i.Find your starting size
Measure without a bra on, tape level all the way around. Breathe out normally before reading the underbust numbers. Enter each measurement to the nearest 0.5 — the guide in the last tab shows exactly where the tape goes.
Sister sizes — same cup volume
What your measurements suggest
ii.Sister-size finder
Cup letters are relative — a 30F, 32E and 28FF hold roughly the same breast volume on different band frames. Find every sister of your size when a band feels too tight, too loose, or your size is sold out.
iii.Fit troubleshooter
Tell us your current size and what's wrong. You'll get a plain-English diagnosis plus a size to try next — based on standard fitting logic, not guesswork.
What's wrong with the fit? (tick all that apply)
iv.US / UK / EU / FR size converter
Shopping internationally? Convert any bra size across the four major systems — or browse the bra size chart tables below. Bands: US and UK share inch numbering; EU uses its own centimeter numbering; French/Spanish adds 15 to the EU band. Cups shift by system — a UK F is a US G and an EU G.
Band conversion chart
| UK / US band | EU band | FR / ES band | Underbust (approx) |
|---|
Cup conversion chart (by difference: bust − band)
| Difference | UK cup | US cup | EU / FR cup |
|---|
Note: US "DDD" is also labelled "F" by some American brands; both mean the same 6-inch difference.
v.Breast-shape style guide
Size is only half the story — shape decides which styles work in your size. This is educational guidance about bra styles, not a medical assessment. Every shape below is completely normal.
vi.How to measure — the right way
Five minutes, one soft tape, no bra on. This is how to measure bra size the right way — stand in front of a mirror so you can check the tape stays level around your back.
1Underbust — three ways
Wrap the tape directly under your breasts where the tissue meets your torso. Take it loose (just resting), then snug (comfortably firm — breathe out first), then tight (as firm as you can without pain). The snug number sets your band; the other two fine-tune the advice.
2Standing bust
Standing straight, wrap the tape around the fullest point of your bust. Keep it level and just firm enough not to slip — don't press it into the tissue or bend it into your cleavage.
3Leaning bust
Bend forward until your back is roughly parallel to the floor and let gravity do the work — this captures tissue that snug bras may have pushed toward your sides. Measure loosely around the fullest hanging point.
4Lying bust
Lie on your back and measure around the fullest point again. Tissue settles differently lying down — averaging all three bust numbers smooths out position quirks no single measurement can catch.
5Sanity-check your numbers
Before calculating: snug should sit between loose and tight; leaning is usually the largest bust number; no bust number should be smaller than your underbust numbers. If something looks off, re-take that one measurement — two minutes now beats a wrong size.
- No soft tape? Use a string or cable, mark it, then measure it flat with a ruler.
- Measure 2–3 times and use the most consistent reading.
- Exhale normally before reading underbust numbers — holding your breath adds phantom inches.
- Re-measure every 6–12 months, and after weight changes, pregnancy, or nursing.
§How the math works
No black box. Exactly what this page computes from your six numbers:
- Band: your snug underbust, rounded to the nearest whole number; if that's odd, rounded up to the next even number (bra bands come in evens). The band is never set below your tight measurement.
- Cup: the average of your three bust measurements, minus the band. The difference is rounded up to the next whole inch — each inch is one cup step (1″=A, 2″=B … 5″=DD, 6″=E, 7″=F, 8″=FF, 9″=G …).
- Systems: US and UK bands share inch numbering; EU bands are numbered differently for the same body (UK 34 ≈ EU 75); French/Spanish bands are EU + 15. Cup letters are translated by the same inch-difference index.
- Sister sizes keep cup volume constant: ±2 in band, ∓1 cup step.
Two honest caveats: cup differences are rounded up, so the result can run slightly generous — if you're between cups, try both. And brands vary: the same labelled size fits differently across manufacturers, which is why this is a starting size, not a guarantee.
?Frequently asked questions
Straight answers about measuring, converting, and fitting.
Why do most old bra size calculators give me the wrong size?
Many traditional calculators still use the outdated "+4 inches" rule, which dates back to when bra fabrics barely stretched. Adding 4 inches to your underbust almost always lands you in a band that is too loose to support you, which is why bands ride up and straps dig in. This calculator skips the +4 rule entirely and starts from your actual snug underbust measurement, the approach recommended by modern fitting communities such as r/ABraThatFits.
How does the 5-measurement method work?
You take three underbust measurements (loose, snug, and tight) plus three bust measurements (standing, leaning over, and lying down). Your band size comes from the snug underbust, rounded to the nearest even number, and your cup size comes from the average of the three bust measurements minus the band, rounded up to the next whole inch. Using several measurements instead of two smooths out tape-position errors and accounts for differences in breast shape.
What are sister sizes, and when should I use them?
Sister sizes hold the same cup volume while trading band for cup: go up a band and down a cup (30F becomes 32E), or down a band and up a cup (30F becomes 28FF). They are useful when your calculated band feels too tight or too loose but the cup volume is right, or when a brand runs small or large in the band. They are not identical bras though — the band, wire width, and strap placement all change slightly, so always try them on.
How do US, UK, EU, and French bra sizes convert?
US and UK bands are both measured in inches, so a 34 band is a 34 band in either system — only the cup letters differ (a UK F is a US G). EU bands use a different numbering for roughly the same centimeters (a UK 34 band is about an EU 75), and French/Spanish bands add 15 to the EU number (EU 75 = FR 90). Cup letters shift too: a UK F equals a US G and an EU G. Use the converter tab above for exact translations.
Why is my size here different from what a store measured me?
Most in-store fittings still use the +4 method or a quick two-measurement estimate designed to fit you into the sizes the store actually stocks. That is also why many people are told they are a 36B when their measurements point to a 30F or 32E — the store simply may not carry those sizes. Treat any calculator result, including this one, as a starting point and judge the final fit by how the bra feels.
My band feels too tight. Should I just go up a band size?
Not by itself — going up a band without changing the cup makes the cups effectively smaller relative to your body. If the band is uncomfortable but the cups fit, try a sister size instead: one band up and one cup down (for example 32E becomes 34D). If both the band and the cups feel tight, go up in the band and keep the cup the same or go up one.
The center gore doesn't touch my chest. What does that mean?
In most cases it means the cups are too small — breast tissue pushes the center panel away from your sternum. Try going up one or two cup sizes in the same band. Exceptions exist: plunge bras have low gores that may never fully tack, and very close-set or very soft breast tissue can keep the gore floating even in the right size.
Why do my cups gape at the top even though the band fits?
Usually the cups are a little too big, so try one cup down. But gaping can also be a shape mismatch rather than a size problem: shallower breast shapes often gape in full or molded cups while fitting the same size in a balconette or demi-cup. If sizing down causes spillage, keep the size and change the style instead.
My band rides up my back. How do I fix it?
A riding band is the classic sign the band is too big — it should sit level and snug on the loosest hook when new. Try a sister size down in the band and up in the cup (for example 36C becomes 34D). If the band still rides up after sister-sizing, that style's band may simply run large.
My straps dig into my shoulders. Should I tighten them more?
Digging straps usually mean the band is doing too little work, so the straps end up carrying weight they were not designed for. Most of a bra's support should come from a snug band, not the straps. Try a sister size with a smaller band and a larger cup (for example 36C becomes 34D) and loosen the straps slightly instead of tightening them.
How often should I re-measure my bra size?
About every 6 to 12 months, and after any significant weight change, pregnancy, nursing, or hormonal shifts. Bands also stretch with wear — a bra that fit perfectly on the loosest hook six months ago may now need the tighter hooks or a smaller band.
Can I measure myself accurately on my own?
Yes. Use a mirror to keep the tape level around your back, breathe out normally before reading the underbust numbers, and take each measurement two or three times to check consistency. The leaning bust measurement is the easiest to do alone, since gravity does the positioning for you.
Do I need a special measuring tape?
A soft tailor's tape is ideal because it follows your body's curves, and it costs very little. In a pinch, a piece of string or a phone-charging cable plus a ruler works fine — just mark the string and measure it flat afterward.
My calculated size isn't sold in regular stores. What now?
That is extremely common, especially above a G cup or below a 32 band — most high-street stores stock a narrow middle range. Specialty online retailers and independent boutiques carry much wider ranges (28–44 bands, AA–K cups and beyond). The sister sizes in the tab above can also work as a fallback when your exact size is unavailable.
Do sports bras and strapless bras use the same size?
Start with your usual size, then adjust by style. Sports bras often run snug in the band, so some people sister-size up one band for comfort. Strapless bras rely entirely on the band, so a firm band is essential — if yours slips down, try your sister size down in the band. Nursing bras use the same sizing; measure while wearing no bra for the most useful number.
Is this a medical tool, and what happens to my measurements?
No — this is an educational fitting aid, not medical advice, and it makes no promise of a perfect fit; bodies vary and only trying bras on confirms the fit. Your measurements never leave your browser: the entire calculator runs locally on this page with no account, no tracking of your inputs, and no data sent anywhere.