Type in two numbers, get a size back — but what's actually happening inside a bra size calculator? It's simpler than it looks, and understanding it makes the result far more useful.
Video: “Bra Size Calculator - How to Find The Perfect Bra” · hosted on YouTube.
The logic behind the letters
Behind every calculator is the same core idea: your band size comes straight from your underbust measurement, rounded to the nearest standard size. Your cup size is the difference between your bust and underbust measurements — not either number on its own.
cup index = round(bust) − band
0=AA · 1=A · 2=B · 3=C · 4=D · 5=DD/E ...
That's why two people with the same bust measurement can land in completely different cup sizes — it all depends on how that bust number compares to their own underbust.
Reading the result
Why the number is a starting point, not a verdict
Sizing isn't standardized
A 34C in one brand can fit like a 34B or 34D in another, depending on cut and pattern.
Projection isn't measured
Two people with the same band and cup can have differently shaped busts that suit different styles.
More than one size can work
If your exact result isn't in stock, a sister size often gives a comparable fit.
Questions
Calculator FAQs
Rounding rules and regional conventions vary slightly between calculators, which can shift the result by a size step either way.
No — a standard calculator only uses two circumference measurements, so shape and projection aren't factored in.
Both are estimates — the calculator is faster and repeatable, but trying a bra on is still the only way to confirm actual fit.
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