Ring Size Calculator
Ring size from finger circumference or diameter.
Enter your values
Enter the measurement in mm.
Choose one of: Inside circumference, Inside diameter.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- US / Canada size
- 6.9
- Inside circumference
- 54 mm
- Inside diameter
- 17.19 mm
- EU / ISO size
- 54
- UK / Australia (approx)
- N
Quick answer
Ring size is set by the inside circumference of the band in millimetres.
US size ≈ (circumference mm − 36.5) ÷ 2.55
At a glance
| What it does | Ring size from finger circumference or diameter. |
|---|---|
| Category | Everyday |
| Inputs needed | Measurement, Measured |
| Main output | US / Canada size |
| Formula | US size ≈ (circumference mm − 36.5) ÷ 2.55 |
| Cost | Free — no sign-up, no download |
How to use the Ring Size Calculator
- 1Wrap a strip of paper around your finger and mark the overlap.
- 2Measure the length in millimetres.
- 3Enter it as the inside circumference.
Inputs explained
- Measurement(mm)
- Enter the measurement in mm.
- Measured
- Choose one of: Inside circumference, Inside diameter.
Worked example
Using the values the calculator loads with:
Inputs
- Measurement54 mm
- MeasuredInside circumference
Results
- US / Canada size6.9
- Inside circumference54 mm
- Inside diameter17.19 mm
- EU / ISO size54
- UK / Australia (approx)N
Frequently asked questions
When should I measure?
Late in the day at normal temperature. Fingers shrink when cold and swell in heat or after salt.
What if I'm between sizes?
Size up, especially for wide bands, which fit more tightly than thin ones at the same nominal size.
What do I need to enter into the Ring Size Calculator?
Just 2 values: measurement, measured. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Ring Size Calculator work out the answer?
Ring size is set by the inside circumference of the band in millimetres. It applies the formula US size ≈ (circumference mm − 36.5) ÷ 2.55 and shows the working so you can check each step by hand.
Is the Ring Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.