Ring Size Calculator
A ring size calculator converts your finger circumference or diameter in millimetres into a ring size using the standard chart relationship — about 2.6 mm of circumference per ring size.
verified_userReviewed by the Calculopedia editorial teamLast updated 2026-08-16
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quizExample
How this calculator works, with real numbers (no JavaScript needed):
Inputs
- Finger circumference
- 55
- Ring diameter
- 0
- Preferred size system
- us
Results
- US ring size
- 7.2
- UK size
- 6.7
- EU size
- 53
- Circumference
- 55
- Diameter
- 17.5
functionsThe formula
Ring sizes are really just circumference measurements in millimetres, compressed onto a size scale. Every size step adds roughly 2.6 mm of circumference. That tidy relationship is the key to any ring size conversion: once you know a finger's circumference (or a ring's inner diameter), the right size is simply arithmetic.
A note on sizing systems
Ring sizing evolves from very practical origins: jewellers needed a standard way to describe a finger's thickness without measuring the actual finger. Over time, different regions settled on their own scales — the US uses numbers, the UK letters, the EU a diameter-based figure. Because they all describe the same physical circumference, converting between them is straightforward, which is exactly what this calculator does for you.
From circumference
US size ≈ (circumference − 36.28) ÷ 2.6
A finger circumference of 55 mm:
US size ≈ (55 − 36.28) ÷ 2.6 ≈ 7.2
So about a US 7.
From an existing ring
Measure the diameter across the inside of a ring that fits. Convert to circumference:
Circumference = diameter × π
A 17 mm inner diameter: 17 × 3.1416 ≈ 53.4 mm → about a US 6.5.
Measuring an existing ring you already know fits is often the most reliable home method — it skips the guesswork of how tight a string should be.
Converting between systems
| System | Relationship |
|---|---|
| US | Baseline size from circumference |
| UK | US − 0.5 |
| EU | ≈ US × 2 + 39 |
The US and UK scales are offset by half a size, while the EU scale tracks roughly double the US number plus a constant — all three are just different rulers measuring the same finger.
Measuring tips
- Measure at the end of the day — fingers swell slightly, so a morning measurement may turn out too tight.
- Use a non-stretch string or paper strip, snug but not tight — a stretchy tape reads loose.
- If between sizes, order the larger size (better to be loose and adjustable than tight and unusable).
- Left and right hands differ; measure the correct hand and finger, and remember your ring finger of choice (engagement rings are usually worn on the left in the US but the right in some countries).
Sizing differs slightly between manufacturers and styles (wide bands fit tighter, and metal thickness matters). When in doubt — especially for a gift you cannot return — get professionally sized first.
helpFrequently asked questions
question_markHow do I calculate my ring size?
Measure your finger circumference in millimetres and convert: US size ≈ (circumference − 36.28) ÷ 2.6. You can also measure the inner diameter of an existing ring and multiply by π.
question_markWhat is the average ring size?
For women, US 5–7 is typical; for men, US 9–11. That corresponds to roughly 49–55 mm and 59–66 mm finger circumferences respectively.
question_markHow do I measure my ring size at home?
Wrap a non-stretch string or paper strip around the finger, mark where it overlaps, lay it flat and measure the length in millimetres — then convert it with this calculator.