The tape measure only results in a well fitted bra for about 1 in 5 women. According to the tape measure guides I'm a 38D, but in real life I'm a 34FF (going on G atm!).
You should really go for a fitting and don't leave until you have found a bra that fits. If it's a store that uses a tape measure make sure you ask them to try other sizes until you find the best fit.
Also remember that different manufacturers have differences in size, so always try a bra on before buying it, don't just plump for the same size once you have a better idea of what is the right size for you.
http://www.bravissimo.com/perfectfit...t/default.aspx