Bra Size Calculator
Find your bra size from two measurements: your underbust and your bust size. Enter them in inches or centimetres and this bra cup size calculator gives you a band size, a cup size, your sister sizes, and the bras Raks actually lists in that size.
Bra size chart — every size we stock
Bands down the side, cups across the top. Tap any size to see the bras listed in it.
Sister sizes
Sister sizes sit diagonally from each other on this chart — one band down is one cup up, so 34C, 32D and 36B all hold about the same amount. They are what to try when the cup fits but the band does not.
| Band | A | B | C | D | DD | E | F | G |
|---|---|---|---|---|---|---|---|---|
| 32 | 32G | |||||||
| 34 | ||||||||
| 36 | ||||||||
| 38 | ||||||||
| 40 | ||||||||
| 42 | 42A | |||||||
| 44 | 44A | |||||||
| 46 | 46A |
- Your size
- Sister size
- Available — tap to shop
- Not stocked
How to measure your bra size at home
You need a soft tape measure and a mirror. It takes about five minutes.
Take your bra off, or wear a soft unpadded one
A padded or push-up bra adds to your bust measurement, which lands you in a cup size larger than you need.
Measure your underbust
Run the tape directly under your bust, where the band of a bra sits. Pull it snug — firm enough that it stays level when you lower your arms — and breathe out. Note the number.
Measure your bust
Run the tape around the fullest part of your bust, keeping it level across your back. This one stays loose: it should rest on you without pressing in.
Enter both numbers above
The calculator turns your underbust into a band size and the difference between the two into a cup size, and shows the bras listed in it.
Bra measurement chart
The same two steps the calculator runs, written out, so you can work a size out from a tape measure alone. Your underbust gives the band; the difference between your bust and that band gives the cup.
Underbust → band
Add 4 inches, round up to even
| Inches | cm | Band |
|---|---|---|
| 27-28 | 69-71 | 32 |
| 29-30 | 74-76 | 34 |
| 31-32 | 79-81 | 36 |
| 33-34 | 84-86 | 38 |
| 35-36 | 89-91 | 40 |
| 37-38 | 94-97 | 42 |
| 39-40 | 99-102 | 44 |
| 41-42 | 104-107 | 46 |
Bust − band → cup
One inch of difference is one cup letter
| Inches | cm | Cup |
|---|---|---|
| 1 | 3 | A |
| 2 | 5 | B |
| 3 | 8 | C |
| 4 | 10 | D |
| 5 | 13 | DD |
| 6 | 15 | E |
| 7 | 18 | F |
| 8 | 20 | FF |
| 9 | 23 | G |
One inch is 2.54 cm. Raks lists cups A, B, C, D, DD, E, F, G — DD sits between D and E, which is the UK ladder these brands are labelled on. A dashed cup above is a real size on that ladder that Raks does not carry.
Signs your bra is the wrong size
A tape measure gets you close. The mirror confirms it. Check these after the bra is on and settled, with the straps adjusted.
The band rides up your back
The band is too loose.
Go down a band and up a cup — 38C becomes 36D.
The band digs in or leaves deep marks
The band is too tight.
Go up a band and down a cup — 34D becomes 36C.
Your breast spills over the top or the side
The cup is too small.
Keep the band, go up one cup letter.
The cup wrinkles or gapes at the top
The cup is too large.
Keep the band, go down one cup letter.
The straps carry the weight, or slip off
The band is doing too little.
The band should carry most of the support. Try a band size down.
The centre panel does not sit flat against you
The cup is too small, or the shape is wrong for you.
Try a cup up first; if it still lifts away, try a different style.
How accurate is this calculator?
The maths is exact: the same two measurements always give the same band and cup, and this page, the chart above and the size filters on the shop all run off one calculation. What a tape measure cannot capture is breast shape, or how a particular style is cut — two bras in the same size from different brands will not fit identically.
So treat your result as the size to try first. If the band is right and the cup is not, move a cup letter. If the cup is right and the band is not, move to a sister size. Raks exchanges unworn items with tags on within 15 days.
Frequently Asked Questions
How do I measure my bra size at home?+
You need a soft tape measure and two numbers. First measure your underbust, directly beneath your bust where the band sits, pulled snug and level. Then measure your bust across the fullest part, keeping the tape level and loose enough not to press in. Enter both into the calculator on this page and it gives you a band and cup size. Measure without a padded bra on, or the bust number comes out too large.
What is my bra size if my underbust is 30 inches and my bust is 36 inches?+
That is a 34B on the sizing this calculator uses. The band comes from the underbust: 30 inches is even, so 4 is added to give a 34 band. The cup comes from the difference between the bust and the band — 36 minus 34 is 2 inches, which is a B cup.
How is a cup size worked out?+
Each inch of difference between your bust measurement and your band size is one cup letter. No difference is AA, 1 inch is A, 2 inches is B, 3 inches is C, 4 inches is D, 5 inches is DD, 6 inches is E, 7 inches is F. The cup letter on its own means nothing without the band — a 32D and a 40D hold very different amounts.
What is a sister size?+
A sister size is a different band with a cup that holds about the same amount. Go down one band and up one cup, or up one band and down one cup: 36C, 34D and 38B are all sister sizes. They are the first thing to try when the cup fits but the band does not — if the band rides up your back, go down a band and up a cup; if it digs in, go up a band and down a cup.
Does this calculator work in centimetres?+
Yes. Switch the toggle to centimetres and enter both measurements in cm. The calculation is done in inches internally, because band and cup sizes are inch-based, and one inch is 2.54 cm.
Is a bra size calculator accurate?+
It gives you a reliable starting point, not a guarantee. A tape measure captures two numbers; a bra also has to suit your breast shape, and cut varies between brands and between styles from the same brand. Treat the result as the size to try first, check the fit against the signs listed on this page, and use your sister sizes if the band is right but the cup is not, or the other way round.
What bra sizes does Raks carry?+
Raks lists bands 32 to 46 and cups A, B, C, D, DD, E, F, G, though not every style is made in every combination. The calculator shows you which bras are actually listed in your size, so you see the real range rather than a chart.
What if the size I calculate does not fit when it arrives?+
Raks exchanges unworn items with their tags on within 15 days, so a size that turns out wrong can be swapped. Getting the measurement right first is still cheaper for everyone than an exchange.