Bra sizes - someone help me!!!

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So I've been wearing crop top type bras for the past year or so and now need to start wearing proper bras again but my Rib cage has expanded since having kids so none of my old bras are really that comfy
I googled how to measure for a bra but nothing overly makes sense - I am 29 inch underneath, probably 30 to be on the safe side then 35 inches round the fullest part - depending on which article I read it said to either take the number as is or to add 4 / 5. To my thinking this either makes me 30 dd or 34 A - I am currently wearing a gossard bra which is 36D and this feels a bit small (in both areas) so what's going on????
Can anyone help?

What dress size are you?
I'm a 20/22 on the top and wear a 36 back so if you are a smaller dress size then you're more likely to need a 30 back than a 36.

Different brands may come up smaller or larger do its best to try a load on. I'd start with the 30DD first and see how it fits.

Try a 30E from boux Avenue or Ann summers - I measured the same way as you and I’m so happy to finally have bras that fit me!

Thanks both - I'm a size 12 and tend to carry weight since kids starting just below the under bust down to top of the tummy - I think this makes me an apple shape. I'm hoping a proper size bra will help distract from the 'pregnant belly' look

I am 27 inches underbust and wear a 30 band. So maybe try a 32DD or E. Bravissimo fittings are worth a try, or Beija if you are in London.

Google boob or bust and use ther guide. I've never had such well fitting bras.

Go and get fitted at bravissimo - you can try on different styles.

The modern view is you don’t add the 4/5 inch thing. So you’re looking at either a 30E or a 32D but you’ll need to try quite a few on. The band size thing is really up to you. I’m supposed to be a 30GG but they absolutely kill me, I don’t like them that tight so I actually wear a 34F even though Bravissimo etc would tell me I’m wrong, I’m the one wearing the bra, I’ve got to be comfortable. (And nope it’s not the brand / style of bra, I think it’s actually a sensory thing).

I've never found measuring to work for me. Just have to try them on.