Bra Size Calculator
Enter two measurements — your bust and your band (underbust) — and this calculator estimates your bra size in US, UK, and EU labeling, along with a sister-size table. Bra sizing is not standardized across brands, so treat the result as an accurate starting point rather than a guarantee.
How bra sizing works
A bra size has two parts. The band is set by the firm measurement around your ribcage just under the bust — that number becomes the 32, 34, 36 you see on the label. The cup comes from how much bigger the bust measurement is than the band: each additional inch is roughly one cup letter. Because the two measurements combine, several size combinations can enclose the same breast volume — those are sister sizes, handy when a particular band feels off or is unavailable.
How it’s calculated
The calculator works in inches (converting from centimeters if needed). The band size is taken from the underbust measurement, rounded to the nearest even number as is conventional. The cup is the rounded bust-minus-band difference: under 1 in is AA, 1 in is A, 2 in B, 3 in C, 4 in D, then the larger US and UK letters diverge (US: DD, DDD/F, G…; UK: DD, E, F, FF…). The EU band is the US band plus 41 (so US 34 → EU 75), following EN 13402. Sister sizes step the band up or down by two while shifting the cup one letter to keep volume constant.
Sizing varies widely between brands and even styles, and these formulas are an approximation. Always try a bra on to confirm fit; a professional fitting can help with hard-to-fit shapes.
Cup size by difference
| Bust − band | US cup | UK cup |
|---|---|---|
| <1 in | AA | AA |
| 1 in | A | A |
| 2 in | B | B |
| 3 in | C | C |
| 4 in | D | D |
| 5 in | DD / E | DD |
| 6 in | DDD / F | E |
| 7 in | G | F |
| 8 in | H | FF |
US and UK cup letters diverge above a D cup.
Worked example
Someone measures a 38 in bust and a 34 in band. The band is 34; the difference is 38 − 34 = 4 in, which is a D cup — so a US 34D (UK 34D, roughly EU 75D). The sister sizes are 32DD (smaller band, larger cup) and 36C (larger band, smaller cup), all holding about the same cup volume.
Common mistakes
- Measuring the band too loosely — it should be firm and level for an accurate frame size.
- Choosing a larger band and smaller cup, the most common fitting error.
- Assuming a size transfers between brands; the same label can fit differently.
- Measuring the bust while braless or leaning over, which distorts the number.
Where it is used
- Shopping online where you cannot try before buying.
- Converting a known size between US, UK, and EU systems.
- Finding sister sizes when your usual band is out of stock.
- Checking whether a poorly fitting bra points to the wrong size.
Frequently asked questions
How do I measure for a bra?
Take two measurements in inches. The band (frame) is the firm measurement snugly around your torso directly under the bust. The bust is the loose measurement around the fullest part of the chest while wearing a well-fitting bra. Measure to the nearest quarter inch for the best result.
How is cup size calculated?
Cup size comes from the difference between the bust and band measurements. Each inch of difference is one cup: a 1-inch difference is roughly an A cup, 2 inches a B, 3 a C, 4 a D, and so on. A 38-inch bust with a 34-inch band is a 4-inch difference, giving a US 34D.
What are sister sizes?
Sister sizes are bras with the same cup volume but a different band. Go down one band and up one cup, or up one band and down one cup: 34D shares volume with 32DD and 36C. They are useful when your band size is out of stock or the band feels too tight or loose.
Why do bra sizes differ between the US, UK, and EU?
Each region uses its own labeling. The US and UK share band numbers but diverge on cups above D (UK uses DD, E, F, FF; US uses DD, DDD, and letters). The EU (EN 13402) numbers the band differently, so a US 34 band is roughly an EU 75. This calculator lists the equivalents.
How do I know if a bra fits?
The band should sit level and snug without riding up, the cups should fully contain the breast without gaps or overflow, and the center panel should lie flat against the chest. Because sizing varies by brand, treat calculated size as a starting point and try before you buy.