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Ring Size Calculator

Measure a ring that already fits well, and convert that measurement into a standard US size.

Inner diameter (mm)

Measure an existing ring that fits well, across the inside edge.

US ring size

54.3 mm circumference

What you entered

Inner diameter: 17.3 mm

1
Convert diameter to circumference
17.3 × π= 54.3496 mm
2
Match against the standard US ring size chart
interpolate nearest chart entries= US size 6.9806

Result

US ring size: 7

A 17.3 mm inner diameter (54.3 mm circumference) corresponds to a US ring size of about 7.

US ring size from inner diameter

Circumference is the inner diameter times pi, and US size is derived from that circumference.

Inner diameter (mm)	Circumference (mm)	US size
14	43.98	3.0
15	47.12	4.1
16	50.27	5.4
17	53.41	6.6
18	56.55	7.8
19	59.69	9.1
20	62.83	10.3

Each millimetre of diameter is roughly 1.2 US sizes, so measuring accuracy matters - half a millimetre is over half a size. Measure an existing ring across its inner edge rather than measuring your finger, which is more reliable. Fingers swell in heat and after salt or alcohol and shrink in cold, so measure at the end of the day at normal room temperature, and size up slightly for a wide band, which sits tighter than a thin one at the same diameter. If between sizes, the larger is usually the safer choice.

How to measure

Measure the inner diameter of a ring that fits the intended finger comfortably — straight across the inside, not around the outside. A piece of string wrapped around the finger and measured for circumference works too, using the circumference-to-size relationship directly.

Finger size changes throughout the day

Fingers are typically slightly larger when warm (like at the end of the day) and smaller when cold — measure at a 'typical' temperature and time of day, and consider sizing for the more common case rather than an extreme.

Frequently asked questions

I measured my ring's inner diameter at 17.3 mm — what US size is that?

A 17.3 mm inner diameter corresponds to approximately a US size 7. The standard chart goes from size 3 (14.0 mm) to size 13 (22.3 mm) in roughly 0.4 mm increments per half size.

Should I measure with a string or a ruler?

If you have a ring that already fits, measure its inner diameter with a ruler or caliper — that's the most accurate method. The string-around-the-finger technique measures circumference instead, which requires dividing by π (3.14) to get diameter. Both work, but a caliper on an existing ring is more precise.

How much does finger size actually change during the day?

Fingers can swell by up to a full ring size between morning and evening, and even more in hot weather or after exercise. Jewelers recommend measuring in the afternoon at a comfortable room temperature for the most representative fit.

What if my measurement falls between two sizes?

Go with the larger size. A ring that's slightly loose can be sized down by a jeweler for $20–$50, but a too-tight ring is uncomfortable and difficult to remove. For wide bands (6mm+), size up by an additional half size — wider bands fit tighter.