Have you ever had two bras, but one is an I cup, and the other is a G cup? The bra fit specialist assures you that these bras are the same size, so why is one labelled with an I and the other a G?
The short answer is that the I cup is a bra that was either made by an American or European company, whereas the G cup is likely a bra that was made in the United Kingdom.
The UK, the US, and the EU all use different sizing systems to categorize their bras. So, to determine which cup size in each country's sizing system will accommodate your breast volume, you need to know how to translate between countries.
Here's a chart to help you out:
As you can see, A to D are the same across the board, but once you get above a D-cup, the bra sizes change.
How Do I Figure Out My Bra Size?
If you measure yourself and use a bra calculator (like this one here), the calculator should tell you your bra size and indicate what sizing system it's using for the cup size. Additionally, most online stores will indicate whether the bra you're looking at is a UK, US, or EU model. So then, all you have to do is keep a copy of the above chart handy, and you'll know what size bra to purchase.
If you get fitted by an expert, you should ask them what measurement system they're using. Depending on their inventory, you may be offered bras from multiple sizing systems or a single sizing system. Some stores convert their inventory to one system, while others translate the sizes depending on who the bra maker is. So, for example, if they measure you as a 34F in US/EU sizing, but they want to give you a bra from a UK brand, they know to provide you with a 34E.
However, if you're shopping solo, with only your measurements, you may need to Google search the brand of the bra you're interested in. This is the best way to know what country the brand originates from so you know what sizing system they use. For instance, if you search for Wacoal, you'll learn that they're an American company, and so they use the US sizing system.
A Note About UK, US, and EU sizing
In theory, while the letters are different, the corresponding cup sizes in each system are meant to represent the same volume. However, just because you fit a 34E in a UK bra doesn't necessarily mean you'll fit in a 34F in a US bra, as there are often subtle differences in cup construction between different brands. For this reason, there's no 100% guarantee that you'll wear the same bra size in every brand.
If you're still confused about what size you should be wearing, book a virtual or in-store consultation with a bra-fit specialist.