Neeeeever use an online calculator. Hell I wouldn’t even use a tape measure! The way bras will fit will fluctuate depending on the style, the brand, what they’re made of, even the colour, one colour can fit great, same bra, another colour can fit completely differently!
Its for this reason I love the way bravissimo fit women for bras because they base it off the bra the customer is currently wearing (which I know isn’t ideal when we’re all in sports bras but I’m speaking on a usual day to day basis here.)
If I were you, I’d just wait until you can go somewhere to be measured/fitted.