You can measure a ring size at home in two minutes with paper, and get it right, provided you avoid four specific mistakes that make almost every home measurement come out one size too small.
The short version
Wrap a strip of paper round the base of the finger, mark where it overlaps, measure the length in millimetres, and find that number in the chart below. Measure at the end of the day, at room temperature, on the hand you will wear it on — and check the strip slides over the knuckle before you trust it.
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Method 1: paper strip (the most accurate free method)
Cut a strip of paper about 5 mm wide and 100 mm long. Printer paper works; card does not, because it will not follow the curve of the finger.
Wrap it snugly round the base of the finger — snug, not tight enough to indent the skin. Mark with a pen exactly where the end overlaps the strip. Unwrap it, lay it flat, and measure from the end to the mark in millimetres. That number is the circumference.
Then do the thing everyone skips: before unwrapping, slide the loop up over the knuckle. If it will not pass, the ring will not either, no matter what the finger measurement says. Enlarge until it just slides over, and use that measurement instead.
Method 2: an existing ring
If they already own a ring that fits the right finger, measure the inside diameter across the widest point, in millimetres, with a ruler. Take it from inner edge to inner edge, not across the metal.
This is more reliable than paper for buying a gift, because it removes the person entirely from the process. It is also the only method that works when the ring is a surprise.
Method 3: buy the actual tool
If you buy rings more than occasionally, or the ring is expensive enough that being one size out matters, skip the measuring entirely. A metal ring sizer set with a mandrel, around $15, removes every source of error on this page: the rings are real sizes in real metal, so you try them on instead of measuring and converting.
Get the metal version rather than a plastic strip sizer. Plastic flexes, which reintroduces exactly the problem you were trying to avoid, and it consistently reads small.
The chart
| Inside diameter (mm) | Circumference (mm) | US size | UK size | EU size |
| 14.1 | 44.2 | 3 | F | 44 |
| 14.9 | 46.8 | 4 | H | 47 |
| 15.7 | 49.3 | 5 | J½ | 49 |
| 16.5 | 51.9 | 6 | L½ | 52 |
| 17.3 | 54.4 | 7 | N½ | 54 |
| 18.1 | 57.0 | 8 | P½ | 57 |
| 18.9 | 59.5 | 9 | R½ | 60 |
| 19.8 | 62.1 | 10 | T½ | 62 |
| 20.6 | 64.6 | 11 | V½ | 65 |
| 21.4 | 67.2 | 12 | Y | 67 |
If your measurement falls between two sizes, go up. A ring slightly loose is wearable; a ring that will not pass the knuckle is not.
The four mistakes
1. Measuring when cold. Fingers shrink noticeably in cold weather — up to half a size. Measure in a warm room.
2. Measuring in the morning. Fingers are at their smallest on waking and largest in the evening and after exercise. Late afternoon is the honest average.
3. Using string. String stretches, and it is the single biggest cause of undersized results. Paper does not stretch. If you must use something flexible, cut a strip from an envelope.
4. Forgetting the knuckle. Some people have knuckles significantly wider than the base of the finger. For them the finger measurement is irrelevant — the knuckle decides.
Two things about the finger itself
Sizes differ between hands. The dominant hand is usually half a size larger, so measure the exact finger the ring will go on.
Wide bands fit tighter than thin ones at the same nominal size, because they cover more of the finger. If the ring is 6 mm wide or more, go up half a size.
Buying as a surprise
Borrow a ring they wear on the right finger and measure its inside diameter — the only method that does not involve asking. Failing that, ask a friend with similar hands to try rings on, or buy an adjustable design and remove the problem entirely.
Most of the rings in our animal ring collection are adjustable for exactly this reason: the band is open at the back, so one ring covers a range of sizes. It is the safest option when you cannot measure.
Related
For necklaces the equivalent problem is length, and it has the same fix: where each necklace length actually sits.
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