Short answer: measure the inside of a ring that already fits, or wrap a strip of string or paper around your finger, get the number in millimeters, and read it straight off the chart below — the circumference in millimeters is the conversion, and every US, UK, and EU size is just a label for that one measurement. You do not need a jeweler or a sizing kit to order a ring online with confidence. You need a ruler marked in millimeters, two minutes, and the table further down this page. Below I walk through both at-home methods, give you a verified mm-to-size chart across the US, UK, and EU systems, and cover the fit details that trip people up — knuckles, temperature, and band width — so the ring fits the first time.
Key takeaways
- Millimeters are the real measurement; sizes are labels. Get your inside circumference (or diameter) in mm, then read across the chart to your US, UK, or EU size.
- Two reliable home methods: measure the inside diameter of a ring that already fits, or wrap string or paper around the finger and measure the length where it overlaps.
- Diameter and circumference are linked by pi: circumference = diameter × 3.14159. If your tool gives you one, you can find the other.
- When you fall between two sizes, size up — a fraction loose is comfortable and stays on; too tight will not pass the knuckle.
- Adjust for real life: measure at the end of a warm day, size for the knuckle if it is wider than the base, and go up a quarter to a half size for bands 6 mm or wider.
✦ Interactive tool
Ring size calculator
Enter what you measured — we’ll convert it instantly. (Same data as the chart below.)
Try one:
Tip: between two sizes? Size up — especially for wider bands.
The fastest method: measure a ring that already fits
If you own a ring that fits the finger you are buying for — or, for a surprise, one you can quietly borrow — this is the most accurate home method, because you are measuring a finished ring rather than a soft finger.
- Lay the ring flat and measure the inside diameter: the straight-line distance across the inner edge, in millimeters. Measure edge to edge of the hole, not across the metal.
- Read it off the chart. An inside diameter of 17.3 mm is a US 7; 16.5 mm is a US 6. Match your number to the closest row.
- Prefer circumference? Multiply the diameter by 3.14159 (pi). A 17.3 mm diameter is a 54.4 mm circumference — the same US 7, read from the circumference column.
Measuring across the inside is more reliable than trying to trace the ring on paper, where the line thickness alone can cost you half a size.
No ring to measure? Use the string-and-paper method
This is the method most people reach for, and it works well as long as you measure the length rather than pull it tight.
- Wrap a strip of string, floss, or thin paper snugly around the base of the finger — firm enough to sit where the ring will live, not cinched.
- Mark the exact point where the end overlaps the start.
- Measure the length from the start to your mark against a millimeter ruler. That length is your inside circumference.
- Convert. Read the circumference straight off the chart, or divide it by 3.14159 to get the diameter. A 54 mm circumference lands at about a US 7; a 51.9 mm circumference is a US 6.
Measure two or three times and take the consistent reading. The single biggest error here is pulling the string tight like a tourniquet, which reads a size too small — snug is the goal.
The mm-to-size conversion chart (US, UK, EU)
Find your millimeter measurement, then read across. The inside circumference and inside diameter are physical measurements; the US number, UK letter, and EU size are three different naming systems for the same finger. The EU size is simply the inside circumference in millimeters, rounded, under the ISO 8653 standard.
| US size | Inside diameter (mm) | Inside circumference (mm) | UK | EU |
|---|---|---|---|---|
| 4 | 14.9 | 46.8 | H | 47 |
| 5 | 15.7 | 49.3 | J | 49 |
| 6 | 16.5 | 51.9 | L | 52 |
| 7 | 17.3 | 54.4 | N | 54 |
| 8 | 18.1 | 57.0 | P | 57 |
| 9 | 18.9 | 59.5 | R | 60 |
| 10 | 19.8 | 62.1 | T | 62 |
| 11 | 20.6 | 64.6 | V | 65 |
| 12 | 21.4 | 67.2 | X | 67 |
| 13 | 22.2 | 69.7 | Z+1 | 70 |
- Whole US sizes step by about 0.8 mm of diameter — roughly 2.55 mm of circumference — so a half size is just over 0.4 mm of diameter. Small numbers make a real difference.
- The US system uses numbers; the UK uses letters; the EU uses the raw millimeter circumference. That is why a US 7 reads as a UK N and an EU 54.
- For the average woman, sizes 5 to 7 cover most fingers, with 6 to 7 the most common; men cluster around 9 to 11. Use it as a sanity check, never as a substitute for measuring.
Once you have your number, you do not have to hunt for it in every listing. Prefer to skip the guesswork? You can browse the full Stylr rings collection and filter to your size from there.
Fit details the calculator does not show you
The chart gets you to the right number. These four adjustments get you to the right fit.
- Measure at the end of the day, when your hands are warm. Fingers shrink in the cold and swell with heat, activity, and salt. A morning or cold-hands measurement reads small; a warm, end-of-day reading reflects how the ring will actually wear.
- Size for the knuckle if it is wider than the base. The ring has to pass the knuckle. If your knuckle is noticeably larger, measure both spots and choose a size in between — large enough to slide over the knuckle, snug enough that it will not spin or slip off at the base.
- Go up for wide bands. A wide band touches more of your finger and fits more snugly than a thin one at the same labeled size. For bands roughly 6 mm or wider, order a quarter to a half size up. Thin bands fit true to size.
- When you are between sizes, round up. A ring that is a hair loose is comfortable and adjustable; one that is too small will not get past the knuckle at all.
A simple band to size right
Thin Dome Ring
A slim, refined dome silhouette built for everyday wear, on a hypoallergenic 316L stainless steel base with an 18k gold PVD finish. Waterproof and tarnish-free, and offered in US sizes 5 through 8 — so once you have your number, you can order with confidence.
Shop this ring →Answering the three worries that stop people from ordering
- "The string method is inaccurate." It is accurate to within about a half size when you measure the length at a snug wrap and do not pull it tight — good enough for the vast majority of rings. Soft string can stretch, so use a firmer strip of paper if you have it, measure two or three times, and where it matters most, measure a ring that already fits instead.
- "What if I am between two sizes?" Round up. The gap between half sizes is under half a millimeter of diameter, and a slightly larger ring is comfortable and stays on. A too-tight ring cannot clear the knuckle and is the harder problem to fix at home.
- "International sizes confuse me." They are the same finger with different labels. Anchor on the millimeter circumference: it is identical everywhere, and the EU size is that circumference. Read the mm value off the chart, then take whichever country's column you need.
Frequently asked questions
How do I calculate my ring size from millimeters?
Take your inside circumference in millimeters and read it off a conversion chart, or divide it by 3.14159 to get the inside diameter and read that. For example, a 54.4 mm circumference equals a 17.3 mm diameter, which is a US size 7. If you measured the diameter instead, you can multiply it by 3.14159 to get the circumference.
How accurate is the string method for measuring ring size?
It is accurate to roughly within half a size when the string or paper is wrapped snugly, not pulled tight, and the length is measured carefully. Soft string can stretch and read small, so use a firmer paper strip, measure two or three times, and take the consistent number. For the highest accuracy, measure the inside diameter of a ring that already fits the finger.
What should I do if I am between two ring sizes?
Round up to the larger size. A whole US size is only about 0.8 mm of diameter, so a half size is a very small difference, and a slightly loose ring is comfortable and stays on. A ring that is too small will not pass the knuckle, which is the harder fit problem to solve without resizing.
Why are US, UK, and EU ring sizes different numbers?
They are three naming systems for the same physical finger. The US uses a numeric scale, the UK uses letters, and the EU uses the inside circumference in millimeters directly under the ISO 8653 standard. A US 7 is a UK N and an EU 54, because all three describe a finger of about 54.4 mm circumference.
Should I order a larger size for a wide band ring?
Usually yes. A wide band contacts more of your finger and feels tighter than a thin band at the same labeled size, so for bands about 6 mm or wider, order a quarter to a half size up. Thin and standard-width bands fit true to size, so no adjustment is needed.
Measure in millimeters, read across the chart, and round up when you are on the line — that is the whole method, and it is reliable enough to buy a ring online without ever visiting a jeweler. For a printable version and more on getting the number right, see our guides on the ring size chart and how to measure ring size at home.
When you have your size, browse the Stylr rings collection — waterproof, tarnish-free styles built for everyday wear.
Part of our complete guide to jewelry styling and sizing.
