Your size combines two numbers: a band measurement (underbust, rounded to the nearest even inch) and a cup letter (the difference between bust and band). Whatever you call the process — working it out, calculating it, finding it, knowing it — the method behind every phrasing is exactly the same.
The Band and Cup Formula
Measure snugly under your bust for your band number, then measure around the fullest part of your bust. Subtract band from bust for the difference in inches, then match that difference to a cup letter using the table below. Round the band to the nearest even number and you have your full UK size.
| Bust − Band | Cup Letter |
|---|---|
| 0" | AA |
| 1" | A |
| 2" | B |
| 3" | C |
| 4" | D |
| 5" | DD |
| 6" | E |
How to Calculate Your Bra Size
"Calculating" your bra size means exactly this same formula — band first, then the bust-band gap for your cup. If you'd rather skip the manual maths entirely, plug your two measurements straight into our bra size calculator and get your band, cup and sister sizes instantly.
How to Find Your Bra Size
"Finding" your size starts with the same two measurements, then a simple lookup against a size chart — see our full UK bra size chart for every band and cup combination in one table.
How to Know Your Bra Size
However you phrase it — how do I know my bra size, how do I know what bra size I am, how can I tell what bra size I am — the answer is the same. "Knowing" your size is really the same question asked a different way. Once you've measured accurately — snug band, no padding, level tape — you can trust the result with confidence, and you'll know your right bra size the same way every time you check it again.
How to Figure Out or Determine Your Bra Size
Figuring out or determining your bra size uses the identical two-step process: band measurement, bust measurement, subtract, match to a letter. If you're asking how to determine bra size and cup at the same time, that's exactly what the difference step above does — one calculation gives you both. Don't guess your bra size by eye; even a rough guess is usually off by a full cup or more, since band and cup aren't something most people can judge accurately without a tape.
How to Check or Get the Right Bra Size
Checking or getting your size right means going one step further than the calculation: trying the bra on and confirming the fit. If you want to check bra size against a bra you already own, compare the label size to your calculated result — a mismatch usually means the label has stretched with wear rather than that you measured wrong. Read our signs your bra doesn't fit guide to know exactly what to look for once you try it on.
What If My Measurements Don't Match a Standard Size?
Round the band to the nearest even number and the cup to the nearest whole inch of difference. If you're still between two results, a sister size — same cup volume, different band-cup combination — is usually the better starting point. See our sister sizes guide to find yours.
Frequently Asked Questions
Can I work out my bra size without a tape measure?
A flexible tape gives the most accurate result, but a piece of string or ribbon marked against a ruler works as a backup. Avoid rigid tape measures — they won't curve properly around your ribcage or bust.
Why do I get a different result every time I measure?
Small variations of half an inch are normal and usually don't change your size. Bigger swings usually mean the tape wasn't level, was pulled too tight, or you measured over a padded bra.
Is an online result as reliable as a professional fitting?
An online result is accurate as a starting point using the same formula a fitter would use. A hands-on fitting adds one extra check — how the bra actually sits and moves on your body — which a calculation alone can't confirm.
What if my measurements land between two sizes?
Round the band to the nearest even number and the cup to the nearest whole inch of difference. If you're still between two results, try the closest sister size first.
What's a sister size, and why does it matter?
A sister size holds the same cup volume as your true size but in a different band-cup combination — for example, 34C and 36B. It's the closest match when your exact size is out of stock or doesn't exist in a brand's range.
Ready to find your perfect bra size? Try our free UK calculator.
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