Enter your band and bust measurements in inches to estimate your bra size using the standard US sizing method.

How to Use the Bra Size Calculator

Measure straight across, snug but not compressed: band directly under the bust, bust around its fullest point. Type both numbers into the fields above and band size, cup letter, and the combined size appear immediately — no unit toggle to fuss with, since inches are the standard convention here.

Get either measurement off by an inch and the output size drifts noticeably — technique determines the result here more than the arithmetic does.

The Add-4 Method

Band Size = round(Underbust) + 4  (if even)
Band Size = round(Underbust) + 5  (if odd)
Cup Difference = round(Bust − Underbust)

This calculator uses the US "add-4" convention: round the underbust measurement to the nearest whole inch, then add 4 or 5 to land on an even-numbered band size, since commercial bra bands are only manufactured in even sizes. The cup then follows from how much larger the bust measurement is than the underbust — each additional inch of difference moves up one cup letter, starting at AA for no difference and running up the alphabet from there.

Here's the part that trips people up shopping online: a C cup isn't a fixed size, only a fixed difference. A 30C and a 40C share a cup letter but not a cup volume — the fabric around a 40-band cup holds meaningfully more than a 30-band cup at the same letter, because cup volume scales with band size even when the letter stays put.

Brand and country add another layer of inconsistency on top of that. UK, US, and EU sizing systems don't map cup letters onto each other cleanly, and even within the US, one brand's C runs noticeably differently than another's — sister sizing (moving up a band and down a cup, or the reverse) is often necessary to find an actual fit within a single brand's line. Weight change shifts the band measurement more than it shifts the cup difference, too, which is part of why a size that fit two years ago might not fit now even without buying anything new — it's worth re-measuring occasionally rather than assuming old numbers still hold.

A starting point, not a guarantee of fit. This calculator gives a reasonable estimate from a standard formula — actual fit varies enough by brand and style that an in-person or brand-specific fitting is worth it whenever precision matters.