Bra Size Calculator: Find Your US & UK Size at Home
Most women are wearing the wrong bra size — not because they chose badly, but because they were sized years ago, with an outdated method, and never re-measured. A bra that fits changes how every top, dress and knit sits: the waist reappears, shoulders relax, and clothes stop needing constant adjusting.
This bra size calculator uses the modern fitting method: a snug underbust measurement for your band, and the difference between bust and band for your cup. No adding four inches, no guessing. Enter two measurements in inches or centimetres and you get your US size, your UK equivalent, your sister sizes and a simple four-point fit check.
Treat the result as your best starting size, not a verdict. Brands vary, styles vary, and the fit check at the end is how you confirm a size on your actual body.
Contents
- Bra size calculator
- How to measure accurately
- Common mistakes and the +4 myth
- How the calculator works
- Sister sizes explained
- Signs of a bad fit
- When to re-measure
- Keep exploring
- Frequently asked questions
Bra Size Calculator
Enter two measurements — in inches or centimetres. This calculator uses modern US sizing (no adding 4 inches), and shows the UK equivalent too.
How to measure: tape snugly around your ribcage, right under your bust, keeping it level. Breathe out before you read it — snug, not tight enough to dig in.
How to measure: tape around the fullest part of your bust, usually across the nipple line, keeping it level and loose enough not to compress you. Measure in a thin, unpadded bra or no bra.
How to Measure Accurately
You need a soft tape measure, a mirror, and a thin unpadded bra — or no bra. Stand up straight and measure each number twice.
Underbust (for your band)
Wrap the tape around your ribcage, directly under your bust, where the band of a bra sits. Keep it level all the way round and pull it snug — this is the one measurement that should feel firm. Breathe out naturally before you read it, so your ribs are at their resting size. A loose underbust reading is the fastest route to a band that rides up all day.
Bust (for your cup)
Wrap the tape around the fullest part of your bust, usually across the nipple line, keeping it level and flat but not tight. It should rest on you without compressing tissue. If you are between postures, stand normally — do not lean forward or push up; the calculator assumes an everyday, standing measurement.
Common Mistakes and the +4 Myth
Measuring over a padded bra. Padding adds to your bust reading and inflates your cup. Measure in something thin and unpadded, or in no bra at all.
A loose underbust tape. The band does most of the supporting work, so it is sized from a snug reading. A tape floating over a jumper will put you in a band one or two sizes too big.
Measuring once, years ago. Bodies change. A size from five or ten years ago is a historical document, not a fitting.
The +4 myth. Older sizing told women to add four (or five) inches to their underbust to get their band size — so a 28-inch ribcage was sold a 32 band. That rule dates from an era of non-stretch fabrics and limited size ranges, and it lingered in fitting rooms for decades. Modern bras use stretch bands and a far wider range of sizes, and modern fitters dropped the rule because it puts most women in a band too loose to support them and a cup too small to hold them — the classic combination behind straps digging in while the band rides up. This calculator does not add anything: your band is your underbust, rounded to the nearest even number.
How the Calculator Works
Band: your underbust, rounded to the nearest even number, halves up. An underbust of 31.5 becomes a 32 band; 33 becomes 34. Bands come in even numbers, and rounding (rather than always rounding up) keeps the band firm, which is where support comes from.
Cup: the difference between your full bust and your band, in inches. Under half an inch is AA, then roughly one inch per letter: 1 = A, 2 = B, 3 = C, 4 = D and 5 = DD. US and UK letters agree up to D and then diverge, which is why the calculator labels both. In US sizing, 5 = DD (sometimes written E), 6 = DDD (F), 7 = G, 8 = H, 9 = I and 10 = J. In UK sizing, 5 = DD, 6 = E, 7 = F, 8 = FF and 9 = G. The number stays the same on both sides of the Atlantic — only the cup letters change.
Worked example: underbust 81 cm is 31.9 inches, so the band is 32. Bust 91 cm is 35.8 inches. Difference against the band: about 3.9 inches, which rounds to a D cup — US 32D, UK 32D.
Sister Sizes Explained
Sister sizes are sizes with roughly the same cup volume on a different band. Go down a band and up a cup letter, or up a band and down a cup letter, and the cup holds about the same amount — only the fit around your ribs changes. They are genuinely useful: if a 32D has perfect cups but the band digs in, its sister 34C keeps the cup volume on a looser band; if the 32D band rides up, sister 30DD keeps the volume on a firmer band.
| If your size is | Sister: firmer band | Sister: looser band |
|---|---|---|
| 28D | — | 30C |
| 30DD | 28DDD | 32D |
| 32D | 30DD | 34C |
| 34C | 32D | 36B |
| 36B | 34C | 38A |
| 38A | 36B | 40AA |
| 40C | 38D | 42B |
US sizing shown. Sister sizes are a fitting tool, not an excuse for a shop to sell you whatever is in stock — the band should still feel right.
Signs of a Bad Fit
The band rides up your back. Your band is too big (or stretched out). Support has moved to your straps, which is why your shoulders ache.
You spill over the cups or under your arms. Your cup is too small, or the style is too shallow for you. Try a cup up before you try a band up.
The cups gape or wrinkle. Your cup is too big, or the style is too projected. Try a cup down, or a different shape.
The gore floats away from your chest. In a wired bra the centre should sit flat. A floating gore usually means the cups are too small.
Straps dig in or slip constantly. Digging usually means the band is doing too little work; slipping often means straps are over-tightened to compensate, or simply need adjusting after washing.
When to Re-Measure
Re-measure whenever your body changes noticeably: weight gain or loss of more than a few pounds, pregnancy and after nursing, starting or changing exercise that builds your back, and hormonal shifts. After 50, many women find their band stays similar while cup volume and tissue firmness change — softer tissue often fits better in a slightly different style at the same calculated size, and a yearly re-measure catches drift early. Bras themselves also age: a band that has lost its stretch will feel like the wrong size even when it is not, so judge a new size in a new bra.
Keep Exploring
See all of our calculators on the tools page, pair this with our body shape calculator for the full picture, and build outfits on your new foundation with elevated casual outfits that look expensive, quiet luxury outfits for women over 40 and timeless old money outfit ideas.
Frequently Asked Questions
Should I add 4 inches to my underbust?
No. The add-4-inches rule is outdated. Modern fitting sizes your band directly from your snug underbust, rounded to the nearest even number — which is what this calculator does.
What is my UK size if I am a US 30DD?
US and UK letters match through D, then diverge. A difference of five inches is DD in both systems, so US 30DD is UK 30DD; at six inches the US wears DDD while the UK wears E.
What are sister sizes?
Sister sizes hold roughly the same cup volume on a different band: down a band and up a cup letter, or up a band and down a letter. For example, 30DD, 32D and 28DDD are sisters.
Why does my calculated size feel too tight?
A new band should feel firm on its loosest hook — most of your support comes from the band. If the cups are right but the band digs, try your looser sister size; if you are spilling out, try a cup size up first.
How often should I measure my bra size?
About once a year, and whenever your weight changes, after pregnancy or nursing, and around and after menopause, when volume and firmness often change even if your band does not.
Can I measure in centimetres?
Yes. Switch the toggle to centimetres and enter cm values — the calculator converts them to inches (dividing by 2.54) before working out your band and cup.
