Ring Size Calculator: Convert mm to US, UK & EU
What's my ring size? Measure your finger circumference in mm and instantly convert to US, UK and EU ring sizes. Free calculator + full conversion chart and how-to-measure guide.
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Reviewed by: Martín Rodríguez (editorial policy ) · Last reviewed:
Result
EU / Argentina Size
54.4 mm circumference = EU 54 = US 7 = UK N = 17.3 mm diameter.
US Size
EU Size
UK Size
Inside diameter
Summary
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For guidance only — not medical advice. Consult a licensed healthcare professional.
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Your result
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Buying a ring without knowing the size is a gamble. This calculator converts your finger circumference in mm to ring sizes in the US, UK and EU systems instantly. Just wrap a string around your finger, measure the length in millimeters, and get all three sizes plus the inside diameter.
When to use this calculator
- You want to buy an engagement ring but don't know your partner's size.
- Buying a ring online and need your exact US, UK or EU size first.
- Converting a ring size from one system to another for an international purchase.
- Giving a ring as a surprise gift and need the size without asking.
- You're a jeweler and need a quick circumference-to-size reference.
Ring Size Conversion Chart: mm → US / UK / EU
| Circumference (mm) | Inside Diameter (mm) | EU / ISO | US | UK |
|---|---|---|---|---|
| 44.2 | 14.1 | 44 | 3 | F |
| 46.8 | 14.9 | 47 | 4 | H |
| 49.3 | 15.7 | 49 | 5 | J |
| 51.9 | 16.5 | 52 | 6 | L |
| 54.4 | 17.3 | 54 | 7 | N |
| 57.0 | 18.1 | 57 | 8 | P |
| 59.5 | 19.0 | 60 | 9 | R |
| 62.1 | 19.8 | 62 | 10 | T |
| 64.6 | 20.6 | 65 | 11 | V |
| 67.2 | 21.4 | 67 | 12 | X |
| 69.7 | 22.2 | 70 | 13 | Z |
Fuente: ISO 8653:2016 — Jewellery, ring sizes, definition, measurement and designation (iso.org/standard/69624.html). EU/ISO size = inside circumference in mm; US size = (circumference mm − 36.5) ÷ 2.55; UK uses letters A–Z (~1.25 mm per letter).
How it works
How to measure your finger
String or paper method
1. Cut a thin strip of paper or a piece of string.
2. Wrap it snugly around your finger where the ring will sit (at the base, just below the knuckle).
3. Mark the spot where it overlaps.
4. Measure that length with a ruler in millimeters.
5. That number is your inside circumference — enter it above.
From a ring that already fits
1. Take a ring that fits the right finger well.
2. Measure its inside diameter in mm (edge to edge across the hole).
3. Multiply by π (≈ 3.1416) to get the circumference, or just look up the diameter in the chart below.
Ring size conversion chart (mm → US / UK / EU)
| Circumference (mm) | Inside diameter (mm) | EU / ISO | US | UK |
|---|---|---|---|---|
| 44.2 | 14.1 | 44 | 3 | F |
| 46.8 | 14.9 | 47 | 4 | H |
| 49.3 | 15.7 | 49 | 5 | J |
| 51.9 | 16.5 | 52 | 6 | L |
| 54.4 | 17.3 | 54 | 7 | N |
| 57.0 | 18.1 | 57 | 8 | P |
| 59.5 | 19.0 | 60 | 9 | R |
| 62.1 | 19.8 | 62 | 10 | T |
| 64.6 | 20.6 | 65 | 11 | V |
| 67.2 | 21.4 | 67 | 12 | X |
| 69.7 | 22.2 | 70 | 13 | Z |
The exact formulas
Average ring sizes
Important tips
Example: 54.4 mm circumference
EU / ISO: equals the circumference → size 54.
Inside diameter: 54.4 ÷ π = 17.3 mm.
US: (54.4 − 36.5) ÷ 2.55 = 7.0 → size 7.
UK: from the chart → N.
54.4 mm circumference = EU 54 = US 7 = UK N = 17.3 mm diameter.
Disclaimer: Los resultados son orientativos y no reemplazan la consulta médica profesional. Antes de tomar decisiones con impacto, consultá con un médico, nutricionista o profesional de la salud matriculado.
Frequently asked questions
How do I measure my ring size at home?
Wrap a string or thin strip of paper around the finger where you'll wear the ring, at the base just below the knuckle. Mark where it overlaps, then measure that length with a ruler in millimeters. That measurement is your inside circumference — enter it in the calculator to get your US, UK and EU sizes.
What is my ring size if my finger circumference is 54 mm?
A 54 mm circumference is EU/ISO size 54, US size 7, UK size N, with an inside diameter of about 17.2 mm. EU sizes match the circumference in mm directly, and US size is (54 − 36.5) ÷ 2.55 = about 7.
What's the difference between US, UK and EU ring sizes?
All three measure the same finger but use different scales. EU/ISO sizes equal the inside circumference in mm. US sizes are numbers that run roughly from 3 to 13+ (each full size ≈ 2.55 mm of circumference). UK sizes use letters A–Z. Converting between them is just mapping the same circumference onto a different scale.
How do I convert millimeters to a US ring size?
Use US size = (inside circumference in mm − 36.5) ÷ 2.55. For example, 57 mm gives (57 − 36.5) ÷ 2.55 ≈ 8, so a US size 8. If you only have the diameter, multiply it by π first to get the circumference.
What's the average ring size for men and women?
For women the most common US sizes are 6 to 7; for men they're 9 to 11. These are just population averages and vary widely by hand size and genetics, so always measure the actual finger rather than guessing.
Why does my ring size change during the day?
Fingers swell and shrink with heat, cold, exercise, salt intake, pregnancy and time of day. It's normal for your size to vary by half a size to a full size. Measure in the late afternoon or evening at a comfortable temperature for the most representative result.
Can a ring be resized if it doesn't fit?
Most precious-metal rings (gold, silver, platinum) can be resized up or down by 1–2 sizes by a jeweler, and making a ring smaller is usually cheaper than enlarging it. Rings made of titanium, tungsten or with channel-set stones generally can't be resized.
Should I size up or down if I'm between two sizes?
Choose the larger size, especially for a ring you'll wear daily. A slightly loose ring is easy to secure with sizing beads or a small resize, while one that's too tight can be uncomfortable or get stuck.
How do I find someone's ring size for a surprise gift?
Borrow a ring they already wear on the correct finger and measure its inside diameter in mm (then multiply by π for circumference), or trace the inside circle on paper. You can also ask a friend or family member who knows, or note the finger and compare it to an average size as a starting point.
Sources & references
Methodology & trust
Editorial
Calculadora de salud revisada por el equipo editorial de Hacé Cuentas, contrastada con ISO 8653:2016 — Jewellery, ring sizes, definition, measurement and designation, según nuestra política editorial y metodología.
Updates
Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.
Privacy
Calculations run 100% in your browser. We do not store or transmit your data.
Limitations
Indicative results. For critical decisions, consult a professional.
📌 How to cite this calculator
Rodríguez, M. (2026). Ring Size Calculator: Convert mm to US, UK & EU. Hacé Cuentas. https://hacecuentas.com/ring-size-finger
Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.