How do cup sizes on bras work?

I had a job in a store doing bra fit measurements. Most ppl think a 34D=36C. This isnt the case. Over 80% of women wear the wrong size bra and thats one reason why. Misinformation. When u lose weight, ur cup size could change. Ur band size could change. All women are different. Get a flexible tape measure and have a girlfriend measure u. (wearing clothes can effect ur sizing accuracy.) Measure around the back and straight across ur nipples. Then measure around ur back and under ur breasts. Subtract the 2 numbers. If the difference is 1-ur an a, 2-ur a b, 3- ur a c, 4-ur a d and so on. Hope I.helped.