Stop second-guessing your partner’s finger size – this guide hands you the exact international ring size conversion chart you need, plus fool-proof measuring steps so the ring fits first time.
Use the universal chart below to convert UK, US, EU and Asian sizes or translate millimetres into letters and numbers in seconds. You’ll also get DIY measuring tricks, jeweller-grade tools, quick answers to “What is a UK L in US?” and expert fitting tips so you can order with confidence worldwide.

Different countries label rings with letters, numbers or millimetres because their industries developed in isolation. A difference of 0.3 mm can make a band pinch or spin, especially on wider styles. Master accurate conversion before you click “buy” – or commission a bespoke piece at our Hatton Garden workshop – and you’ll avoid costly resizing and the awkward proposal wobble.

Understanding Ring Size Systems Worldwide

Before you start juggling letters, numbers and millimetres, it helps to know how each sizing system works. Every standard has its own logic, and mixing them up is the fastest way to botch a ring size conversion.

UK alphabetical sizing (A–Z+)

Britain and much of the Commonwealth use letters, with half-sizes indicated by “½” and extra-large sizes marked “+”. Each step equals roughly 1.25 mm of inner circumference. Example: J½ sits around 48 mm, while the popular N is about 54 mm.

US & Canada numerical sizing

Across North America rings run from 000 to 16 in whole and half numbers. The jump between half-sizes is tighter than the UK system—about 0.8 mm of circumference—so a US 7 isn’t just “medium”; it’s a precise 54.4 mm circle that can feel snug or loose with only a half-size shift.

European ISO/metric sizes

Most of mainland Europe follows ISO 8653, labelling rings by their inner circumference in millimetres: 40 mm to 76 mm. Older French, German or Italian charts sometimes differ by one or two millimetres, but the principle is identical—what you see is the exact measurement.

Asian ring size systems

Japan and China number rings 1–30, calculated as “inner circumference (mm) minus 40”, so a Japanese 13 equals 53 mm. Hong Kong uses diameter-based fractions such as 15.5, meaning 15.5 mm across.

Why these systems differ

Local jewellers historically chose what matched their tools—letters for UK guilds, inches split into eighths for the US, metric for post-war Europe. Some count diameter, others circumference, and that 0.3 mm mismatch is enough to create a half-size discrepancy when converting between them.

Master Ring Size Conversion Charts

A single master chart is the fastest way to swap between systems because it lists the hard numbers (diameter and circumference in millimetres) that never change. Match the figure you have to the system you need and you’re done.

Universal cross-reference chart

Below is an abbreviated extract of our two-page master table (A–Z+6, US 000–16). Use it online, then grab the printable PDF mentioned further down for the full list.

Rounding rule of thumb

• Slim bands (< 4 mm): round down if you sit between two sizes.
• Wide bands (> 6 mm): round up by half a UK size to avoid the “ring glove” effect.

Quick-lookup mini charts

Most shoppers only need a narrow band of sizes, so we’ve sliced the big table into two pocket guides.

Women (popular UK J–P)

Men (popular UK P–Z)

Downloadable & printable sizing aid

Click the “Download PDF” button at the top of the page to grab:

• The full universal chart (UK A–Z+6, US 000–16, EU 40–76, Asian systems)
• A built-in 100 mm ruler and a “credit-card check” line so you know your printer hasn’t scaled the page
• A fold-up ring sizer belt you can cut out and reuse

Print at 100 % on A4, measure the verification line with any bank card and you’ll have jeweller-grade accuracy without leaving home.

How to Measure Your Ring Size Accurately at Home

Measuring at home saves a trip to the jeweller and gives you instant numbers to plug into the ring size conversion chart above, but only if you follow a method that removes the usual guess-work. Pick one of the three options below, run through the steps twice, and compare the results—any mismatch larger than 0.3 mm means you should measure again.

String or paper strip method

1. Cut a 1 cm-wide strip of non-stretch paper or dental floss long enough to wrap around your chosen finger’s widest point (usually the knuckle).
2. Wrap it so it sits snugly but can still slide over the knuckle without force; mark where the end overlaps.
3. Lay the strip flat and measure the distance between the marks in millimetres.
4. Match that circumference to the universal chart—e.g., 54 mm = UK N, US 7.

Pros

• Costs nothing and works anywhere.

Cons & tips

• Paper absorbs moisture; repeat the process three times and average the result.
• Measure late afternoon when fingers are warm and slightly larger.
• Never pull tight; constriction can shave off half a UK size.

Printable ring sizer belt

Download the A4 PDF, set printer scaling to 100 %, then:

1. Cut along the dotted outline and pierce the slot.
2. Thread the tail through the slot, creating a mini belt.
3. Slip over the knuckle, tighten until comfortable, and read the size shown in the window.

Avoid common errors: if the credit-card check line on the printout is off by more than 1 mm, your entire reading will be wrong.

Measuring an existing ring

1. Choose a round, unworn ring that fits the intended finger.
2. Use a steel ruler or digital callipers to record the inner diameter in millimetres (to one decimal place).
3. Cross-reference that diameter in the diameter column—e.g., 17.3 mm = UK N, US 7.

Double-check the ring isn’t oval from wear; even a slight squish can throw the reading off and, by extension, your conversion.

Professional Tools and Services for Precise Sizing

DIY tricks are great for a quick estimate, but if you want diamond-grade accuracy—especially before commissioning a bespoke ring—invest in purpose-built tools or let a jeweller size you under controlled conditions. A fraction of a millimetre here really is the difference between “glides on” and “won’t budge”.

Metal ring gauge sets

Stainless-steel gauges look like a bunch of mini rings on a loop. Slip them on until one turns with gentle resistance and read the engraved size.

• Cost: £15–£40 for a full UK A–Z+ kit.
• Pros: virtually indestructible, precise to half-size, reusable for friends.
• Cons: initial outlay and you still need to convert if the set isn’t dual-marked.

Plastic multisizer loops

Think of a cable-tie printed with letters. Feed the tail through, tighten, read, repeat.

• About £3 online or free from many jewellers.
• Light, post-friendly and covers UK A–Z.
• Plastic can flex with warmth, so take two readings and average them.

Visiting a jeweller: what to expect

A professional will warm your hands, measure both fingers with calibrated gauges of different widths and note the preferred fit for the ring style you’re ordering. Ask about:

1. Temperature and time-of-day adjustments.
2. Band width allowances.
3. Complimentary resizing policies—most bespoke makers, including A Star Diamonds, include at least one free resize.
   The five-minute appointment often costs nothing and eliminates all conversion guesswork.

Common Ring Size Conversion Questions Answered

Below are the quick-fire answers our gemologists give most often. Screenshot them for instant reference. Figures assume a standard 2 mm band unless noted.

What is a UK size L in US?

UK L converts to roughly US 5¾, measuring about 16.3 mm inner diameter or 51.2 mm circumference. For bands over 6 mm wide, round up to a US 6.

What UK size equals a US 7?

A true US 7 aligns with UK N (sometimes N½ on slim shanks), giving a 17.3 mm diameter and 54.4 mm circumference.

Converting centimetres vs millimetres

Jewellers work in millimetres for precision. Quick rule: circumference (cm) × 10 = mm. Example: 5.6 cm → 56 mm, which matches UK P. Avoid rounding 0.05 cm; it can shift half a size.

Do men’s and women’s sizes convert differently?

The charts are unisex—the maths is identical. Differences come from style: wider men’s bands often feel tighter, so many gents order a quarter to half a size larger.

Tips to Get the Perfect Fit Every Time

Charts and callipers give hard numbers, but your body isn’t static. Build a small safety-margin into your ring size and you’ll dodge the two most common problems—pins-and-needles tightness in summer and surprise spins in winter.

Factor in daily finger swelling and temperature

Fingers are slimmer in the morning and when you’re cold. Measure once when you wake and once after work; if they differ, choose the larger value or go up a quarter-size for wide bands.

Seasonal changes and life events

Heatwaves, pregnancy, medication and weight shifts can add half a UK size overnight. If any of these apply, delay final sizing until your hands stabilise.

Adjusting for wide bands, stacking and eternity rings

Anything over 6 mm sits tighter because there’s more metal against the skin. Add ¼–½ UK size for a comfort fit, and remember full eternity rings can’t be resized later.

When resizing is your best option

If the ring leaves a deep indent or spins past the knuckle, professional resizing is cheaper than losing the stone. UK jewellers charge roughly £40–£80 for gold and platinum; tungsten and titanium often can’t be altered at all.

Ready to Find Your Perfect Fit?

You now know how the size systems work, how to convert them, and how to measure like a pro. Use the universal chart to cross-check any online listing, follow the at-home measuring tips to nail your numbers, and keep our fit advice in mind when life—or the weather—makes your fingers fluctuate. Still want absolute certainty before you invest in a once-in-a-lifetime ring? Our Hatton Garden specialists will size you with calibrated gauges, factor in band style and lifestyle, and guide you through diamond or lab-grown options.

Book a complimentary virtual or in-studio fitting today with the family team at A Star Diamonds and walk away knowing your ring will fit perfectly from the first slide on.