How Our Bra Size Calculator Works

Method version 0.7. Band comes from the snug underbust measurement. Cup comes from the full bust minus the midpoint of the band range, at roughly 2 cm per cup step. Nothing else is used – no age, no shape questions, no hidden adjustments.

The two inputs

The calculator takes a snug underbust and a full bust, in centimetres or inches. Inches are converted to centimetres at 2.54 before any lookup happens, so both units follow exactly the same path through the arithmetic and cannot produce different answers.

There is one optional question about whether your current band feels firm on the loosest hook. It changes the guidance text shown alongside the result. It does not alter the calculated size, and we say so on the form.

Step one: band

The snug underbust is matched against a table of five-centimetre intervals. 73–77 cm gives AU 12, 78–82 cm gives AU 14, and so on.

Scroll the table sideways on a narrow screen.

Snug underbust measurement to band size (method 0.7)
Snug underbust	AU / NZ band	UK band	US band	EU band
63–67 cm	8	30	30	65
68–72 cm	10	32	32	70
73–77 cm	12	34	34	75
78–82 cm	14	36	36	80
83–87 cm	16	38	38	85
88–92 cm	18	40	40	90
93–97 cm	20	42	42	95
98–102 cm	22	44	44	100
103–107 cm	24	46	46	105
108–112 cm	26	48	48	110
113–117 cm	28	50	50	115
118–122 cm	30	52	52	120
123–127 cm	32	54	54	125

We do not add anything to the underbust measurement. Instructions to add 10 or 12 come from an era of much less elastic fabric and generally produce a band several sizes too loose in a modern bra.

Step two: cup

We subtract the midpoint of the matched band interval from the full bust, and look the difference up in the cup table.

Scroll the table sideways on a narrow screen.

Bust minus band midpoint to cup size (method 0.7)
Difference	AU / UK cup
10 – under 12 cm	A
12 – under 14 cm	B
14 – under 16 cm	C
16 – under 18 cm	D
18 – under 20 cm	DD
20 – under 22 cm	E
22 – under 24 cm	F
24 – under 26 cm	FF
26 – under 28 cm	G
28 – under 30 cm	GG
30 – under 32 cm	H
32 – under 34 cm	HH
34 – under 36 cm	J
36 – under 38 cm	JJ
38 – under 40 cm	K

Using the midpoint rather than the raw underbust is a deliberate choice. It means everyone assigned to the same band is measured against the same reference point, so two people in AU 12 with the same bust get the same cup. Using the raw underbust would make the cup drift by up to two steps within a single band.

Boundary handling

When the difference lands within 0.7 cm of the edge of a cup interval, we flag the result as a close call and show the adjacent cup alongside it. We do this because measurement error at home is comfortably larger than 0.7 cm, so presenting a single confident answer at a boundary would be false precision.

Sister sizes

One band step down pairs with one cup step up, and one band step up pairs with one cup step down, using the AU cup sequence A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K. If a sister size would fall outside the supported bands or off the end of the cup sequence, we omit it rather than showing something that does not exist.

Out of range

The calculator covers underbust measurements from 63 cm to 127 cm and cup differences from 10 cm to under 40 cm. Outside that, we say so and point to measuring guidance and specialist fitting. We never round an out-of-range measurement into the nearest supported size – a wrong answer presented confidently is worse than no answer.

International conversions

Band and cup conversions are held as two separate lookup tables, because they behave differently. Band conversion is arithmetic: AU plus 22 gives UK and US, and EU runs on its own centimetre-derived scale. Cup conversion is convention, and it diverges materially in the US and EU above D. Every conversion page states which convention it uses and links back here.

What this method cannot do

It cannot account for breast shape, tissue density, root width or asymmetry, all of which affect fit as much as volume does.
It cannot predict how a specific brand will grade its cups.
It cannot tell you whether a bra is comfortable. Only wearing it does that.
It is not a medical tool and makes no claims about breast health.

The result is a starting size: the first size to try on, with two alternatives and a checklist. That is the whole promise.

Versioning and review

Every threshold, label and conversion in the calculator lives in a versioned configuration file, not in code. The active version is printed with every result and shown at the top of this page. Changing a chart changes the version. We review the conversion tables and the core algorithm quarterly and record what changed.

The method is published transparently for informational use and remains open to independent expert review. We will name a reviewer only after they have completed a review, and we will describe their relevant experience accurately.

If you are an expert fitter and wish to be a reviewer of our fitting guidelines, please contact us via our contact page and we can have a chat.

Published test cases

These cases are asserted in the calculator’s automated tests. If any of them ever stop holding, the calculator is broken.

Method 0.7 acceptance cases
Case	Input	Expected
Band lower boundary	63.0 cm underbust	AU 8 band
Band upper boundary	67.0 cm underbust	AU 8 band
Next band	68.0 cm underbust	AU 10 band
Decimal input	74.4 cm	Accepted, deterministic rounding
Inches	29.5 in	Converted before lookup
Invalid relationship	Bust ≤ underbust	Inline error, no result
Below range	Underbust under 55 cm	Out-of-range guidance
Above range	Bust over 175 cm	Out-of-range guidance
Cup boundary	Within 0.7 cm of a threshold	Adjacent cup shown
Smallest sister	AU 8 band result	No smaller-band sister
Largest sister	AU 32 band result	No larger-band sister
Privacy	Any completed calculation	No raw values in network, URL or analytics