How Our Bra Size Calculator Works
A quick estimate, with the assumptions visible
The quick calculator uses two measurements. An optional detailed calculator uses six measurements, independently implementing the ABTF community manual with the product choices below. Neither is a medical assessment or a prediction of a particular bra’s fit; this site is not affiliated with ABTF.
For consistent input, wear a non-padded bra, stand relaxed and measure snugly below the breasts and gently around the fullest part. Do not mix measurements taken in different postures or support conditions.
The calculation — quick-v1
1. Convert centimeters to inches using exactly 2.54 cm per inch. Keep full input precision until rounding.
2. Round underbust to the nearest even integer. At the halfway point, round up: 33 inches becomes band 34.
3. Round bust to the nearest whole inch, halfway up. Subtract the band label from this rounded bust. A difference of 0 maps to AA, 1 to A, 2 to B, 3 to C and 4 to D; the remaining sequence appears in the bra size chart.
Example: 34-inch underbust and 38-inch bust give band 34 and a difference of 4: 34D. For 81.28 cm and 91.44 cm, the converted measurements are 32 and 36 inches: 32D.
What is sourced and what is our choice?
BRABAR’s public fit guide describes nearest-even underbust, whole-inch bust and the bust-minus-band approach, including 34/38 → 34D and 33 → band 34. We implemented these ideas independently; this site is not affiliated with BRABAR.
Tie handling, the AA extension at zero difference, the supported limits and the reminder within 0.15 inch of a rounding boundary are explicit product conventions, not claims of a universal sizing standard. The boundary reminder is not a confidence score.
We use a common US sequence AA–A–B–C–D–DD–DDD–G–H–I–J–K–L and UK sequence AA–A–B–C–D–DD–E–F–FF–G–GG–H–HH. US brands may use other letters. Check the brand chart.
Quick-method limits and sister sizes
This release supports underbust 26–50 inches, bust up to 64 inches and a rounded cup difference of 0–12 inches. Bust must not be smaller than underbust. Outside those limits we do not clamp or guess a size. These are limits of this tool, not a judgement about a body.
A nearby sister size changes the band by 2 and the cup step in the opposite direction. It is useful to compare when cup capacity feels right but the band needs adjusting. It does not preserve all dimensions or guarantee fit.
Shape, materials and construction matter. Use the fit checks rather than treating gaps or pressure as a one-step size diagnosis. Shape information does not change this calculator’s cup result.
Optional detailed method — six-manual-v1
Measure loose, snug and tight underbust, plus standing, leaning and lying bust, without a bra. Keep the tape level for underbust and do not compress breast tissue for bust. Tight measurement must not hurt. Do not carry bra-supported measurements over from the quick method.
The standard pathway follows the manual’s female-at-birth pathway: use leaning bust unless leaning minus standing is greater than 2.5 inches, then average all three bust measurements. The alternative male-at-birth pathway uses (2 × standing + leaning + 2 × lying) ÷ 5. These are community method assumptions, not identity or body assessments, and are not validated for every body.
Take the nearest even snug underbust as band, with halfway values upward. Subtract loose underbust from the effective bust. A nonnegative difference below 1 maps to AA; otherwise retain both neighboring whole cup steps when fractional. Round the absolute loose-underbust-minus-band difference to a whole inch, halfway upward, then halve it. Add that correction when loose underbust exceeds band, or subtract when below. Keep adjacent whole cup steps for fractional endpoints. For example: loose 35.25, snug 34, tight 32, standing 42, leaning 43.25 and lying 42 inches yield UK 34FF / 34G.
Snug-minus-tight below 2 inches prompts a band-comfort check; at least 4 inches prompts comparing a firmer band only if fit warrants it. These reminders do not automatically change the band. Smaller leaning measurements prompt a recheck, not an automatic rejection.
Our product limits: snug underbust 26–50 inches; all measurements positive and no greater than 64 inches; loose ≥ snug ≥ tight; each bust ≥ snug; every resulting cup step within 0–12. We reject unsupported ranges instead of clipping them. Stable decimal handling, midpoint rules, expanding fractional endpoints and these limits are explicit implementation choices. The manual does not specify all software edge cases; output may differ from the ABTF online calculator. Numerical tests do not establish real-world fitting accuracy.
Review and sources
Method versions quick-v1 and six-manual-v1. Reviewed September 29, 2026. Automated checks cover published examples, unit equivalence, rounding boundaries, invalid inputs and sister-size limits. These checks verify implementation, not real-world fitting accuracy.
International bra size chart — international-v1
Calculation uses UK band and cup steps; international labels are converted afterward.
Choose the sizing used by the brand, wherever you live. EU, FR and AU/NZ results are label references from Freya and Wacoal—not new measurements or a guarantee of the same fit.
Read band and cup separately. These brand reference labels do not guarantee equal fit or availability. For EU, FR and AU/NZ we cover UK bands 28–46 and cups AA–HH. A band label such as EU 75 is not a conversion of your measured underbust to centimeters.
There is no universal country-to-country conversion. Japanese, Italian and Chinese labels need further brand-specific validation. Even official charts can disagree, particularly for larger cups. Use the chart from the brand you are buying.