Methodology
How Our Bra Size Calculator Works
Our calculator estimates a starting size from body measurements. It does not claim to predict the exact fit of every bra, because brands, styles, fabrics and breast shape change how a labelled size behaves.
Current algorithm version: 2.0.0
What the calculator can and cannot do
It can
- Estimate a starting band and cup from your measurements.
- Show a wider range when your inputs sit near a size boundary.
- Convert that size across UK, US, EU, French, Australian/NZ and Japanese labels.
- Show nearby sister sizes with a similar cup volume.
- Flag when it can't produce a confident result rather than guessing.
It cannot
- See cup depth, projection, wire width or fabric stretch.
- Guarantee any specific bra, in any brand, will fit.
- Diagnose pain, asymmetry causes, or any medical condition.
- Replace trying the bra on and checking the fit in person.
Quick Size Check method
Inputs: your chosen unit, a snug underbust measurement, and a standing bust measurement.
- Convert your entered values to inches internally.
- Round the snug underbust to the nearest even band number.
- Take the difference between your bust measurement and that band number.
- Round that difference to the nearest whole number to get a cup position.
- If your underbust sits within 0.3" of the midpoint between two band sizes, or your bust-to-band difference sits within 0.2" of a cup boundary, we show a neighbouring size alongside the main result rather than picking one silently.
Quick mode never returns "high" confidence - two measurements alone can't rule out a large bust spread, a rounded input, or a band-comfort issue the way six measurements can. See "measurement confidence" below.
Advanced Fit Analysis method
Advanced mode adds loose and tight underbust readings (for a band comfort range) and leaning and lying bust readings (for a more stable cup estimate).
- Your loose, snug and tight underbust readings must be in that order (loose ≥ snug ≥ tight). If they aren't, we ask you to recheck rather than silently averaging them.
- If your snug-based band would sit below your tight underbust, or above your loose underbust by more than 0.3", we suggest a firmer or looser starting band than the snug reading alone would give, and show both the raw and adjusted band so you can see the difference.
- For the bust, we compare your standing, leaning and lying readings. When they're close together (under 1"), we use their average. When they spread further apart (up to 2.5"), we weight standing more heavily. Beyond that spread, we don't collapse the result to a single cup - we show a range instead, because a single number would overstate how sure we are.
International size systems
Internally, every size is stored as a band position and a cup position on a shared numeric scale, independent of any one country's labels. Each sizing system (UK, US, EU, French, Australian/NZ, Japanese) is just a set of display labels mapped onto that same scale - so a size never gets silently relabelled through a shortcut conversion, only through this shared scale.
The US market has no single agreed cup letter past a D cup - our tables show one common label per position (for example "DD" at one step) and flag the accepted alternative (for example "E") with a note that brands vary, rather than presenting either one as the only correct label.
How sister sizes are calculated
A sister size moves one step in band size while moving one step in the opposite direction in cup position, which keeps the approximate cup volume the same while changing how firm the band is. We never claim a sister size is an identical fit - cup shape and wire geometry can still differ even at a matching volume.
What "measurement confidence" means
We show one of three labels, never a percentage:
- High - only possible in Advanced mode, when all six measurements are present, consistent with each other, and not close to a size boundary.
- Moderate - the normal Quick-mode result, or an Advanced result near a boundary, with a large bust spread, or with rounded-looking inputs.
- Limited - inputs outside our supported measurement range, or a result outside our current size tables.
Confidence describes how internally consistent your measurements and the method were - not the probability that a specific bra will fit. We do not publish, and will not publish, a fabricated accuracy percentage.
Known limitations
- Asymmetry between left and right isn't something circumference measurements can capture.
- Very soft or very firm tissue can behave differently than the average the formulas assume.
- Pronounced projection, and unusually narrow or wide roots, affect fit in ways a band/bust measurement alone doesn't fully describe.
- Pregnancy, postpartum and post-surgical bodies can change faster than a single measurement captures.
- Brands with non-standard sizing, and styles without conventional cups, may not map cleanly onto any of the systems above.
- Measurement error - a tape that's not level, or pulled too tight or too loose - affects the result more than any part of the formula does.
Test cases and version history
The calculation engine is covered by an automated test suite that checks specific worked
examples, boundary cases and error conditions on every change, so a formula change can't silently
alter results without the tests reflecting it. Current version: 2.0.0.
Editorial and expert review
Guides on this site are published under the Bra Size Calculator Pro Editorial Team byline. See our editorial policy for how content review works, or get in touch to report a calculation or conversion issue.