Believe it or not, at least 80% of women wear an incorrectly sized
bra! Most people wear a bra too large in the back and too small in the
cups. And although the average bra size is often quoted as being 36C,
this is just the most popular size that women buy. Most people who wear
an "average" 36C bra should actually be wearing somewhere around a 32DD
or even 30E.
So how do bra sizes work? Most people know that bra sizes are made up
of two parts - a number and a letter. The number is known as the "band
size" or "back size," and the letter is the "cup size." However, what
many people do not realise is that cup sizes are in proportion to the
band size, so a D cup, for example, is not the same size in every bra. A
32D is the same size as a 34C or 36B, but on a smaller frame. A 28F is
actually five cup sizes smaller than a 38F, so it's not as big as it
sounds!
[1]If you're fairly slim, then you may well need a large cup size even though your bust doesn't look any bigger than average.
Larger women may still need a small band size because this relates to
the size of your ribcage only - you can still be curvy everywhere else!
And losing or gaining just a few pounds is likely to have an effect on
your bra size but it's not often considered even when other clothing is
resized. And then there are the times when you go for so long wearing a
certain size that you don't even realise it doesn't fit well anymore and
you stop noticing the discomfort and treating it as "normal"! If you're
looking for a better fit, here's how to find your true bra size.

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more...
6 Steps to measure your bra size