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Bra Size Calculator

Estimate a starting bra size from underbust and bust measurements, with US, UK, EU and Australian equivalents and sister sizes.

Level: Grades 4–5

Printable worksheet

Bra Size Calculator

Inputs
Measure in

Snug around the ribcage, just under the bust, tape level and breathing out.

Loosely around the fullest part of the bust, tape level.

Result

US size

32D

Measurements of 31.0 in under the bust and 36.0 in at the bust suggest a US 32D, UK 32D, EU 70D.

UK size
32D
EU size
70D
Australia / NZ size
10D
US sister sizes (same cup volume)
30DD and 34C
Band size
32
Bust minus band
4 in

See the step-by-step solution ↓

Step-by-step solution

  1. Band: round the underbust to the nearest inch. Bands come in even numbers, so move up to the next one

    31 in=31=31+1=band 32

  2. Cup: subtract the band from the bust and round to whole inches. Each inch of difference is one cup size

    36−32=4=≈ 4 → cup D

  3. Convert for other countries. The UK keeps the same band but uses its own letters after D. EU bands are 2.5 × band − 10, in cm. Australian bands are the band minus 22

    2.5×32−10=EU band 70

  4. Put the band and cup together

    US size=32D

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find a good starting bra size from two tape-measure readings, with the matching UK, EU and Australian sizes. A bra size has two parts. The band number is the size of the ribcage. The cup letter shows how much larger the bust is than the band. You take one reading snug under the bust and one around its fullest part. Different countries write the same fit in different ways, which makes ordering from abroad confusing.

This calculator turns your two measurements into a starting US size. It also shows the UK, EU and Australian sizes, and your sister sizes. Those are sizes with the same cup volume on a different band. Treat the answer as a place to start trying on, not a final verdict. Brands, styles and fabrics all fit differently.

Where it's used

Why this math matters outside the classroom:

  • Online shopping. Shoppers measuring 31 in under the bust and 36 in at the bust start with a US 32D.
  • Ordering abroad. Buyers convert a US 34C into an EU 75C by using 2.5 × 34 − 10 = 75 for the band.
  • Fitting rooms. Fitters suggest the sister size 34C when a 32D band feels too firm.
  • Rounding practice. Students round a 33.4-inch measurement to 33, then up to the even band 34.

How to use it

  1. Choose inches or centimeters.
  2. Measure your underbust. Wrap the tape snugly around your ribcage just below the bust, level all the way round, and breathe out.
  3. Measure your bust. Wrap the tape loosely around the fullest part and keep it level. Wear a non-padded bra, or none.
  4. Read the US size and its equivalents. If a band feels tight or loose, try a sister size.

What the inputs mean

Underbust
Measure snugly but comfortably. The band does most of the supporting, so a slightly firm measurement is more useful than a loose one.
Bust
Stand straight and keep the tape level across your back. If the tape slopes down a little, it adds an inch or more.

Formula and method

band = underbust rounded to the nearest inch, then up to the next even number if odd
cup index = round(bust − band), in inches: 0 = AA, 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD …
EU band = 2.5 × band − 10   (cm steps of 5: 32 ↔ 70)
Australia / NZ band = band − 22   (32 ↔ 10)
sister sizes: band − 2 with one cup up, band + 2 with one cup down

In these lines, band is the band number, underbust and bust are your two readings, and cup index counts cup sizes from AA. round( ) means round to the nearest whole number.

US and UK bands are even numbers in inches. The calculator rounds the snug underbust to the nearest inch. If that's odd, it moves up to the next even number. Some brands still add 4 or 5 inches to the underbust. That older method usually gives a band that's too loose, and this calculator doesn't use it.

Each inch of difference between the bust and the band is one cup size. Up to D, the letters match everywhere. The US and UK also share DD, while EU charts call that cup E. After that the lists split. US charts often go on DD, DDD, G, H. UK charts go DD, E, F, FF, G, GG, and EU charts go E, F, G, H. The calculator uses these common chart sequences.

EU bands are in centimeters, in 5 cm steps (65, 70, 75 …). Each one matches a 2-inch US or UK band. Australian and New Zealand bands use dress-size numbers, 22 less than the UK band.

A cup letter only means something together with its band. Moving the band down one size and the cup up one keeps about the same cup volume. That's why 30DD, 32D and 34C are called sister sizes.

Worked example

Someone measures 31 inches under the bust and 36 inches around the fullest part.

  1. Band: 31 is odd, so it rounds up to 32.
  2. Cup: 36 − 32 = 4 inches, which is a D cup.
  3. UK: same band and letter, 32D. EU: 2.5 × 32 − 10 = 70, so 70D. Australia: 32 − 22 = 10, so 10D.
  4. Sister sizes: 30DD (smaller band) and 34C (larger band).

Result: A starting size of US 32D, which is UK 32D, EU 70D and Australia 10D, with sister sizes 30DD and 34C.

Understanding the result

A well-fitting band sits level and stays put without riding up. If it feels tight on the loosest hook, try the next band up and one cup down (a sister size).

If the cups gape or wrinkle, try one cup smaller. If the bust spills over the top or sides, try one cup larger.

Many people find their size changes with weight, hormones, pregnancy and age. So it's worth measuring again every so often.

Assumptions and limitations

  • Measurements are taken with a flexible tape, level around the body.
  • Band sizing follows the modern method (underbust rounded to an even number). Cup letters follow common US, UK and EU chart sequences.
  • Brands and styles size differently, so the result is a starting point only.
  • Covers bands 28 to 48 and cup differences up to 10 inches (US J). Outside that range, a specialist fitting is the better guide.
  • Breast shape, spacing and fullness affect fit, and two measurements can't capture them.

Frequently asked questions

Why is my size different from what I usually buy?

Many stores and older charts add 4 or 5 inches to the underbust to get the band. That gives a larger band and a smaller cup letter. Others size by their own patterns. Use the result here as a starting point, and adjust by sister sizes.

What are sister sizes?

Sizes with about the same cup volume on a different band: one band size down and one cup up, or one band up and one cup down. 32D, 30DD and 34C are sisters.

Is a UK DD the same as a US DD?

Yes, up to DD the letters match. After that they differ: a US DDD is usually a UK E and an EU F. The calculator shows each region's letter so you can order from any of them.

How do I convert a UK size to a US size?

The band number stays the same. For cups up to DD, the letter also stays the same. Above DD, match the cup step rather than the letter: UK E is US DDD, and UK F is US G.

How this calculator is checked

Method: Common underbust-to-even-band method with one cup per inch of bust-minus-band difference; regional cup letter sequences and EU (5 cm) and Australian band conversions from widely used size charts. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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