Can you help me with bra sizing please?

[poshfrock]

I've thought for a while that I'm probably wearing the wrong bra size so decided to check online how to measure myself. I am 33 inches under boob and 37 inches over boob. So according to the sites I should add 5 inches to the under boob measurement to get the band size and subtract the over on measurement to get the cup size. The problem is that this gives me a negative number so I am something like a 38AAA. A size which does not seem to exist. What am I doing wrong ? Any ideas what size I should be?

Also DD15 is a 32D. She wants a strapless bra to wear under summer tops. Received wisdom seems to be that she should go down a band size and up a cup size for a secure fit. This would make her a 30E. If this is correct then I can't find any bras on this size at all.

Can anyone help please?

Please don't follow the 'add 5"" rule, it's really outdated and doesn't work. Try following the guide here: thebetterbracampaign.blogspot.com/p/fitting-advice.html

I've never heard of wearing a different size in a strapless bra. But if she does want to do that, it would be 30DD that would be the 'sister size' to 32D.

There are over 20 bras to choose from in those sizes on Figleaves alone so don't worry about not being able to find one.

Sorrel is right - adding 5 is an absolute no-no (because of modern stretch fabrics)

Wonderbra do an excellent strapless bra which comes up small on the band. Would be worth a look for your DD although she'll probably want to check her size properly first

What does your DD actually measure? And did you measure yourself leaning down for the overbust?
Don't add anything. With those measurements, I would try sizes around 32DD

I advise you and your DD to check your size using booborbust.com/
The website also lists DD+ bra retailers