About the Bra Size Calculator
A bra size has two parts: the band, which is the number, and the cup, which is the letter. Both come from two measurements taken with a soft tape: around your ribcage just under the bust, and around the fullest part of the bust. This calculator turns those two numbers into a US size, the UK equivalent and the EU band, and it shows the difference in inches that decides the cup. Type your underbust and bust measurements, in inches or centimetres and the bra size appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Bra Size Calculator
- 1Enter your underbust and bust measurements, in inches or centimetres.
- 2Pick the units or options from the menus if the bra size tool offers them.
- 3Read the bra size in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the bra size
Many people wear the wrong size because they have never measured, or because sizes change with brands. A calculation gives you a sensible size to try first, which saves time in a shop or when ordering online. It is especially useful after weight change, pregnancy or sports.
Measure without a bra or in a thin, unpadded one. Keep the tape level and snug but not tight for the underbust, and loose enough not to squash for the bust. Stand upright, breathe out, and take each measurement twice.
How the bra size is worked out
Band = underbust rounded to the nearest even number of inches. Cup = bust − band, with one cup letter per inch of difference: 1 inch is A, 2 is B, 3 is C, 4 is D, and so on. With an underbust of 32 inches and a bust of 36 inches, the band is 32 and the difference is 4 inches, so the size is 32D in both US and UK sizing. An underbust of 75 cm and a bust of 92 cm gives a band of 30 and a difference of 6 inches, which is 30DDD in the US and 30E in the UK; the EU band is 75.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- US, UK and EU sizes
- Inches or centimetres
- Cup difference shown
Mistakes to avoid with bra size
The result is a starting point only. Brands cut differently, and sister sizes, such as 34C and 32D, hold the same cup volume. If the band rides up or the cups gape, move to a neighbouring size. A professional fitting is best, particularly for large cup sizes.
On phone and computer
On a phone, type into the boxes one after another and read the bra size underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the bra size move. The bra size calculator works the same on both. Fitting is easier when you know your other measurements. Use Height Converter for centimetres and inches, BMI Calculator and Body Fat Calculator if you are tracking health goals, and Shoe Size Converter when you are buying clothes from another country.
Features
- Keep the tape level
- Try sister sizes
- Brands differ
- Clear messages when the bra size inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate bra size?
Band = underbust rounded to the nearest even number of inches. Cup = bust − band, with one cup letter per inch of difference: 1 inch is A, 2 is B, 3 is C, 4 is D, and so on. With an underbust of 32 inches and a bust of 36 inches, the band is 32 and the difference is 4 inches, so the size is 32D in both US and UK sizing. An underbust of 75 cm and a bust of 92 cm gives a band of 30 and a difference of 6 inches, which is 30DDD in the US and 30E in the UK; the EU band is 75.
What is the formula for bra size?
Band = underbust rounded to the nearest even number of inches. Cup = bust − band, with one cup letter per inch of difference: 1 inch is A, 2 is B, 3 is C, 4 is D, and so on. The bra size calculator applies it exactly and rounds only the displayed answer to six significant figures.
What is a sister size?
A size with the same cup volume but a different band. Going up a band size and down a cup size, or the other way round, gives a sister size.
Why do UK and US cups differ?
The two systems use different letters after D. US goes DD, DDD, G and UK goes DD, E, F, FF, G.
Is this a medical tool?
No. It is a sizing guide for clothing, and a fitter can give you a more accurate result.
Is this bra size calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.