Buying a ring from overseas, or helping a partner abroad pick the right fit, means dealing with two completely different sizing systems. The UK uses letters (A through Z), while the US uses numbers (1 through 16). Without an accurate ring size conversion UK to US, you risk ordering a ring that’s either too tight or slides right off.

At A Star Diamonds, our Hatton Garden team helps customers navigate this exact issue every week. Whether you’re ordering a bespoke engagement ring for someone in the States or simply trying to make sense of international sizing charts, getting the conversion right matters, especially for a piece you’ll wear for a lifetime.

This guide gives you a complete UK to US ring size conversion chart, along with practical tips for measuring at home and advice on getting the best possible fit. No guesswork required.

What UK and US ring sizes mean

Before you attempt any ring size conversion UK to US, it helps to understand what each system is actually measuring. Both systems are based on the internal circumference or diameter of a ring, but they express that measurement in completely different ways. Knowing the logic behind each system makes the conversion process far less confusing.

How the UK system works

The UK ring sizing system uses letters of the alphabet, starting at A and running through to Z, with half sizes shown as A½, B½, and so on. Each full letter step represents a difference of about 1.25 mm in internal circumference. A size L, for example, has an internal circumference of roughly 49.3 mm. The system is also used across Ireland, Australia, and South Africa, so you may encounter it more widely than you expect.

If you already know your UK ring size in letters, you are halfway to a clean conversion; each letter maps directly to a fixed circumference measurement.

Key facts about UK sizing:

• Starts at A and runs to Z, with half sizes throughout
• Based on internal circumference in millimetres
• Shared with Ireland, Australia, and South Africa

How the US system works

Unlike the UK approach, the US ring sizing system uses numbers, typically ranging from 1 to 16, with half and quarter sizes also available (such as size 6.5 or 7.25). Each full size step represents a change of roughly 0.8 mm in internal diameter, which is a slightly different increment to the UK system. A US size 7, for instance, corresponds to an internal diameter of approximately 17.35 mm. Canada uses the same numbering system, so a US size works directly in Canadian jewellery shops as well.

Key facts about US sizing:

• Uses numbers from 1 to 16, with half and quarter sizes
• Based on internal diameter in millimetres
• Shared with Canada

Step 1. Measure your ring size accurately at home

Before using any ring size conversion UK to US chart, you need a reliable base measurement. An inaccurate starting figure leads to an inaccurate conversion, so spending two minutes measuring correctly at home prevents a lot of frustration later.

Using a strip of paper or string

Wrap a thin strip of paper tightly around the base of your ring finger. Mark the point where the two ends meet, then lay the strip flat against a ruler. That measurement in millimetres is your internal circumference, which is what both UK and US sizing systems use at their core.

Measure your finger at the end of the day when it is slightly warmer and naturally a little larger, as finger size changes with temperature.

Steps to follow:

1. Cut a paper strip roughly 5 mm wide and 10 cm long
2. Wrap it snugly around the base of your ring finger
3. Mark where the strip meets itself
4. Measure that length in millimetres with a ruler

Matching your measurement to a UK size

Once you have your circumference figure, use this quick reference to find the closest UK letter size before moving on to the full conversion chart below.

Step 2. Use the UK to US ring size chart

Once you have your circumference measurement and UK letter size, the table below gives you a direct ring size conversion UK to US for the most common sizes. Find your UK letter in the left column, then read across to the corresponding US number on the right.

Always verify your conversion against the millimetre circumference column, as this is the single most reliable reference point across both systems.

Full conversion chart: UK letters to US numbers

The chart below covers UK sizes J through T, which account for the vast majority of adult ring sizes. If your size falls outside this range, use your circumference in millimetres as the common anchor point and match it to the closest US size.

Step 3. Check the fit before you order a ring

Once you complete your ring size conversion UK to US, do not place an order immediately. A converted size is a starting point, not a guaranteed fit. Before committing, run two quick checks to make sure the size you have works for your specific finger shape and the ring style you plan to buy.

The knuckle test

Your knuckle is often wider than the base of your finger, which means a ring that slides over your knuckle comfortably may feel slightly loose once it settles at the base. To test this at home, cut a strip of paper to match the circumference of your converted ring size, form it into a loop, and slide it over your knuckle. It should pass over with light resistance, then sit snugly at the base without spinning freely.

If the paper loop spins freely at the base of your finger, go up half a size; if it won’t clear your knuckle at all, go down half a size.

Wide band allowance

Wide ring bands, typically anything over 6 mm, fit tighter than narrow bands at the same numerical size. If your chosen ring has a wide shank, order half a size up from your converted US figure to account for this difference before placing your order.

Common sizing problems and quick fixes

Even with a careful ring size conversion UK to US, a few common issues can still catch you out. Knowing what to look for, and how to fix it quickly, saves you the cost and delay of a resize.

Your ring feels too tight after ordering

Swelling from heat, exercise, or diet can make a ring feel noticeably tighter than it did when you measured. Before requesting a resize, wear the ring for two to three days and measure your comfort level in the morning as well as the evening. If the ring consistently feels too snug throughout the day, contact your jeweller to discuss a half-size adjustment rather than jumping straight to a full size.

Most reputable jewellers, including A Star Diamonds, offer free resizing as part of their aftercare, so do not panic if the fit is slightly off on arrival.

Your converted size falls between two US sizes

Some UK half sizes convert to a point between two standard US sizes. In this situation, always round up rather than down, as a slightly loose ring is far easier to resize than one that will not clear your knuckle. Ask your jeweller whether quarter sizes are available, as many custom ring makers can accommodate them.

Next steps

You now have everything you need to complete a ring size conversion UK to US with confidence. Start by measuring your finger at home using the paper strip method, match your circumference to the chart in Step 2, and then run the knuckle test before placing any order. If your size lands between two US figures, round up and ask about quarter sizes rather than guessing.

One thing worth remembering: a conversion chart gives you a reliable starting point, but every finger is slightly different. Band width, finger shape, and daily swelling all affect how a ring ultimately fits in real life, so build in a small margin for error and choose a jeweller who offers resizing as standard.

If you are looking for a bespoke engagement ring or wedding band crafted by hand in London, visit A Star Diamonds to book a consultation with our Hatton Garden team.