Measurements are a guide, not a gospel. Once you've got your size, try on a bra in that range. Put it on the loosest hook first (bra bands stretch over time, so you want room to adjust). The band should be snug but not painful—you should be able to slide one finger underneath it easily.
Check the straps: they should be adjusted to hold the cups against your chest, but not bear all the weight. If your straps are digging trenches into your shoulders, the band is too loose. The band should do 80% of the work. Also, the center gore (that little piece between the cups) should lie flat against your sternum. If it's floating in space, the cups are too small.
And finally, breathe. Move around. Raise your arms. Jump a little. If the underwire jabs you, or the band rides up in the back, it's a no. Walk away. There are other bras in the sea. I promise you, finding the right fit feels like a hug for your torso, and you deserve that feeling. Now go forth and measure—your boobs will thank you.
