We recommended your top/tank size as a starting point - this should be used as a guide for your sports bra size. Remember that sports bras are meant to fit snug on the body - wearing it too loose won't provide you with the support you need. Bra support comes mostly from the under bust band so it's important that it fits firmly and close to your ribcage.
You can find your perfect size using our fit guide here. You can always size up or down to nail your sports bra fit.