How Do I Convert a 34G UK Bra Size to US Sizing?
If you have ever tried to buy a bra from a US brand while knowing your size only in UK terms, you have run into one of the most confusing quirks in lingerie: a 34G does not mean the same thing on both sides of the Atlantic. The band number stays the same, but the cup letter shifts, and getting it wrong can mean a bra that is a full cup off. The good news is that the conversion is completely predictable once you understand the pattern, and brands like Parfait that list both systems together make it easy to confirm. Here is exactly how a UK 34G translates to US sizing, and why.
The Short Answer
A UK 34G converts to a US 34I. The band stays 34 in both systems, and the UK G cup becomes a US I cup. That difference of two apparent letters is not a mistake; it is the direct result of the two countries counting cups differently once you go past a D. So if your best-fitting bra is a UK 34G, you would look for a 34I when shopping a US-labeled brand that uses the standard American cup sequence. Keep reading to see the full chain, because understanding the why makes you far less likely to slip up on other sizes.
Why the Band Stays but the Cup Changes
The band measurement is the easy part: a 34 band is a 34 band in both UK and US sizing, because both are based on the same underbust measurement. The cups are where the systems diverge. Both start the same through C, D, and DD, but after that the UK system inserts letters that the US system labels differently. The UK counts up as DD, E, F, FF, G, GG, H, while the US counts DD, DDD, then continues with single letters G, H, I, J, K. Because the US skips the doubled-letter steps that the UK uses, the same physical cup ends up with a letter that looks bigger in the US system even though the volume is identical.
The Full UK-to-US Cup Chain
Here is the sequence Parfait uses on its size chart, which follows the standard conversion. UK C, D, and DD match US C, D, and DD exactly. From there: UK E equals US DDD, UK F equals US G, UK FF equals US H, UK G equals US I, UK GG equals US J, UK H equals US K, and UK HH equals US L. Reading down that chain, you can see how UK G lands on US I, which is why a UK 34G is a US 34I. The same logic applies at any band: a UK 36G is a US 36I, and a UK 32G is a US 32I, because only the cup letter conversion matters once the band is fixed.
Confirming It on a Real Size Chart
The safest way to trust a conversion is to check it against a brand that prints both systems. Parfait does exactly this on every product page, showing each cup as a US and UK pairing so you never have to convert blind. On a style like the Elise Underwire Full Bust Everyday T-Shirt Bra, the cup selector lists the US and UK labels together, so you can find UK G and immediately confirm it corresponds to US I. Seeing the pairing spelled out on the actual product removes any doubt, and it is far more reliable than memorizing a rule, especially if you are shopping across several sizes for different styles.
Do Not Forget to Re-Check the Fit
A conversion gets you to the right label, but it does not guarantee a perfect fit, because grading still varies between brands. Once you have translated UK 34G to US 34I, treat that as your starting size and check the key fit points when you try the bra: the band should be firm and level, the wires or cup edges should sit behind your breast tissue rather than on it, and the cup should contain you without gaping or spilling. If something is slightly off, sister sizing lets you adjust by moving a band size and a cup letter in opposite directions while keeping the same cup volume. The conversion and a quick fit check together will land you in the right place.
Common Mistakes When Converting
The most frequent error is assuming a UK G is the same as a US G, since the letters look identical. In fact a US G is the equivalent of a UK F, one full step smaller than a UK G, so treating them as the same size lands you in a cup that is too shallow. A second common slip is converting the cup but forgetting to keep the band fixed, which leads people to change the number unnecessarily; the band stays at 34 throughout. A third is stopping at the conversion and skipping the fit check, then blaming the size when the real issue is a brand that grades firmer or softer than expected. Knowing that a UK 34G is a US 34I is the anchor, but pairing that knowledge with the band-stays, cup-shifts logic keeps you from making these avoidable mistakes.
Shopping Confidently Across Systems
Once you know your size in both systems, cross-brand shopping gets much less stressful. Write down both your UK and US sizes, for example UK 34G and US 34I, and keep them handy. When a brand shows both, like Parfait does across everyday styles such as the Simplicity Full Bust Everyday Bra and the Holly Wire-Free Full Bust Padded Bra, you can order in whichever system you find first and know it is right. When a brand shows only one, you will already have the equivalent ready to go, which means you can shop widely without second-guessing every label.
The Bottom Line
Converting a UK 34G to US sizing is simple once you know the pattern: the band stays 34 and the cup becomes a US I, giving you a US 34I. The difference comes from the two systems counting cups differently past a D, following the chain where UK F is US G, UK FF is US H, and UK G is US I. To confirm any conversion with confidence, check it on a brand that lists both, like the Elise Everyday T-Shirt Bra, then verify the fit when it arrives. Know both your sizes, keep them written down, and you can shop UK or US brands without ever guessing again.
