There is an infallible method. Measure round your ribs under your boobs for the bra size. If it is an even number add four. If it is an odd number, add five. That's your bra size.
Then measure yourself around the boobs. Write that measurement down.
So, suppose your rib measurement was 29inches and your boobs measurement was 35 inches, here's what you do.
Add five to 29. That gives you 34 – your bra size.
If your boob measurement was 34 you'd need an A cup.
But it is 35, so you need a B cup.
If it had been 36 you'd need a C cup
37 – D cup, etc.
I suppose if it's 37+half, that might be a DD
Whether a bra fits well also depends on the style, how far apart the cups are (or not), and other stuff like that, so it may still take you a while to find a bra that's right for you.