You’re better off to measure yourself hun, a lot of places use a method that involves adding 4-6ins to your back band which is inacurate and leaves your bra unsupporttive (I’m a clothing/lingerie designer) there’s a really good bra size calculator here :http://www.sophisticatedpair.com/bracalculator.htm
Just grab a measuring tape, enter your across bust in inches and underbust and it gives the correct supportive size in UK, US and EU sizes