# Bra size calculator

> Bra size calculator: band and cup from underbust and bust in inches or cm, in US, UK, EU, FR/BE and AU sizes, with sister sizes and the method stated.

Interactive version: https://www.calcopenly.com/everyday/bra-size-calculator
Subject: Everyday calculators

Band size comes from the snug measurement around the ribcage under the bust, and cup size from how much the bust measurement exceeds the band. The default plus-zero method, used in Triumph's UK fitting guide, rounds the underbust to the nearest even inch and counts one cup per inch of difference: 0 is AA, 1 A, 2 B, 3 C, 4 D and 5 DD. The traditional plus-four method and the European standard EN 13402, which works in centimeters, are the other two options.

An underbust of 31.5 inches and a bust of 37 inches give a 32 band and a 5-inch difference, a US and UK 32DD. The same size is a 70E in European sizes, 85E in French and Belgian sizes and 10DD in Australia, from the Fantasie and Freya conversion charts.

Brands size differently and the methods disagree by one or two band sizes, so treat the result as a starting point for a fitting. The sister sizes hold the same cup volume on a band one size up or down.

## Inputs

- **Method** (options: Plus zero (modern fitting), Plus four (traditional), EN 13402 (European standard, cm))
- **Underbust (snug, under the bust)**: Level around the ribcage, directly under the bust, pulled snug.
- **Bust (fullest part)**: Around the fullest part, tape level and not pulled tight.
- **Show first** (options: US, UK, EU, FR/BE, AU/NZ)

## Results

- US size — main result
- UK size
- EU size
- FR/BE size
- AU/NZ size
- Band (UK/US) (in)
- Cup difference
- Sister size, smaller band
- Sister size, larger band

## Formula

$$
\text{band} = 2\,\operatorname{round}\!\left(\tfrac{\text{underbust (in)}}{2}\right),\qquad \text{cup} = \operatorname{round}(\text{bust} - \text{band})\ \text{inches} \;(0 = \text{AA},\ 1 = \text{A},\ \dots)
$$

## Worked examples

### 31.5 in underbust, 37 in bust (plus zero)

- Method: Plus zero (modern fitting)
- Underbust (snug, under the bust): 31.5 in
- Bust (fullest part): 37 in
- Show first: US
- **US size: 32DD**
- **UK size: 32DD**
- **EU size: 70E**
- **FR/BE size: 85E**
- **AU/NZ size: 10DD**
- **Band (UK/US): 32 in**
- **Cup difference: 5 in**
- **Sister size, smaller band: 30DDD**
- **Sister size, larger band: 34D**
- Checked against: Triumph UK method: 31.5 → 32 band, 37 − 32 = 5 in = DD; conversions from the Fantasie chart (UK DD = EU/FR E = AU DD = US DD; 32 = EU 70 = FR 85 = AU 10)

### EN 13402 worked example: 89 cm and 108 cm

- Method: EN 13402 (European standard, cm)
- Underbust (snug, under the bust): 89 cm
- Bust (fullest part): 108 cm
- Show first: EU
- **EU size: 90D**
- **UK size: 40D**
- **FR/BE size: 105D**
- **Cup difference: 19 cm**
- Checked against: EN 13402 summary: 108 − 89 = 19 cm = D, 89 → 90, size 90D; EU 90 = UK 40 = FR 105 on the Fantasie chart

### Sister sizes of 34B

- Method: Plus zero (modern fitting)
- Underbust (snug, under the bust): 34 in
- Bust (fullest part): 36 in
- Show first: US
- **US size: 34B**
- **Sister size, smaller band: 32C**
- **Sister size, larger band: 36A**
- Checked against: b.tempt'd by Wacoal: “If you're usually a 34B, your sister sizes would be 32C and 36A”

### Plus four: 30 in underbust, 36 in bust

- Method: Plus four (traditional)
- Underbust (snug, under the bust): 30 in
- Bust (fullest part): 36 in
- Show first: US
- **US size: 34B**
- **Band (UK/US): 34 in**
- **Cup difference: 2 in**
- Checked against: Plus-four rule as described in Wikipedia's Bra size article: even underbust + 4 = 34; 36 − 34 = 2 in = B

### UK 34E is a US 34DDD and EU 75F

- Method: Plus zero (modern fitting)
- Underbust (snug, under the bust): 34 in
- Bust (fullest part): 40 in
- Show first: UK
- **UK size: 34E**
- **US size: 34DDD**
- **EU size: 75F**
- **AU/NZ size: 12E**
- Checked against: Fantasie chart row UK E = EU F = FR F = AU E = US DDD; band 34 = EU 75 = AU 12

### Edge: odd whole-inch underbust rounds up, zero difference is AA

- Method: Plus zero (modern fitting)
- Underbust (snug, under the bust): 33 in
- Bust (fullest part): 34 in
- Show first: US
- **UK size: 34AA**
- **Band (UK/US): 34 in**
- **Cup difference: 0 in**
- Checked against: Triumph table: 0 in = AA; 33 in is equally near 32 and 34 and is rounded up, the convention calculator.net also uses (29 in → 30 band)

## Questions

### How do I measure my bra size?

Measure snugly around the ribcage directly under the bust for the band, and around the fullest part of the bust for the cup, with the tape level. Triumph's UK guide rounds the underbust to the nearest even inch, then subtracts that band from the bust: each inch is one cup. A 33.5-inch underbust and 37-inch bust give a 34 band and a 3-inch difference, a 34C.

### What are sister sizes?

Sister sizes share the same cup volume on a different band: go up one band size and down one cup, or down one band and up one cup. Wacoal's b.tempt'd fit guide gives 32C and 36A as the sister sizes of a 34B. They help when a band feels too tight or loose but the cups fit, or when a style doesn't come in your usual size.

### Should I add 4 inches to my band measurement?

The plus-four rule adds 4 inches to an even underbust and 5 to an odd one, so a 30-inch ribcage wears a 34 band. Wikipedia's Bra size article records Kohl's using it in 2013. Triumph's UK guide instead rounds the measured underbust to the nearest even inch, which for a 30-inch ribcage and 36-inch bust gives a 30E instead of the plus-four 34B.

### How do US, UK and European bra sizes convert?

Bands follow a fixed pattern: UK and US 32 is 70 in Europe, 85 in France and Belgium and 10 in Australia, and each band step adds 5 cm. Cups match up to D; after that the UK uses DD, E, F, FF, the US DD, DDD, G, H, and Europe E, F, G, H, one letter per step, according to the Fantasie and Freya charts. A UK 34E is a US 34DDD and an EU 75F.

### How does the European standard EN 13402 size bras?

EN 13402 labels a bra with the underbust in centimeters, rounded to the nearest 5, and a cup letter from the difference between bust and underbust in 2 cm steps: AA is 10–12 cm, A 12–14, B 14–16, C 16–18 and D 18–20. An 89 cm underbust and 108 cm bust have a 19 cm difference, so the standard's size is 90D, which converts to a UK 40D.

### How accurate is the bra size calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “31.5 in underbust, 37 in bust (plus zero)” is checked against Triumph UK method: 31.5 → 32 band, 37 − 32 = 5 in = DD; conversions from the Fantasie chart (UK DD = EU/FR E = AU DD = US DD; 32 = EU 70 = FR 85 = AU 10).

### Where does the method come from?

Triumph UK — Bra size calculator and fitting guide (round the underbust to the nearest even inch; 1 inch per cup); EN 13402-3 bra labelling (underbust to the nearest 5 cm; AA = 10–12 cm … H = 26–28 cm), summarised with the 90D worked example; Fantasie — International size guide (UK, EU, FR, AU/NZ, US band and cup conversion); Wacoal America — Size charts (US and UK cup sequences); b.tempt'd by Wacoal — Fit calculator and sister sizes.

## Sources

- [Triumph UK — Bra size calculator and fitting guide (round the underbust to the nearest even inch; 1 inch per cup)](https://uk.triumph.com/bra-size-calculator-fitting-guide)
- [EN 13402-3 bra labelling (underbust to the nearest 5 cm; AA = 10–12 cm … H = 26–28 cm), summarised with the 90D worked example](https://en.wikipedia.org/wiki/Joint_European_standard_for_size_labelling_of_clothes)
- [Fantasie — International size guide (UK, EU, FR, AU/NZ, US band and cup conversion)](https://www.fantasie.com/us/en/size-guide/)
- [Wacoal America — Size charts (US and UK cup sequences)](https://wacoal-america.com/pages/size-charts)
- [b.tempt'd by Wacoal — Fit calculator and sister sizes](https://btemptd.wacoal-america.com/fit-calculator)