How to use it
- Measure snugly around your ribcage, just under the bust, after breathing out.
- Measure loosely around the fullest part of the bust.
- Enter both measurements in inches.
- Read your US size, other systems and sister sizes.
How to measure
Wear a non-padded bra or none, and use a soft tape measure.
- Underbust (band): wrap the tape around your ribcage directly under the bust. Keep it level and snug, and measure after breathing out.
- Bust: wrap it around the fullest part of the bust, level across the back, without squeezing. Leaning forward slightly can help if you have softer breast tissue.
Round to the nearest half inch. The calculator converts your measurements to centimeters internally and works out the band and cup with the European EN 13402-3 method, then converts to US and other systems.
How band and cup sizes are worked out
- Band: the underbust is rounded to the nearest European band size (5 cm steps: 65, 70, 75 …), which converts to a US/UK band (30, 32, 34 …).
- Cup: the difference between bust and underbust, in 2 cm (about 3/4 in) steps: AA 10–12 cm, A 12–14, B 14–16, C 16–18, D 18–20 cm, and so on.
Example: underbust 29 in, bust 35 in. 29 in is about 74 cm, which rounds to EU band 75 = US 34. The difference is 6 in (15.2 cm), a B cup. Result: 34B (EU 75B, UK 34B, France 90B).
US bands are traditionally a few inches larger than the underbust measured in inches; that's why a 29 in ribcage maps to a 34 band in the standard conversion tables. Some brands now size closer to the actual measurement, so if the band rides up, try a band size down.
Size conversion chart
| US | UK | EU | France, Belgium, Spain | Italy |
|---|---|---|---|---|
| 32A | 32A | 70A | 85A | 2A |
| 34B | 34B | 75B | 90B | 3B |
| 34C | 34C | 75C | 90C | 3C |
| 36D | 36D | 80D | 95D | 4D |
| 36DD/E | 36DD | 80E | 95E | 4E |
| 38DDD/F | 38E | 85F | 100F | 5F |
Up to D, cup letters match across US, UK and EU. After that they split: the US uses DD (or E), DDD (or F), then G, H…; the UK uses DD, E, F, FF, G, GG…; the EU continues E, F, G…. Always check the brand's own chart.
Sister sizes
Sister sizes hold about the same cup volume on a different band. Going one band size up and one cup letter down (or the reverse) keeps the volume: 34B, 32C and 36A are sister sizes.
- Band too tight, cups fit: go up a band and down a cup (34C → 36B).
- Band too loose, cups fit: go down a band and up a cup (34C → 32D).
The calculator lists your sister sizes, with your own band highlighted.
Fit check
A well-fitting bra:
- has a band that sits level and snug, about the width of two fingers under it at most; the band gives most of the support;
- has cups that hold all the breast tissue without gaping or spilling over;
- has a center panel that sits flat against your breastbone (on underwire styles);
- has straps that stay put without digging in.
New bras should fit on the loosest hook, so you can tighten them as the band stretches. Sizes vary between brands and styles, so treat the result as a starting point and try on when you can.
Frequently asked questions
How do I calculate my bra size?
Measure your underbust and bust in inches and enter both. The calculator finds the band from the underbust and the cup from the difference.
What is my bra size if my underbust is 30 and my bust is 36?
Enter them in the calculator: a 6 in difference is about a B cup, and a 30 in underbust maps to about a 34 band in standard conversion tables, so 34B is a typical starting point.
Are US and UK bra sizes the same?
Bands are the same and cups match up to D. Above D they differ: US DDD/F is about UK E, for example.
What are sister sizes?
Sizes with about the same cup volume on a different band: one band up and one cup down, or one band down and one cup up.
Why does my size differ between brands?
Brands use different fits, cutting and sizing methods. Use the result as a guide and check each brand's chart.
Last updated: November 12, 2026