As much as it may not seem like it, with all those random numbers and letters and every company offering free professional sizing to get you through the door, bras are only of the only types of traditionally female fashion that have standard sizing you can work out at home. Whilst in dress sizes you may be a size 8 in one store and a 14 in another, with bras you will usually know what you need. This isn't always the case, different brands will cut their patterns in slightly different ways, different bra shapes will lead to tightness in some places and looseness in others, and different materials will have different levels of stretch. If you always wear a different size in a certain brand, style, or material, that will be a better judge of fit then your calculated size. But if you're starting from scratch, this is what you need to know. First off, there's one main thing that needs to be explained. Those letters aren't random, they aren't arbitrary. With each cup increase, chest size increases by approximately one inch in circumference. Cups are not a regular measurement - they are a comparative measurement. When you ask what a cup is, you are asking "how much bigger is my chest than my torso?"
Think about it this way - if your torso is 30 inches, and your chest is 31 inches, that is one cup. If your torso is 30 inches, and your chest is 32, that's two cups. If your torso is 36 inches and your chest is 38, that is still just two cups.
So how do you calculate your own? Easy.
1. Measure your underbust. You want the tape measure to be snug, but not tight. It should lie flat against the skin without pulling inwards - just like you don't want your bra to dig in. Simple enough, this measurement is your underbust. If your number is an odd number round upwards. A measurement of 31 inches means your underbust number will be 32. 2. Measure your chest. Again, snug but not tight. You don't want the breasts to be flattening or bending around the tape, but you want the tape to be flush against your skin where possible. Remember this number.
3. Subtract the underbust from the chest. This is the only bit of maths you need to do. Chest was 38 and underbust was 34 inches? You're left with 4.
4. Find your cup size. Each cup is equal to one inch difference. This chart uses the UK cup system. I use this for two main reasons: 1) this blog is based in the UK, and 2) UK bra sizes have some of the largest variety of size, especially for bigger sizes. If you're American or European don't worry, you haven't wasted your time, there are conversion charts at the bottom of the page and the cups still work the same way.
5. Put your underbust measurement in front of your cup size. Let's take our hypothetical 38 inch chest and 34 inch underbust person. They have a 4 inch difference, which means a cup size D, and a 34 inch underbust. Thus: 34D. Easy as that.
"Oh but wait," you say, "this company doesn't have my size! What can I do?" This is where the concept of sister sizes come into play. The comparative nature of bra measurements make it very easy to find a different size that still has the same total amount of volume. If you need to take off a band size, you simply need to add a cup size, and vice versa. Our 34D can also become a 32DD or a 36C. Most of the time, going one step in either direction should be fine, but the further removed you get the more the shape will change and worse the bra will fit. Below are a series of charts. Firstly, if you're struggling with the maths you can find your bra size on here just from the two measurements you took. Secondly, you can work out your sister sizes. Just go diagonally up/left or down/right - one set of sister sizes is marked in green to show you.
Below that, we have two conversion charts. The first is for cup letters between the UK, US, and Europe. In the US, sometimes the sizes above D are simply DD, DDD, DDDD ad infinitum. If you see this sizing anywhere, all you need to remember is that each D is an extra inch. The second chart is to convert UK/US band sizes to European, as theirs is based on the metric system.