Ring Size Calculator
Enter one real measurement and get an exact size across every major sizing system, plus a confidence level so you know how much to trust the result.
Enter a measurement and select Calculate my size to see every size equivalent here.
US size
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UK / AU
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EU
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Japan
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Diameter
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Circumference
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Knuckle size
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Circumference vs. diameter
Not sure which measurement you've got? Here's what each one means — and how to read either one straight off your screen.
Diameter
The straight-line distance across the circle, edge to edge through the center. For a ring, that's its inner hole; for a finger, that's straight across its width.
Circumference
The distance all the way around the circle. For a ring, that's its inner edge unrolled flat; for a finger, that's what a thread wrapped snugly around it would measure.
The two are linked by a simple formula — circumference = diameter × π (about 3.14) — so the calculator above accepts either one.
Measure with your screen
No paper or printer needed — calibrate with a card you already have, then use the ruler below for either measurement method.
Why calibrate?
Step 1 — Calibrate your screen
Pick what you're using below, hold it up to the screen against the box, then drag the slider until the box matches it exactly. This corrects the ruler below for your screen.
Step 2 — Measure in millimeters (mm)
Each small mark = 1mm. Use one of the two methods below against this ruler, starting at 0.
Method 1 — Diameter: your finger
If you have a ring that already fits, place it flat on the screen instead and measure across its inner edge — that's more accurate. Enter it above and choose "An existing ring."
Method 2 — Circumference: a thread
When to see a jeweler
Frequently asked questions
Which measurement method is most accurate?
Measuring the inner diameter of a ring that already fits well is the most reliable method, since it's a hard surface that's easy to measure precisely. Finger circumference — wrapping a strip of paper or string around the finger — is a close second. Measuring finger diameter directly with calipers is the least reliable, since fingers compress slightly under pressure.
What if I don't have a ring or calipers?
Use our Printable Ring Sizer — it includes a ruler and a set of circles you can compare a ring against, plus a credit-card calibration check to make sure your printer didn't scale the page.
My result is between two sizes — which should I choose?
Choose the larger size. Rings can almost always be sized down for a small fee, but sizing up — or replacing a ring entirely — is more expensive and isn't always possible, especially for eternity bands.
Why do I need to account for band width separately?
A wider band covers more of your finger's natural taper and presses against more surface area, which makes it feel tighter than a thin band at the same nominal size. Use the Ring Width Adjustment tool after calculating your base size if you're choosing a wide band.
Does temperature affect my measurement?
Yes — fingers can swell by up to half a size in heat, after exercise, or in the evening, and shrink slightly in cold weather. For the most consistent result, measure at room temperature in the afternoon, when you haven't been exercising. See the Weather & Swelling Adjustment tool for details.
Go further
Fine-tune your result or shop with confidence.