Bra Sizing Methodology

What this page documents

This page describes the current calculation logic used by BraSizePRO's main Bra Size Calculator. The goal is transparency: you can see which measurements affect the result, how the arithmetic works, and where the result stops being a calculation and becomes a garment-fitting question.

BraSizePRO is not treating this method as a universal industry standard. Clothing sizing depends on measurement definitions, sizing systems, population data, and garment design. ISO 8559-1 provides standardized terminology and guidance for clothing-related body measurements, while ISO 8559-3 describes how body-measurement tables and intervals can be constructed and notes that body shape and proportions vary between populations. ISO 8559-1 · ISO 8559-3

Inputs used by the main calculator

Input	Used in calculation?	Role
Snug underbust	Yes	Determines the starting band value and forms the cup-difference calculation.
Standing bust	Yes	Forms the bust-minus-underbust difference used for the cup index.
Tight underbust	No	Optional reference measurement shown to help you understand your measurement range.
Leaning bust	No	Optional reference measurement shown for comparison; it does not change the main result.
Measurement unit	Yes	Centimeter inputs are converted to inches internally before calculation.
Target sizing system	Yes	Controls the cup-label sequence used for the displayed starting size.

Step 1: Calculate the band starting point

The calculator takes the snug underbust measurement in inches.

Band input → nearest whole inch → next even number when needed.

For example, a snug underbust of 32.2 inches becomes 32. A measurement of 31.2 inches rounds to 31 and then moves to the next even value, 32.

The current accepted calculation range is 27 through 51 inches. The displayed band therefore runs from 28 through 50. This is a boundary of BraSizePRO's calculation model, not a claim that bodies outside this range cannot be fitted.

Step 2: Calculate the cup index

The cup calculation uses the difference between standing bust and snug underbust:

Cup difference = standing bust − snug underbust

The difference is rounded to the nearest whole inch and used as an index into the selected cup sequence. Negative differences are treated as zero.

Rounded difference	US sequence	UK sequence	EU sequence	AU sequence
0	AA	AA	AA	AA
1	A	A	A	A
2	B	B	B	B
3	C	C	C	C
4	D	D	D	D
5	DD	DD	E	DD
6	DDD/F	E	F	E
7	G	F	G	F
8	H	FF	H	FF
9	I	G	I	G
10	J	GG	J	GG
11	K	H	K	H
12	L	HH	L	HH
13+	Clamped to final available label	Clamped to final available label	Clamped to final available label	Clamped to final available label

These sequences are calculator mappings, not a claim that every manufacturer uses the same cup progression. Published research has found measurable differences in cup volume between manufacturers, with larger differences at some larger cup sizes. Moscarelli et al.

Worked example

Suppose the entered measurements are:

• Snug underbust: 32 inches
• Standing bust: 36 inches
• Target system: US

The band calculation keeps 32 as the even band value. The bust difference is 36 − 32 = 4 inches. A rounded difference of 4 maps to D in the US sequence. The calculator therefore displays 32D as the measurement-based starting point.

That does not mean every 32D bra will fit the same way. The actual garment may differ in cup shape, wire geometry, stretch, depth, and grading. Research comparing bra-fitting criteria also demonstrates that different fitting approaches can produce different recommended sizes. White & Scurr

International label mapping

After calculating the US/UK-style numeric band, BraSizePRO maps the band to common EU, AU, FR, IT, and JP reference values where a mapping is available. For the main calculator's displayed conversions, the shared band reference is:

US / UK band	EU / JP	AU / NZ	FR	IT
28	60	6	75	0
30	65	8	80	1
32	70	10	85	2
34	75	12	90	3
36	80	14	95	4
38	85	16	100	5
40	90	18	105	6
42	95	20	110	7
44	100	22	115	8
46	105	24	120	9
48	110	26	125	10
50	115	28	130	11

International labels should still be treated as reference mappings. A retailer's or manufacturer's chart is the appropriate final reference when purchasing a particular garment.

How sister sizes are generated

For the supported cup sequences, the calculator moves the band by 2 and moves one cup position in the opposite direction:

• Snugger-band sister: band − 2, cup + 1 position.
• Looser-band sister: band + 2, cup − 1 position.

For example, a US 32D produces 30DD and 34C when both alternatives remain inside the calculator's supported boundaries.

These are adjacent sizing references, not guarantees of identical fit. Cup construction, wire shape, fabric, and manufacturer grading can change how the garment feels and fits.

What the calculator does not claim

• It does not establish a medically or anatomically correct bra size.
• It does not claim that one sizing formula works for every manufacturer.
• It does not use tight underbust or leaning bust to override the primary calculation.
• It does not determine whether a specific bra's cup shape, wire, straps, or fabric will fit your body.
• It does not replace the current size chart supplied by the manufacturer of the garment you are considering.

Why BraSizePRO documents the method

A calculator is more useful when its assumptions are visible. BraSizePRO therefore treats its result as a reproducible sizing reference: the same inputs and selected system produce the same result, and the calculation can be inspected rather than presented as a black-box recommendation.

This approach is also consistent with the broader reality of clothing sizing. ISO's clothing-sizing methodology recognizes that body shape and proportions differ across populations and that measurement tables are constructed for particular sizing purposes. The fitting literature likewise shows that different fitting criteria can lead to different recommendations. ISO 8559-3 · White & Scurr

Using the result

1. Measure consistently using the instructions on the calculator.
2. Use the result as a starting point rather than a universal answer.
3. Check the current manufacturer's size chart for the garment you want.
4. Evaluate the actual garment's band, cups, wires, straps, and overall comfort.
5. If needed, use the Sister Size Calculator or Bra Fit Checker to explore adjacent sizing and fit considerations.