A bra size calculator is highly accurate for the math itself, but only as accurate as the two measurements you enter. Most sizing errors come from measurement technique, not the formula.
The Formula Is Standard, Not Secret
The band-and-cup formula this site uses (underbust rounded to the nearest even number, cup determined by the bust-minus-band difference) is the same industry-standard method used across most professional fittings. You can see the exact steps on Our fitting process page.
Where Accuracy Actually Breaks Down
Measuring over a padded bra, measuring while slouching, or pulling the tape too tight all throw off the input numbers before the math even runs. A perfect formula can’t fix an inaccurate measurement.
How to Get the Most Accurate Result
Measure without padding, standing naturally, with the tape snug but not compressing. Re-measure a second time to confirm. If the two readings differ by more than half an inch, take a third measurement and use the most consistent number.
Then double-check the result with the Sister size calculator- if a sister size feels notably better in-store, that’s useful feedback, not a sign the calculator failed.
What Calculators Can’t Account For
No calculator can see how a specific bra’s cut, wire shape, or fabric stretch will interact with your individual shape. The size it gives you is an accurate mathematical starting point, not a guarantee for every single style and brand you try.
How to Improve Your Own Accuracy
Measure without a bra or in a thin, unpadded one, keep the tape parallel to the floor front and back, and measure the bust at the fullest point while breathing normally rather than holding your breath. These small technique details affect the result more than which calculator you use.
Treat any calculator’s result (this one included) as a confident starting point for trying bras on, not a guarantee that skips the fitting-room step entirely.
What “Accurate” Actually Means Here
This calculator applies the standard, publicly documented band-and-cup method that fitters have used for years. Given accurate measurements, the math itself doesn’t introduce error. The real variable in accuracy is almost always the measurement, not the calculation.
Where Estimates Naturally Diverge
Two different measuring attempts on the same body, even minutes apart, can produce slightly different numbers depending on tape tension, posture, and exactly where the tape sits. This is normal and not a flaw in the tool. It’s why re-measuring once if a result feels surprising is worth doing before assuming the calculator is wrong.
What This Tool Can’t Account For
Brand-specific cut variation is real and the calculator gives you a measurement-based starting size, not a guarantee for every specific product. See the Brand Size Decoder for known tendencies at specific retailers. Breast shape and projection also affect how a mathematically-correct size actually feels in a specific style, which is a fit question the numbers alone can’t fully answer.
What Affects Accuracy Most
Measuring technique affects accuracy far more than which specific calculator or formula you use. A tape that’s too loose, too tight, or measured at the wrong body position will throw off any calculation method equally. Getting the measuring technique right is a bigger accuracy lever than debating which sizing formula is theoretically best.
Comparing to Professional Fittings
Independent comparisons between self-measured calculator results and professional in-store fittings generally find good alignment when the self-measurement is done carefully, with larger discrepancies concentrated among people with less-typical body proportions or measuring technique issues rather than in the underlying math itself.
What We Do to Support Accuracy
Publishing the exact math behind every calculation (rather than a black-box result) lets you sanity-check the logic yourself, which is a level of transparency not every sizing tool offers.