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Category Women\'s Health

Measurements

Unit System

Inches (in) Metric (cm)

Band / Underbust

Snug fit, exhale before measuring.

Bust / Overbust

Fullest part, usually across nipples.

1. Where is your breast tissue fuller?

2. When leaning forward (90°), your shape is:

3. Root Width (Where tissue ends on sides):

∅

Enter your measurements or take the shape quiz to see your personalized fit profile.

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About

Finding the correct bra size is a combination of art and engineering. Most women wear the wrong size due to outdated measuring techniques (such as the "+4 method") and inconsistent sizing standards across brands. This tool utilizes the Modern Fit Method, which prioritizes a snug band for support and calculates cup volume based on the relative difference between the bust and underbust measurements.

Accuracy depends on precision. A bra's band provides 80% of the support, meaning the Band measurement is the critical anchor. The Cup is not a static size but a ratio; a 34C has a smaller volume than a 36C. This calculator accounts for these volumetric relationships, offering Sister Sizes (volume equivalents) to provide options when a specific brand fits tighter or looser than standard.

Formulas

The core algorithm relies on two primary variables: the Snug Underbust (U) and the Standing/Leaning Bust (B).

{

Band ≈ RoundEven(U) for US/UKBand ≈ RoundFive(U_cm) for EU

The Cup Size is derived from the delta:

Index = B − Band

Where the Index maps to a regional alphanumeric sequence (e.g., 1="A", 5="DD"). Sister sizes utilize the conservation of volume principle:

Vol(Band, Cup) ≡ Vol(Band + 2, Cup - 1)

Reference Data

Measurement Diff (Inch)	Measurement Diff (CM)	US Standard	UK Standard	EU Standard	Volume Index
< 1.0	< 2.5	AA	AA	AA	0
1.0	2.5	A	A	A	1
2.0	5.0	B	B	B	2
3.0	7.5	C	C	C	3
4.0	10.0	D	D	D	4
5.0	12.5	DD / E	DD	E	5
6.0	15.0	DDD / F	E	F	6
7.0	17.5	G	F	G	7
8.0	20.0	H	FF	H	8
9.0	22.5	I	G	I	9
10.0	25.0	J	GG	J	10
11.0	27.5	K	H	K	11
12.0	30.0	L	HH	L	12

Frequently Asked Questions

Many stores use the outdated "+4 method" (adding 4 inches to your underbust) to fit you into a limited stock range. Modern fitting prioritizes a snug band because the band, not the straps, should carry the weight. If you measure 30 inches, a 30 (or 32) band is mechanically superior to a 34.

Cup sizes are relative to band sizes. The volume of a 34C is exactly the same as a 32D and a 36B. These are "Sister Sizes". If a 34C fits well in the cup but the band is too loose, you should try the 32D (down a band, up a cup).

For projected shapes or softer tissue, the leaning measurement is often more accurate as it captures the full breast tissue. We recommend averaging the standing and leaning measurements if the difference is greater than 2 inches for the most balanced starting point.

Yes, "Sticker Shock" is common. In mainstream media, "DD" is often portrayed as "huge", but in proper fitting, DD simply means a 5-inch difference between bust and band. Average properly-fitted sizes are often closer to 32F or 34E than the ubiquitous 34B.