What bra size? (teen)

[Pigletpoglet]

DD14 is autistic and cannot go and be fitted for a bra (and I guess that would be out of the question at the moment anyway!). I have looked at loads of online calculators, and they are all saying different things.
Her measurements are 72cm under bust, and 86cm over bust. That is with a bra on but unfastened (which is the best I could do to measure her). She's currently in a 28D, which is starting to look at bit small (through her t-shirt).
Best guesses at a size?

You say the measurements are not accurate, so it’s pointless trying to work anything out from them. Just go by look. If the 28D Is looking too small, is it too small in the band or the cup?
If the band is still okay you could try going up a cup size (28 DD - that will be getting harder to find)
if the band is too small but the cups are okay go up a band size and down a cup size (30 C)?
If both look too small try 30D ...

With those measurements you'd want to keep the 28 band but go up in the cup.

Never do the 'add four inches' thing, it's a load of rubbish!

Just be aware that if the cup is too small it'll make the band seem snug. Try the bra back to front. If the band is ok - you should be able to fit a couple of fingers under, no need to go up as long as you can- then it's up in the cup.

Personally, I find one cup too small is quite subtle and tends to show up as a slight bump when I breathe in or under a snug top, or as a bit of an underarm bulge. So if it's noticeably small I'd order 2 cups up as well