Ring Size Calculator

Wrap string or a paper strip around your finger, measure the length, and enter it below to get your US ring size plus UK, EU, and Japan conversions. You can also measure an existing ring's inner diameter or convert a known US size.

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Half the people searching for ring sizes are not sizing their own finger. They are trying to figure out someone else's size without ruining a surprise, which is a genuinely tricky problem and we cover it below (short version: borrow a ring they already wear and measure its inner diameter with the second option above). If you are sizing your own finger, all you need is a piece of string or a strip of paper and five minutes.

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How to measure your ring size at home

Wrap a piece of string or a thin strip of paper snugly around the base of the finger the ring will live on. Mark the exact point where the end meets the wrap, then lay the string or paper flat against a ruler and measure from the end to the mark. That length is your finger circumference. Enter it above in millimeters or inches and we do the rest.

Two accuracy warnings that matter more than the measuring itself. First, measure at the end of the day: fingers swell as the day goes on, and a ring sized to a cold morning finger can pinch by evening. Second, check your knuckle. If your knuckle is noticeably larger than the base of your finger, measure the knuckle too and size for it, because a ring that cannot clear the knuckle never reaches the base at all.

Sizing from an existing ring

This is the surprise-gift method, and honestly the more accurate one. Borrow a ring the person already wears on the correct finger (left and right hands differ, and each finger differs), lay it on a ruler, and measure the inside of the band straight across at its widest point, edge to edge. That is the inner diameter. Switch the calculator to the existing-ring option, enter the number, and you have their size without a single suspicious question.

The formula

US size = (circumference in mm − 36.537) ÷ 2.5535

Circumference is the inner circumference of the ring in millimeters, which is also the EU and ISO size under the ISO 8653 standard. If you measured a diameter instead, circumference = diameter × π. We round the US result to the nearest quarter size, since that is the finest increment jewelers actually stock.

Worked example

Say your string measures 54.4 mm around your finger. US size = (54.4 − 36.537) ÷ 2.5535 = 7.0, so you wear a US 7. That same ring is a UK N 1/2, an EU 54.4 (the EU size is simply the circumference in millimeters), roughly a Japan 14, and has an inner diameter of 54.4 ÷ π = 17.3 mm. Measuring an existing ring at 17.3 mm across gives you the same answer from the other direction.

Why printable ring charts mislead

The internet is full of printable sizers and print-at-actual-size circle charts, and they fail for one boring reason: printers scale. "Fit to page," borderless mode, and PDF viewer zoom each shrink or stretch the chart by a few percent, and a few percent is a full ring size. A string and a ruler cannot be scaled by a print dialog, which is why the low-tech method beats the printable one every time. A plastic ring sizer beats both, if you want to spend a few dollars.

US 3 to 13 conversion table

Every value below comes from the same ISO 8653 math the calculator uses. Japanese sizes are approximate (roughly the circumference in millimeters minus 40), so confirm with the jeweler before ordering from a JP chart.

USDiameter (mm)EU / ISO (mm)UKJapan
314.144.2F 1/24
3 1/214.545.5G 1/25
414.946.8H 1/26
4 1/215.348.0I 1/28
515.749.3J 1/29
5 1/216.150.6K 1/210
616.551.9L 1/211
6 1/216.953.1M 1/213
717.354.4N 1/214
7 1/217.755.7O 1/215
818.157.0P 1/216
8 1/218.558.2Q 1/218
918.959.5R 1/219
9 1/219.460.8S 1/220
1019.862.1T 1/222
10 1/220.263.3U 1/223
1120.664.6V 1/224
11 1/221.065.9W 1/225
1221.467.2X 1/227
12 1/221.868.5Y 1/228
1322.269.7Z 1/229

Between sizes? Size up

Quarter sizes exist precisely because fingers do not land neatly on half sizes. If your measurement falls between two stocked sizes, take the larger one. A slightly loose ring spins a little and comes off when you want it to. A slightly tight ring is uncomfortable all day and can be genuinely hard to remove when fingers swell in heat or during exercise. Wide bands also wear tighter than thin ones, so for bands over about 6 mm wide, going up a quarter to half size from your measured size is standard jeweler practice.

Sizing other things while you are here? Try the hat size calculator or the shoe size converter.

Frequently asked questions

How do I find my ring size without them knowing?

Borrow a ring they already wear on the correct finger and measure its inner diameter, straight across the inside at the widest point. Enter that number in the existing-ring option above and you have their size without asking a single question. If borrowing is risky, trace the inside of the ring on paper while they are out, then measure the circle. Sizes differ between fingers and between hands, so make sure the ring you borrow lives on the finger you are shopping for.

How do I measure my ring size with string or paper?

Wrap string or a thin paper strip snugly around the base of the finger, mark where the end meets the wrap, then lay it flat and measure from end to mark with a ruler. That length is your circumference; enter it above in mm or inches. Measure at the end of the day when fingers are largest, and measure your knuckle too if it is bigger than the finger base.

What is the average ring size for women and men?

The most common women's ring size in the US is around 6 to 7, and the most common men's size is around 9 to 10. Averages are a last resort, though: if a surprise is the goal, measuring an existing ring is far more reliable, and most jewelers will resize within a size or two if the guess misses.

How do I convert my US ring size to UK or EU?

The EU and ISO size is simply your inner circumference in millimeters, so a US 7 is an EU 54.4. UK sizes use letters: a US 6 is a UK L 1/2, a US 7 is N 1/2, and a US 8 is P 1/2, moving half a letter for each quarter US size. Use the convert option above or the full table on this page for any size from US 3 to 13.

Are printable ring size charts accurate?

Usually not, and the reason is printer scaling. Fit-to-page settings, borderless printing, and PDF zoom each change the printed size by a few percent, and a few percent of a ring diameter is a full size. String and a ruler cannot be rescaled by a print dialog, and a cheap plastic ring sizer is more accurate than both.

Should I size up or down if I am between ring sizes?

Size up. A slightly loose ring spins a little but comes off when you want it to; a slightly tight one is uncomfortable and can be hard to remove when your fingers swell in heat or exercise. For wide bands over about 6 mm, jewelers also recommend going up a quarter to half size from your measured size.

When is the best time of day to measure ring size?

The end of the day, when your fingers are at their largest. Fingers swell with heat, activity, and salt, and shrink when cold, so a morning or winter measurement can run a half size small. Measure in the evening at normal room temperature, and take the measurement twice to make sure it repeats.

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