About this Bra Size Calculator
Bra sizing is based on two related measurements rather than on one universal body number: the band measurement around the torso and the difference between that measurement and the fullest part of the bust.
The Bra Size Calculator converts those measurements into a starting size using the sizing convention built into the tool. It is most useful as a fitting reference, not as a guarantee that every bra labeled with that size will fit the same way.
Brand patterns, fabric stretch, cup shape, wire width, padding, and regional sizing systems can all change how a size feels. Taking measurements over light, unpadded clothing—or according to the method specified by the calculator—helps reduce distortion.
If the calculated size feels wrong, the fit of the actual garment is more important than the label.
What Is a Bra Size Calculator?
A Bra Size Calculator estimates a band size and cup size from torso and bust measurements. The band is derived from the circumference under or around the bust according to the calculator’s sizing system.
Cup size is based on the difference between the fuller bust measurement and the band-related measurement, with increasing differences mapped to cup letters. Because cup volume is relative to band size, the same cup letter does not represent the same volume across every band.
This is why sister sizes can have similar cup volume while using different band and cup labels.
Reading Your Results
The figure shown is your Bra Size result, calculated from the values above. The method does not change between runs, so you can rely on it as a fixed reference point.
Try changing a single figure and comparing the two results: the size of the shift is often more informative than the number itself.
How to use this calculator
The calculator subtracts the band measurement from the bust measurement to determine the cup size and combines it with the band size to display the estimated bra size.
Where
Example 1
Example 2
Metric Example
Using the metric cup size chart, a 10 cm difference corresponds approximately to an A cup.
Bra Size ≈ 100A
Band Size from Underbust Measurement
| Underbust (cm) | Underbust (in) | UK/US Band Size | EU Band Size |
|---|---|---|---|
| 63 – 67 | 25 – 26 | 30 | 65 |
| 68 – 72 | 27 – 28 | 32 | 70 |
| 73 – 77 | 29 – 30 | 34 | 75 |
| 78 – 82 | 31 – 32 | 36 | 80 |
| 83 – 87 | 33 – 34 | 38 | 85 |
| 88 – 92 | 35 – 36 | 40 | 90 |
Measure snugly around the ribcage directly beneath the bust, keeping the tape level.
Cup Size from Bust Difference
| Difference (Bust − Underbust) | UK Cup | US Cup |
|---|---|---|
| Under 1 in (2.5 cm) | AA | AA |
| 1 in (2.5 cm) | A | A |
| 2 in (5 cm) | B | B |
| 3 in (7.5 cm) | C | C |
| 4 in (10 cm) | D | D |
| 5 in (12.5 cm) | DD | DD/E |
| 6 in (15 cm) | E | DDD/F |
| 7 in (17.5 cm) | F | G |
Cup letters are relative to the band, so the same cup differs in volume across band sizes.
Sister Size Reference
| Current Size | Smaller Band Equivalent | Larger Band Equivalent |
|---|---|---|
| 32C | 30D | 34B |
| 34B | 32C | 36A |
| 34D | 32DD | 36C |
| 36C | 34D | 38B |
| 38D | 36DD | 40C |
If the band is loose, go down a band and up a cup to keep the same cup volume.
Factors Affecting Your Bra Size Result
Measurement location and tape tension are the biggest controllable factors. The underbust measurement should be level and snug without being painfully tight, while the full-bust measurement should capture the fullest point without compressing breast tissue. Thick clothing or padded bras can increase the measured circumference.
The sizing system also matters because US, UK, EU, and other conventions use different band numbers and cup-letter progressions. Rounding rules can move a measurement to the next band or cup step.
Breast shape, projection, root width, asymmetry, and tissue distribution are not fully represented by circumference alone, so two people with the same calculated size may prefer different styles. Brand construction and fabric stretch can make the practical fit vary even within the same labeled size.
Benefits of Using the Bra Size Calculator
- Turn two tape measurements into a practical starting size without manually working through band rounding and cup-difference charts.
- Understand the relationship between band and cup rather than treating the cup letter as an absolute volume independent of band size.
- Use the result to narrow the first sizes to try, which can reduce trial-and-error when shopping across many nearby size options.
- Check sister sizes when the cup volume feels close but the band is too tight or too loose.
- Separate measurement-based sizing from actual garment fit, helping you use the calculated number as guidance rather than as a rigid rule.
Frequently asked questions
Why can the same bra size fit differently between brands?
Sizing labels are not perfectly standardized. Pattern shape, fabric stretch, wire width, cup depth, padding, and grading methods vary by brand and style, so the measured size should be treated as a starting point for fitting rather than a guaranteed match.
What are sister sizes?
Sister sizes use a different band and cup letter while keeping a similar cup volume. When moving to a larger band, the cup letter usually moves down; when moving to a smaller band, the cup letter usually moves up. Fit still depends on the specific bra.
Should I measure while wearing a bra?
Follow the method specified by the calculator. In general, heavy padding can distort the bust measurement. A thin, unpadded bra or direct measurement may be more suitable depending on the sizing method used.
Why is my calculated cup letter larger than I expected?
Cup letters are based on the difference between measurements and are relative to band size. Popular assumptions about what a particular letter ‘looks like’ are often inaccurate, so a larger letter does not automatically mean an unusually large overall bust.
Which matters more, the calculator or how the bra fits?
Fit matters more. A supportive band, contained breast tissue, comfortable wires, smooth cups, and straps that do not carry the entire load are more useful indicators than the label alone. Use the calculation to choose sizes to try.
Final Words
The point of the Bra Size Calculator is not just to return a number, but to help you turn two tape measurements into a practical starting size without manually working through band rounding and cup-difference charts.
One important consideration is that measurement location and tape tension are the biggest controllable factors. If the situation changes, use the Bra Size Calculator again and understand the relationship between band and cup rather than treating the cup letter as an absolute volume independent of band size.
Disclaimer
This calculator provides estimates for general informational and educational purposes only and should not be treated as financial, medical, legal, or professional advice. Results depend entirely on the accuracy of the figures you enter.
Always verify important decisions with a qualified professional, official documentation, or your financial institution before acting on any result shown here.
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