly interchangeable.
What are bra sister sizes?
Sister sizes are neighbouring combinations of band and cup labels that aim to maintain a broadly similar cup volume.
For example, if a 32DD band feels too tight, a 34D may be worth trying. If the band feels too loose, a 30E may be worth comparing.
These are fitting alternatives, not guaranteed equivalents. Shape, support and comfort may still differ significantly.
How to check whether a bra fits
A size estimate is most useful when followed by a practical fit check. Consider the following:
- Band: It should feel secure and generally remain level around your torso without causing discomfort.
- Cups: Look for a smooth fit without noticeable spilling, cutting in or substantial empty space.
- Straps: They should stay in place without carrying all the support or digging into your shoulders.
- Centre front: Depending on the bra style, check whether the centre sits as intended by the manufacturer.
- Movement: Raise your arms and move normally to check whether the garment stays comfortable and supportive.
UK, US and EU bra sizing differences
UK and US sizing commonly use band numbers such as 30, 32, 34 and 36, but cup-letter sequences may differ. EU sizing generally uses a different band-number convention.
Because brands do not always follow identical sizing systems, avoid assuming that the same cup letter represents the same size everywhere.
For an international purchase, use the specific manufacturer's size chart and return policy.
Common bra measurement mistakes
- Measuring the underbust with a loose or slanted tape.
- Compressing the bust while taking the full-bust measurement.
- Mixing centimetres and inches in the calculator inputs.
- Assuming one size will fit identically across every brand.
- Treating a calculator estimate as a final fitting result.
- Ignoring differences in bra style, fabric and construction.
Limitations of this calculator
The calculator estimates a band size by rounding the underbust measurement to the nearest even inch and uses a simplified cup progression.
Actual bra sizing is influenced by garment design, manufacturer conventions, breast shape, material stretch and personal fit preferences.
Frequently asked questions
How do I calculate bra size from centimetres?
Measure your underbust and full bust in centimetres. This calculator converts the measurements to inches, estimates an even-numbered band and assigns an illustrative UK cup based on the bust-to-band difference.
What measurements do I need?
You need an underbust measurement taken beneath the bust and a full-bust measurement taken around its fullest part. Enter both measurements in centimetres.
Is this a UK bra size calculator?
It provides an illustrative UK-style band and cup estimate. It does not represent every UK retailer's fitting method or size chart.
What does 32DD mean?
The number 32 indicates the band size, while DD is a cup designation relative to that band. It does not describe a fixed cup volume across all band sizes.
Are UK and US cup sizes identical?
Not always. Some UK and US cup-letter progressions differ, especially beyond D. Check the brand's country-specific size chart.
What if my calculated size feels uncomfortable?
Try nearby band and cup combinations, review the garment's fit and compare the brand's sizing guidance. Comfort and support matter more than matching a calculator label exactly.
Can my bra size change over time?
Yes. Measurements and fit preferences may change over time. Re-measuring can be helpful when your existing bras no longer fit comfortably.
Does this tool upload my measurements?
The calculator is designed to process the numerical measurements in your browser. It does not require uploading a photograph. Website analytics and advertising may operate separately.
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Maintained by CalculatorsOnline Editorial Team. Original implementation: . The estimate is informational and does not claim professional fitting certification. Read our editorial policy .