How to measure your bra size

You need two numbers: your underbust and your bust, in centimeters. The European standard for bra labels, EN 13402-3, turns them into a size with two rules. The band is the underbust rounded to the nearest 5 cm. The cup is set by the difference between the two measurements.

An underbust of 76 cm and a bust of 92 cm make a 75C, which is a 34C in the UK and the US. Below: the steps, the arithmetic, sister sizes, and what a store that sells bras should show.

What you need

A soft tape measure, a mirror, and an unpadded bra (or no bra). Padding adds to the bust measurement and moves the cup. No tape? Use a strip of paper or a string, mark where it meets its end, and measure the mark flat against a ruler.

How to measure, step by step

1. Stand straight, arms down, and breathe out normally. Don't hold your breath or pull your stomach in: the band sits on your ribcage while you breathe.
2. Underbust. Wrap the tape snugly around your ribcage, just under your bust, where a bra band sits. Keep it level all the way around, and check in the mirror that it doesn't ride up at the back. Snug means flat on the skin, not digging in.
3. Read the underbust where the tape meets its start, and write it down. If your tape only has inches, multiply by 2.54.
4. Bust. Wrap the tape around the fullest part of your bust, tape level. This time it rests on you without pressing.
5. Read the bust and write it down.
6. Measure both twice. If the readings differ, use the average.

From two measurements to a size

EN 13402-3 works in centimeters, and the arithmetic fits on a sticky note.

Band: your underbust, rounded to the nearest 5 cm. An underbust from 72.5 up to 77.4 cm is a 75 band; from 77.5 cm it's an 80. UK and US bands are in inches and go up by 2 for every 5 cm; French bands are the EU band plus 15.

Cup: your bust minus your underbust. A difference of 10 to 12 cm is AA, then one letter for every 2 cm. The letters part ways after D: the UK goes D, DD, E, F, FF; the US goes D, DD, DDD, G.

The difference uses the underbust you measured, not the rounded band. Near a boundary that changes the letter (see the mistakes below).

A worked example

Underbust 76 cm, bust 92 cm.

• Band: 76 rounds to 75.
• Cup: 92 minus 76 is 16 cm, which falls in C (16 to 18).

That's a 75C in EU sizes. Ruban Size's engine writes the same size in seven systems:

Sister sizes

A sister size keeps the same cup volume on another band: one band down and one cup up, or one band up and one cup down. The sisters of a 75C are 70D and 80B (32D and 36B in the UK and the US). A cup letter isn't a fixed volume: a C on a 70 band holds less than a C on an 80 band, which is why the letter moves when the band does.

When to use them: if the band feels loose on the tightest hook, try the sister one band down (70D). If the band digs in but the cups fit, try one band up (80B). There's more in bra sister sizes explained.

Try your own numbers here. The calculator applies the same EN 13402-3 rules and lists the sisters.

It also has its own page, the bra size calculator, and every system is side by side in the bra size conversion chart.

Common mistakes

• A loose underbust. The band does most of the support, so a tape that hangs or rides up at the back gives a band that's too big.
• Measuring in a padded bra. Padding adds centimeters to the bust, and a letter or more to the cup.
• Adding inches to the band. Some older methods add a few inches to the underbust before picking the band. EN 13402-3 doesn't: it rounds the underbust itself.
• Subtracting the rounded band. With a 77 cm underbust and a 91 cm bust, the band is 75 and the difference is 14 cm: a 75B. Subtract the band instead (91 minus 75 is 16) and you get a C, one cup too big.
• Reading UK and US cups as the same past DD. A UK 34E is a US 34DDD and an EU 75F. Check which system a label uses.
• Measuring once for months. Bodies change. For a maternity bra, measure from the second trimester: the band grows with your ribcage, the cup with your bust.

What it means for a Shopify store

A bra chart has a different shape from a clothing chart. One band covers several cups, and each cup has its own bust range. Shoppers arrive with a size in their own system (a 34C from the US, a 90C from France) or with two measurements, and both should lead to the same answer.

So show the underbust and bust ranges for every band and cup, in centimeters with a switch to inches, and the size in the systems your shoppers use. Brands don't all cut exactly to the standard, so the chart you print is the one that counts: if your 75C covers a different bust range, the table should say so.

Two Ruban Size templates start you there. The Bras template has bands 70 to 80 and cups A to D, with each band cell merged across its cups. Maternity and nursing bras has bands 75 to 90 and cups B to E, with a note to measure from the second trimester and to choose a fit that is comfortable on the loosest hook. Their values are typical ones: put your own before publishing.

How Ruban Size handles bras

On a bra chart, Find my size asks for two measurements, Underbust and Bust, and reads the table as bands and cups, merged cells included. The answer is a size from your chart. On a chart where those measurements land in 75C, it reads "Your size: 75C", followed by two lines:

• "The same size in other systems: FR 90C · IT 3C · UK 34C · US 34C · AU 12C · JP C75"
• "Sister sizes (same cup volume): 70D · 80B"

If your sizes aren't bra sizes (S, M and L for bralettes, say), the first line becomes "Your bra size from these measurements (EN 13402)", worked out with the rules above.

A few details fit bras in particular. A band that's too loose counts against a size as much as one that's too tight (on clothes, too tight weighs more). "Fit I like: Snug, Regular, Loose" moves the target half a centimeter either way. Size systems (EU, UK, US...) sit on their own rows, with a switch for shoppers and a default by country: US for the United States, UK for Great Britain and Ireland, EU everywhere else. Rings, shoes, belts and dogs get their own rules too, described on fit by product type.

Questions

How do I measure my bra size at home?

Measure your underbust snugly and level just under your bust, then your bust at its fullest point, in an unpadded bra. Round the underbust to the nearest 5 cm for the band; the bust minus the underbust gives the cup, AA from 10 cm and one letter for every 2 cm.

What are my bra sister sizes?

One band down and one cup up, or one band up and one cup down. The sisters of a 75C are 70D and 80B, which are 32D and 36B in the UK and the US.

Is a UK E cup the same as a US E cup?

No. US letters go D, DD, DDD, then G, so a UK 34E is a US 34DDD and an EU 75F. Up to DD, UK and US letters match.

Should I measure my bust with a bra on?

Wear an unpadded bra, or none. A padded or push-up bra adds to the bust measurement and gives a cup that's too big.