Ring Size Calculator

Convert finger circumference to US ring size.

Finger circumference

Results update as you type.

US ring size

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Finger diameter—

How it works

Finding your ring size without a jeweler is easy if you can measure your finger's circumference, and this calculator converts it to a US ring size. You enter the inside circumference of your finger in millimeters, and it returns the approximate US ring size and the finger diameter. To measure, wrap a thin strip of paper or string snugly around the base of the finger, mark where it overlaps, and measure the length in millimeters — that is the circumference. The formula maps circumference to the US sizing scale, where each full size corresponds to about 2.55 mm of circumference. For example, a circumference of about 54.4 mm is roughly a US size 7. A few tips for accuracy: measure at the end of the day when fingers are largest, avoid measuring when cold (fingers shrink), measure the specific finger the ring is for since sizes differ between fingers and hands, and if between sizes, size up, especially for wide bands which fit more snugly. This gives a US size; other countries use different scales, and jewelers can convert. For an important purchase like an engagement ring, confirming with a professional ring sizer is wise, but this calculator is great for a quick, reliable estimate. Enter your finger circumference to find your ring size.

Formula

US ring size ≈ (circumference in mm − 36.537) ÷ 2.5535

US ring sizes correspond to the inside circumference of the ring, with each full size about 2.55 millimeters apart. This formula converts a measured finger circumference in millimeters to the approximate US ring size, and dividing the circumference by pi gives the finger's diameter in millimeters.

Source:US ring sizing scale

We review all our tools from time to time to ensure accuracy.

Worked example

Finger circumference:54.4

(54.4 − 36.537) ÷ 2.5535 ≈ 7.0, a US ring size 7 (diameter ≈ 17.3 mm).

How to use this calculator

1
Enter your values
Enter finger circumference.
2
Adjust the options
Fine-tune any options to match your situation.
3
Read your result
US ring size updates instantly as you type — no button to press.

Frequently asked questions

How do I measure my ring size at home?

Wrap a thin strip of paper or string snugly around the base of the finger, mark where it overlaps, and measure that length in millimeters — that is the circumference. Enter it here to get your US ring size. Measure the specific finger the ring is for, since sizes vary between fingers and hands.

How does circumference relate to ring size?

US ring sizes are based on the inside circumference of the ring, with each full size about 2.55 millimeters apart in circumference. The calculator applies the standard formula to convert your measured circumference to the corresponding US size. Half sizes exist for finer fit, which is why the result can include a decimal.

When is the best time to measure ring size?

Measure at the end of the day, when fingers tend to be at their largest, and when you're at a normal temperature — cold shrinks fingers and heat swells them. Measuring under typical conditions gives a size that will be comfortable most of the time. Avoid measuring right after exercise or when hands are very cold or hot.

What if I'm between two ring sizes?

If your measurement falls between sizes, it is generally better to size up, since a ring that is slightly loose is more comfortable and less likely to get stuck than one that is too tight. This is especially true for wide bands, which fit more snugly than thin ones, so consider sizing up a bit for wider rings.

Do other countries use the same ring sizes?

No. This calculator gives US ring sizes, but the UK, Europe, Japan, and others use different sizing scales and units. The underlying measurement — the finger's circumference or diameter — is universal, so a jeweler or conversion chart can translate your size to another country's system if you're buying a ring from abroad.