Fit & sizing

Bra Size Calculator: US, UK, EU & AU/NZ Conversions

Find your bra size from band and bust. Converts to US, UK, EU, AU/NZ. Sister sizes and fit guide.

By Jeff Beem

Updated May 27, 2026

Bra size calculator

Units

Band size

Under-bust measurement, snug and straight. Rounded to nearest even number.

Bust size

Fullest part, tape parallel to floor.

Show legacy (+4) sizing

Your size (US)

34C

Global sizes

Standard US

34C

Standard UK

34C

Standard EU

85C

FR/BE/ES

EU +15

100C

AUNZ

Dress size

12C

UK Dress

Dress size

12C

EU band in cm; US/UK in inches. FR/BE/ES = EU + 15. UK uses double letters (DD, FF, GG).

Sister sizes

32D Down band, up cup36B Up band, down cup

Information hub

How to measure

The tape must be parallel to the floor. Use a mirror to ensure the back isn't sagging, this is the #1 cause of incorrect measurements.

Band

Tape straight across your back, under the bust. Snug but not digging in.

Bust

Over the fullest part (nipples), tape parallel to the floor.

The sister size secret

Same cup volume, different band. Going up a band and down a cup (or vice versa) can find a more comfortable fit.

Example: 34C, 36B, and 32D hold the same cup volume. If your size feels off, try a sister size. Your calculated sister sizes appear in the results card.

The 80% problem

~80% of people wear the wrong bra size. Proper fit: the band provides 80% of the support and the straps only 20%. If the straps are doing the work, the band is too big.

Common signs of a bad fit:

Straps digging into shoulders
Band sliding up your back
Spillage or gaping at the cups
Red marks that linger after removing

If any of these sound familiar, remeasure and try this calculator. Sizing varies by brand, use this as a starting point.

Why is my size different in-store?

Some traditional brands add 4 inches to your band measurement (the "Plus 4" method). Our calculator uses modern industry standards for better support.

If a brand feels too tight, try your sister size instead, same cup volume, different band.

Shape matters more than size

Measurements only account for volume. Factors like breast "projection" (full on bottom vs. full on top) will affect how a cup fits.

If you have gapping at the top but the wires fit, try a balcony or demi-style cup.

Reference tables

Band size mappings across regions (XXS to 5XL). FR/BE/ES = EU + 15.

Size	FR/BE/ES	EU (cm)	US/UK (in)	AU/NZ
XXS	75	60	28	6
XS	80	65	30	8
S	85	70	32	10
M	90	75	34	12
L	95	80	36	14
XL	100	85	38	16
2XL	105	90	40	18
3XL	110	95	42	20
4XL	115	100	44	22
5XL	120	105	46	24

Cup	US/UK (in)	EU (cm)
AA	<1"	<2 cm
A	1"	2 cm
B	2"	4 cm
C	3"	6 cm
D	4"	8 cm
DD	5"	10 cm
DDD/E	6"	12 cm
F	7"	14 cm
G	8"	16 cm
H	9"	18 cm
I	10"	20 cm

34″ band, 37″ bust → about 34C

Three inches over the band is a C cup in US inch sizing after the band rounds to an even 34. Change the tapes and the letter moves; change only the band and sister sizes (36B, 32D) keep similar cup volume. Brands still disagree on labeling above D, which is why the page shows US, UK, EU, FR/BE/ES, and AU/NZ in one place.

Four things worth knowing

Cup is a difference, not a bust measurement

A 37″ bust with a 34″ band is not a “37 cup.” Subtract band from bust: 3″ → C in inch mode. In centimeter mode the tool steps cups every 2 cm on the same difference. Wrong tape level (belly button vs underbust) is the usual reason the math feels “off.”

Sister sizes trade band for cup

Going from 34C to 36B adds band tightness relief without changing volume much. The calculator suggests neighbors when your band is in range (roughly 30–42). It is not a full matrix of every combination, just the one-step swaps people use in stores.

Legacy +4 is optional for a reason

Some older labels sized the band looser than your underbust tape. The checkbox shows that alternate US/UK label without making it the default. If modern sizing feels tight, sister-size up before jumping to legacy numbers.

Style still beats the chart

Full-on-top vs full-on-bottom tissue changes whether a balconette or full-coverage cup lies smooth. The fit checklist in the tool (straps digging, band riding, spillage) is about symptoms, not breast shape labels. No chart replaces trying the bra on.

Bra size calculator: band, bust, and regional labels

34″ underbust and 37″ bust → about 34C in US inch mode. Sister sizes, optional legacy +4, and five regional label styles. Starting point only; try on before you buy.

What this calculator does

You type underbust (band) and fullest bust. It rounds the band to an even inch for US/UK-style numbers, maps the bust-minus-band gap to a cup letter, and prints labels for US, UK, EU (5 cm band steps), FR/BE/ES (EU band +15), AU/NZ, plus UK dress-style band in the global panel.

Outputs:

Primary US size, regional variants, optional legacy (+4) US/UK, one-step sister sizes, cup/band reference tables, and a short fit symptom checklist in the widget.
Limits:

No brand fit database, no breast shape model, no guarantee a mail-order size fits. Padding, stretch fabrics, and label drift above D/DD still require try-on. Runs in the browser only.

The math

Cup steps come from bust minus band after the band is rounded for labeling.

Inch mode:

$\text{cup index} \approx \lfloor \text{bust} - \text{band} \rfloor$
Under 1″ difference → AA. 1″ → A, 2″ → B, 3″ → C (e.g. 37 − 34 = 3 → 34C).
Metric mode:

Same inputs converted to inches internally; cup steps use about 2 cm per letter on the difference (EN 13402 style). Under 2 cm → AA.
Band label:

Measured underbust rounds to the nearest whole inch, then odd values bump up to the next even band (33″ tape → 34 band).
Sister sizes:

34C ↔ 36B ↔ 32D: +2 band with −1 cup step (or the reverse) holds volume roughly steady.
Legacy (+4):

Displayed band = measured even/odd underbust + 4″ or +5″; cup uses bust minus that looser band. Off by default.
Regions:

EU band ≈ US band × 2.54 rounded to 5 cm steps (60–105). FR/BE/ES = EU band + 15. AU/NZ dress band ≈ US − 22. UK cup letters diverge from US above D (DD vs DDD, E vs G, etc.).

Using the calculator

Measure twice, average, same time of day if you are tracking changes. Use the fit checklist when something feels wrong even though the letter matches the chart.

Band tape:

Level, snug, directly under breast tissue.
Bust tape:

Fullest point, parallel to the floor, no crushing.
When sizes disagree:

Try sister size first, then another brand. Toggle legacy only if that brand documents +4 sizing.
Privacy:

Measurements stay on your device.

Bra Size Calculator FAQ

How do I measure my band size?

Snug tape directly under the bust, level around your back. Round to the nearest even inch for US/UK band labels (this tool bumps odd numbers up to the next even). If you land between sizes, try the sister size one band up or down.

How do I measure my bust size?

Over the fullest part of the chest, usually nipple height, tape parallel to the floor. Wear a soft unpadded bra or none. The tape should sit without squeezing tissue flat.

Why is bust larger than band?

Band is under the breasts; bust is the fullest circumference. Cup comes from the gap:

\text{cup step} \approx \text{bust} - \text{band}

in inches (about 1″ per letter) or

2\,\text{cm}

per step in metric mode (EN 13402 style).

What are sister sizes?

Same cup volume, different band. Too tight in 34C? Try 36B. Cups gaping? Try 32D. The results card lists one step up-band/down-cup and the reverse when your measurements allow it.

Do brands match this number?

Rarely exactly. Treat the output as a starting point and try on. UK labels run DD, E, F, FF where US lines often use DD, DDD, G; the global sizes panel shows both conventions.

What does the AA note mean?

Under 1″ difference in inch mode (under 2 cm in metric mode) maps to AA. Many stores do not stock AA; bralettes or brands that specialize in small cups are the usual workaround.

What is legacy (+4) sizing?

Older US methods add 4″ (or 5″ from an odd underbust) to the measured band before picking a cup. Toggle it in the calculator to compare. If the modern size feels tight, try a sister size before assuming you need legacy math.

Mathematical Reference Note

Calculation Logic: This tool uses standard mathematical algorithms. While we strive for accuracy, errors in logic or user input can result in incorrect data.

Verification: Results should be cross-checked if used for important academic, professional, or personal calculations.

Standard Terms: This tool is provided free of charge and as-is. CalcRegistry provides no warranty regarding the accuracy or fitness of these results for your specific needs.

Size	FR/BE/ES	EU (cm)	US/UK (in)	AU/NZ
XXS	75	60	28	6
XS	80	65	30	8
S	85	70	32	10
M	90	75	34	12
L	95	80	36	14
XL	100	85	38	16
2XL	105	90	40	18
3XL	110	95	42	20
4XL	115	100	44	22
5XL	120	105	46	24

Size	FR/BE/ES	EU (cm)	US/UK (in)	AU/NZ
XXS	75	60	28	6
XS	80	65	30	8
S	85	70	32	10
M	90	75	34	12
L	95	80	36	14
XL	100	85	38	16
2XL	105	90	40	18
3XL	110	95	42	20
4XL	115	100	44	22
5XL	120	105	46	24

Size	FR/BE/ES	EU (cm)	US/UK (in)	AU/NZ
XXS	75	60	28	6
XS	80	65	30	8
S	85	70	32	10
M	90	75	34	12
L	95	80	36	14
XL	100	85	38	16
2XL	105	90	40	18
3XL	110	95	42	20
4XL	115	100	44	22
5XL	120	105	46	24