How Brasize.ai Calculates
Every number on this site is derived from two measurements by rules you can read here. No lookup tables, no brand data behind the calculator: a specification, published. Change the inputs below and watch each rule apply.
Two numbers through every rule
Two measurements
| Input | How it is taken | Units |
|---|---|---|
| Underbust | Directly under the bust, snug against the ribcage, level, on a normal exhale | Inches (cm accepted; converted ÷ 2.54) |
| Bust | Around the fullest point, tape level, touching but not compressing; leaning forward for soft tissue | Inches (cm accepted) |
No inches are added. The traditional +4/+5 rule is not used anywhere on the site; where it appears, it is to translate a legacy size back to a snug-measure size.
Band, cup and rounding
| Step | Rule | Example (33″ / 37″) |
|---|---|---|
| Band | Round the underbust to the nearest whole inch; if odd, round up to the next even number; clamp to 28–46 | 33 → 34 |
| Cup index | Round (bust − band) to the nearest whole inch; clamp to 0–11 | 37 − 34 = 3 |
| Cup letter (US) | Index → AA, A, B, C, D, DD, DDD, G, H, I, J, K | 3 → C |
| Size | Band + cup letter | 34C |
| Sister sizes | (band − 2, index + 1) and (band + 2, index − 1) where in range | 32D · 36B |
| +4-rule twin | If a legacy chart produced this size, snug size ≈ (band − 4, index + 2) | 34C → 30DD |
The size pages, cup pages and comparisons state measurement ranges as band −1.5 to +0.4 inches (underbust) and bust ± 0.5 inches, reflecting the rounding above.
Eight systems from one US size
| System | Band formula | Cup alphabet (index 0 →) | US 34C |
|---|---|---|---|
| US | band | AA A B C D DD DDD G H I J K | 34C |
| UK | = US | AA A B C D DD E F FF G GG H | 34C |
| EU / DE | US × 2.5 − 10 | AA A B C D E F G H I J K | 75C |
| FR / ES | EU + 15 | as EU | 90C |
| AU / NZ | US − 22 | as UK | 12C |
| IT | (EU − 60) ÷ 5 | as EU | 3C |
| JP | = EU | ≈ EU (some charts +1 above D) | 75C |
Where a system's practice varies by brand (Japanese cup grading, US brands writing DDD as F) the page says so rather than choosing silently.
What else is computed
Your US size → the brand's system by the formulas above → adjusted by the brand's recorded grading (band firm/true/soft, cup small/true/generous), which produces the "also try" sizes. Grading is editorial, based on brand charts and fitting practice, and stated on every brand page.
Eight breast shapes are modelled on eight parameters; each shape × construction verdict is an editorial matrix drawn on that model. Diagrams are schematic: mechanism, not anatomy.
Brand band and cup ranges reflect core collections and are checked against published charts; "widely stocked" describes bands 32–38 with cups B–DD.
Five bodies through the rules
| Underbust | Bust | Band | Index | Size | Sisters | EU · FR · AU |
|---|---|---|---|---|---|---|
| 28.5″ | 32″ | 28 | 4 | 28D | — · 30C | 60D · 75D · 6D |
| 31″ | 34″ | 32 | 2 | 32B | 30C · 34A | 70B · 85B · 10B |
| 33″ | 37″ | 34 | 3 | 34C | 32D · 36B | 75C · 90C · 12C |
| 35.5″ | 42″ | 36 | 6 | 36DDD | 34G · 38DD | 80F · 95F · 14E |
| 39″ | 44″ | 40 | 4 | 40D | 38DD · 42C | 90D · 105D · 18D |
Declared constants: tension, rounding, baseline
Three choices sit upstream of every formula on this site, and each one changes the answer if made differently, so we state ours outright rather than leave them implied.
Tape tension. Our standard is snug: the tape lies flat against the skin, level front and back, on a normal exhale, compressing nothing. Published brand instructions genuinely disagree here: several majors instruct "as tight as possible," others "firmly," most "snug", and tension alone can move an underbust reading by a full band size before any chart is consulted. Every number on this site assumes the snug reading.
Rounding. Band: underbust rounded to the nearest even inch (33″ → 34; exact odd numbers round up). Cup: bust minus band, to the nearest whole inch. Brands publish at least four different rounding rules (nearest, always-up, up-to-even, even-is-band); ours is fixed, stated, and applied identically everywhere.
Baseline. There is no single authoritative external ladder to normalize against: reference sources and premium houses disagree with each other about where "the EU convention" places its cup boundaries, sometimes by a full letter. All conversions and comparisons on this site therefore normalize against the formulas published on this page (our declared baseline) and wherever a brand's own arithmetic differs, we show the brand's arithmetic and say so.
Two "80%" figures, handled honestly. The claim that "80% of women wear the wrong bra size" traces to a 2008 fitted-population study (Wood, Cameron & Fitzgerald, n=104) and should not be quoted as a general-population fact; at least eight major brands repeat it uncited. The separate claim that "the band provides 80% of a bra's support" is a fitting-room heuristic without a controlled source; we phrase it as "most of a bra's support." Neither figure appears on this site as a precise statistic.
Changes and citation
| Version | Date | Change |
|---|---|---|
| 1.0 | June 2026 | Snug-measure method; rounding rules; sister sizes; eight-system conversion; brand grading; shape model; published. |
Found a rule that doesn't hold? Report it via the editorial policy; corrections are versioned here.
v1.1 (August 2026). Added the Declared Constants section (tape-tension standard, rounding rules, normalization baseline) and sourced or softened both industry "80%" figures, following our 57-entity brand-chart variance study. v1.0 (June 2026): first published edition.
Methodology: answered
Why doesn't Brasize.ai add four inches to the underbust?
The +4/+5 rule dates from non-stretch bands. Modern bands stretch and are drafted to the snug underbust; adding inches produces loose bands and small cups. We use the snug measurement directly and translate legacy sizes back with the +4-rule twin.
How is a sister size calculated?
Band down two, cup index up one (snugger); band up two, cup index down one (looser). Same cup volume by convention; different wire width.
How accurate is the calculator?
It is exact for the rules above; the uncertainty is in the measurement (tape tension, tissue softness) and in brand grading, which is why every result carries sister sizes and every brand page states how it runs. Confirm on the body with the five fit checks.
Can I reuse Brasize.ai's tables and figures?
Yes, with attribution: see the citation block above.
Opens HauteFlair’s own hub page; use your measured size there. HauteFlair is owned by the same company as Brasize.ai; that relationship never changes the sizing math, charts, or fit advice on this page. How this site makes money.