Short answer: To measure your bra size at home, take two numbers with a soft tape. Your band is your ribcage measured snugly right under your bust. Your bust is the fullest part. The band, rounded to an even number, is your band size, and the difference between bust and band sets your cup: about one inch per cup, so a four inch difference is a D. You can get the exact size in seconds with our free bra size calculator.
Getting this right matters more than most people think. In a cross sectional study by researchers at the University of Wollongong, 85 percent of women were wearing an ill fitting bra, and self selected sizes were often far from the correct one (McGhee and Steele, Journal of Science and Medicine in Sport). A few minutes with a tape measure fixes that.
What you need
- A soft cloth tape measure. If you only have a rigid one, use a piece of string and lay it flat against a ruler afterwards.
- An unpadded bra, or no bra at all, so the tape reads your body and not the padding.
- A mirror, so you can keep the tape level all the way around.
How to measure your bra size at home
- Measure your band. Wrap the tape around your ribcage directly under your bust. Keep it snug and level, breathe out, and read the number in inches. This is your underband measurement.
- Measure your bust. Wrap the tape around the fullest part of your bust, usually across the nipple line. Keep it level and do not pull it tight. Standing straight gives the truest reading.
- Do the math, or skip it and enter both numbers into the bra size calculator for your band, cup and sister sizes at once.
How to work out your band and cup size
Your band size comes from the underband number, rounded to the nearest even number. If you measure 31 inches, your band is 32. Some fitters add nothing to the measurement and rely on the stretch of the band, which is the method most modern size charts use.
Your cup size comes from the difference between your bust and your band, at roughly one inch per cup. Subtract the band from the bust and match the gap to the chart below.
| Bust minus band | US cup |
|---|---|
| Less than 1 inch | AA |
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD (E) |
| 6 inches | DDD (F) |
| 7 inches | G |
So a 34 inch band with a 38 inch bust is a four inch difference, which reads as a 34D. The bra size calculator runs this for you and also gives the UK and EU equivalents.
Sister sizes: when to size up or down
A sister size is a different band and cup that holds the same cup volume, so it fits when your usual size is close but not quite right. The rule is simple. If the band feels too tight, go up one band and down one cup. If the band feels loose, go down one band and up one cup. A 34C, a 32D and a 36B are all sister sizes with the same cup volume. This is the fastest fix when a style runs small or large.
How to tell your bra actually fits
The tape gets you close. Fit confirms it. Look for these signs:
- The band is level and firm. It should sit straight across your back, not ride up. If it climbs, the band is too big.
- The cups are smooth. No spillover at the top and no empty gaps. Spillover means size up a cup, gaping means size down.
- The underwire sits flat against your ribcage and around the breast, not on top of breast tissue.
- The straps stay put without digging in. If they carry all the weight, the band is doing too little.
Bra sizes change by brand and country
Two bras labelled the same can fit differently, and US, UK and EU systems do not line up one to one. A US DD is a UK E, and EU bands run on centimetres, so the numbers look nothing alike. When you shop across regions, convert your size with our international size converter, and treat any single label as a starting point rather than a guarantee.
Sizing up fitted clothing is the same skill. If you are between sizes in dresses and tops, our clothing size calculator and our edit of fitted tops and blouses use the same body measurements. For swim, cup fit works a little differently, so start with the swimwear size calculator.
Ready for your number? Enter your band and bust in the free bra size calculator and get your size, sister sizes and international equivalents in seconds. This is general guidance to help you shop, not medical advice.
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