Bra Size Calculator
Most bra size calculators use two measurements and an outdated formula — which is why the result rarely fits. This one uses six measurements across three positions for a more accurate size, plus sister sizes and instant conversion across UK, US, EU, AU, French, and Japanese sizing.
Underbust measurements
Breathe out. Tape loosely under bust.
Comfortably snug under bust — used for band size.
As tight as comfortably possible under bust.
Bust measurements
Arms down, standing, at fullest point of bust.
Lean forward 90°, tape at fullest point.
Lie flat, tape at fullest point.
Your size will appear here
Enter your measurements and click Find my size to see your result plus sister sizes.
Quick Answer
Enter three underbust measurements (loose, snug, tight) and three bust measurements (standing, leaning, lying). Your band size comes from your snug underbust rounded to the nearest even number. Your cup size comes from the difference between your leaning bust and snug underbust. The result is shown in UK, US, EU, AU, FR, and JP sizing, plus sister sizes.
How It Works: Formula & Variables
Band size (UK/US) = snug underbust (in), rounded to nearest even number
Cup index = leaning bust (in) − snug underbust (in), rounded
EU band = snug underbust (cm), rounded to nearest 5
FR band = EU band + 15
AU band = UK band − 16
- Cup diff 1″
- A cup
- Cup diff 2″
- B cup
- Cup diff 3″
- C cup
- Cup diff 4″
- D cup
- Cup diff 5″
- DD (UK) / DD (US)
- Cup diff 6″
- E (UK) / DDD/F (US)
Worked Examples
Example 1: Result in inches
Snug underbust 31″ → rounds to even 32 → band size 32. Leaning bust 37″ − snug 31″ = 6″ → E cup (UK) / DDD cup (US). Result: 32E (UK) / 32DDD (US).
Example 2: Result in centimetres
Snug underbust 79 cm → rounds to nearest 5 → EU band 80. Leaning bust 94 cm → converted to inches: 37″. Snug 79 cm → 31.1″. Difference ≈ 6″ → E cup. Result: 80E (EU) / 65F (FR).
Key Concepts
The +4 method is outdated: Older calculators added 4 inches to the underbust to get the band size — a workaround from an era when bras had limited stretch. Modern bras with elastic bands no longer need this adjustment. Using +4 pushes most people into a band that is too loose and a cup that is too small.
Cup size is relative, not absolute: A D cup on a 28 band is much smaller than a D cup on a 38 band. Cup letters only make sense alongside the band number — which is why sister sizes exist.
Your calculated size is a starting point: Sizing varies between brands and even between styles within the same brand. Always try on a bra and check the fit signs before buying.
Leaning bust is most accurate: When you lean forward, breast tissue falls fully away from the chest wall, giving a true volume reading. Standing bust tends to underestimate cup size for larger busts.
Common Mistakes
Measuring over clothing: Bras, padding, and thick fabric all add bulk. Measure on bare skin or a thin non-padded bra only.
Pulling the tape too tight on the bust: The bust tape should be parallel to the floor and loose enough to breathe comfortably — it should not compress the breast tissue.
Trusting in-store fitters who use the +4 method: Many chain-store fitters still use the outdated formula. If you've been consistently told you're a 34B or 36C and it never quite fits, recalculate with this tool.
Ignoring sister sizes: If a bra in your calculated size doesn't fit perfectly, try the sister sizes before dismissing the result — the band or cup shape may simply suit you better in the adjacent size.
Frequently Asked Questions
Most bra size calculators use only two measurements and apply an outdated "+4 inch" formula to the underbust — a workaround from an era when bras had little stretch. Bra fitting communities and independent research have consistently shown this produces the wrong size for the majority of people. Six measurements (three underbust, three bust) account for natural variation across body positions, giving a far more accurate result.
Sister sizes are bra sizes that share the same cup volume but use a different band size. For example, 34C, 36B, and 32D are all sister sizes — the actual cup is the same, only the band length changes. If your exact size is out of stock, a sister size will fit the cup equally well. Go up a band size and down a cup letter, or down a band size and up a cup letter.
The leaning bust measurement is generally the most accurate because gravity allows the breast tissue to hang fully forward, giving a true reading of the cup volume. This calculator uses your leaning bust as the primary measurement, falling back to standing bust if leaning is not entered.
Probably not. Studies estimate that 70–80% of people wear a band that is too large and a cup that is too small. Common sizes like 34B or 36C are over-manufactured and aggressively sold, so many people are fitted into them regardless of their actual measurements. Trust the numbers, try the suggested size on, and check the fit signs: the band should be level, the underwire should sit flat against your ribcage, and there should be no spillage.
There is no international standard for bra sizing. The US, UK, EU, Australian, French, and Japanese systems all use different scales for both the band and cup. The band number in the US and UK refers to inches, EU uses centimetres rounded to the nearest 5, and France adds 15 to the EU number. Cup letters also diverge after D — UK uses DD, E, F, FF, G while the US uses DD, DDD/F, G. This calculator converts your measurements into all major systems at once.
Yes. The 6-measurement method works for any chest shape, size, or gender. The calculator includes results for very small bands (28 and under) and very large cups (up to UK KK / US S) that traditional store calculators often ignore. If your size falls outside standard retail ranges, sister sizes or specialist lingerie brands are your best route.