Ring Size Chart in MM – Ring Sizes in Millimetres | Diameter & Circumference

Ring Size Chart in MM

Complete ring size chart in millimetres — finger diameter (mm) and circumference (mm) with full US, UK/AU, EU, French, German, Swiss and Japanese conversions. Free mm ring size calculator and step-by-step measurement guide.

📏 Diameter & Circumference in MM 🌍 US · UK/AU · EU · FR · DE · CH · JP 💍 Women's & Men's Charts 🧮 Free MM Calculator 📐 Measurement Formulas

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17.32mm

Average Women's
Diameter (US 7 / UK N½)

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54.40mm

Average Women's
Circumference

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19.76mm

Average Men's
Diameter (US 10 / UK T½)

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π × d

Circumference Formula
Circ = Diameter × 3.14159

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Why Use Millimetres for Ring Sizing?

Millimetres (mm) are the most universally accurate unit for ring sizing — used by jewellers, ring manufacturers, and sizing tools worldwide. Every international ring size system (US numbers, UK/AU letters, EU numbers, French, German, Swiss, Japanese) is ultimately based on finger measurements in mm. Knowing your finger diameter in mm or your finger circumference in mm means you can instantly convert to any ring size system in the world without any ambiguity. The two key mm measurements are: (1) Inside Diameter — measured straight across the inside of the ring or around the finger; and (2) Inside Circumference — the full distance around the inside of the ring, equal to Diameter × π (3.14159). The complete ring size chart in mm below covers all standard sizes from 14.0 mm (US 3 / UK F) through 22.6 mm (US 13½ / UK Z+2) with full international conversions.

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Key Ring Sizes in MM — Quick Reference

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Women's Average

17.32 mm

Diameter · 54.40 mm circumference
US 7 · UK/AU N½ · EU 54

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Men's Average

19.76 mm

Diameter · 62.10 mm circumference
US 10 · UK/AU T½ · EU 62

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Smallest Standard

14.0 mm

Diameter · 44.0 mm circumference
US 3 · UK/AU F · EU 44

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Most Common Women's Range

16.1–18.3 mm

Diameter range
US 5½–8¼ · UK K½–Q

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Most Common Men's Range

19.0–21.4 mm

Diameter range
US 9–12 · UK R½–X½

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Largest Standard

22.6 mm

Diameter · 71.0 mm circumference
US 13½ · UK Z+2 · EU 71

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Ring Size MM Calculator — Convert MM to All Systems

📏 Enter your finger measurement in mm to get your ring size in every system instantly

Your Ring Size in MM

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Complete Ring Size Chart in MM — Diameter & Circumference

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How to Read This Chart

The Diameter (mm) column is the measurement straight across the inside of a ring. The Circumference (mm) column is the total distance around the inside of the ring — this is also the measurement you get when you wrap a strip of paper around your finger. Use the calculator above to find your size instantly from either measurement. The green highlighted rows are the most commonly purchased ring sizes worldwide. Data sourced from Brilliance ring size conversion chart and The Jewellery Room international size guide.

⌀ Diameter (mm)	📏 Circumference (mm)	🇺🇸 US / Canada	🇬🇧 UK / AU / NZ	🇪🇺 EU / DK	🇫🇷 France	🇩🇪 Germany	🇯🇵 Japan	Range
14.0 mm	44.0 mm	3	F	44	44	14	4	Very Small
14.4 mm	45.0 mm	3¼	F½	45	45¼	14½	5½	Very Small
14.7 mm	46.0 mm	3¾	G	46	46½	15	6	Small
15.0 mm	47.0 mm	4	H	47	47¾	15¼	7	Small
15.3 mm	48.0 mm	4½	I	48	48	15¼	8	Below Avg
15.6 mm	49.0 mm	5	J	49	49	15¾	9	Below Avg
15.9 mm	50.0 mm	5¼	J½	50	50¾	16	10	Below Avg
16.1 mm	50.6 mm	5½	K½	51	51	16	10½	Popular ♀
16.3 mm	51.2 mm	5¾	L	51	51½	16¼	11	Popular ♀
16.5 mm	51.9 mm	6	L½	52	51½	16½	12	Popular ♀
16.7 mm	52.5 mm	6¼	M	52	52¾	16¾	12½	Popular ♀
16.9 mm	53.1 mm	6½	M½	53	52¾	17	13	Popular ♀
17.1 mm	53.8 mm	6¾	N	54	54	17¼	13½	Popular ♀
17.32 mm 🏆	54.40 mm	7	N½	54	54	17¼	14	Most Common ♀
17.5 mm	55.1 mm	7¼	O	55	55¼	17¾	15	Popular ♀
17.7 mm	55.6 mm	7½	O½	55	55¼	17¾	15½	Popular ♀
17.9 mm	56.3 mm	7¾	P	56	56¾	18	16	Popular ♀
18.2 mm	57.0 mm	8	P½	57	56¾	18	16½	Popular ♀
18.3 mm	57.6 mm	8¼	Q	58	58	18½	17	Popular ♀
18.5 mm	58.3 mm	8½	Q½	58	58	18½	17½	Larger
18.8 mm	59.0 mm	8¾	R	59	59¼	19	18	Larger
19.0 mm	59.5 mm	9	R½	60	59¼	19	18½	Popular ♂
19.2 mm	60.2 mm	9¼	S	60	60¾	19½	19	Popular ♂
19.4 mm	60.8 mm	9½	S½	61	60¾	19½	19½	Popular ♂
19.6 mm	61.4 mm	9¾	T	62	61¾	19¾	20	Popular ♂
19.76 mm 🏆	62.10 mm	10	T½	62	61¾	20	20	Most Common ♂
20.0 mm	62.7 mm	10¼	U	63	62¾	20¼	21	Popular ♂
20.2 mm	63.4 mm	10½	U½	63	62¾	20¼	22	Popular ♂
20.6 mm	64.6 mm	11	V½	64	64¼	20¾	23	Popular ♂
21.0 mm	65.9 mm	11½	W½	66	66	21	24	Popular ♂
21.4 mm	67.2 mm	12	X½	67	67¼	21¼	25	Popular ♂
21.8 mm	68.0 mm	12½	Z	68	68	21¾	26	Large
22.2 mm	69.0 mm	13	Z+1	69	69	22	27	XL
22.6 mm	71.0 mm	13½	Z+2	71	71	22.6	29	Custom

🏆 17.32mm = Most common women's size (US 7 / UK N½)

🏆 19.76mm = Most common men's size (US 10 / UK T½)

📐 Circumference = Diameter × π (3.14159)

💡 Sources: Brilliance · The Jewellery Room · Shiels AU

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Ring Size MM Formulas — Diameter & Circumference

📐 Essential Ring Size Millimetre Formulas

Diameter → Circumference (the key formula)

Circumference (mm) = Diameter (mm) × 3.14159 (π)

Example: 17.32 mm × 3.14159 = 54.40 mm → US 7 / UK N½ (most common women's size)

Circumference → Diameter

Diameter (mm) = Circumference (mm) ÷ 3.14159 (π)

Example: 62.10 mm ÷ 3.14159 = 19.76 mm → US 10 / UK T½ (most common men's size)

Inches → MM (diameter)

Diameter (mm) = Diameter (inches) × 25.4

Example: 0.682 inches × 25.4 = 17.32 mm → US 7 / UK N½

EU Size → Circumference in MM (EU size IS the circumference)

EU Ring Size Number = Finger Circumference in mm (exactly)

EU size 54 = 54 mm circumference = 17.19 mm diameter = US 7 / UK N½. The EU system is the simplest — the number directly equals the circumference in mm.

MM Circumference → US Ring Size (approximate)

US Size ≈ (Circumference mm − 36.5) ÷ 2.55

Example: (54.40 − 36.5) ÷ 2.55 = 7.0 → US 7. For precision always use the full chart above.

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How to Measure Your Ring Size in MM at Home

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Two Measurements — Circumference MM or Diameter MM

There are two equally valid ways to get your ring size in mm: (1) Measure your finger circumference in mm — wrap a strip of paper or string around your finger, mark the overlap, and measure with a ruler in mm. This is your circumference in mm. (2) Measure the diameter of an existing ring in mm — lay the ring flat and measure straight across the inside from inner wall to inner wall. Both measurements are accepted by all jewellers worldwide. Enter either measurement into the MM calculator above to instantly display all size equivalents. Measure at room temperature — finger size fluctuates with heat (larger), cold (smaller), and time of day (larger in evening).

Method 1 — Paper Strip: Measure Circumference in MM

Cut a non-stretchy paper strip approximately 10 cm × 1 cm

Use printer paper, dental floss, thin ribbon, or cotton string — nothing elastic. Stretchy materials over-measure and produce a ring that feels too tight. Do not use tape or sticky materials that may grip the skin unevenly.

Wrap snugly around the base of your finger

Wrap around the base of the specific finger and hand the ring will be worn on. The strip should be snug — not tight — and still able to pass over the knuckle with gentle pressure. If your knuckle is larger than the base of your finger, size for the knuckle (add about 1 mm to the base-finger reading).

Mark the overlap and measure in mm with a ruler

Mark where the strip ends meet with a fine pen. Lay it flat against a metric ruler and read the measurement in millimetres. This number is your finger circumference in mm — enter it into the calculator above with "Circumference (mm)" selected.

Take three readings at the right time of day and average them

Finger size is smallest in the morning and largest in the evening (by up to 1–1.5 mm circumference). Measure two or three times across one session in the afternoon for the most accurate year-round result. Always go up ½ a size if your measurement falls between two sizes on the chart.

Method 2 — Existing Ring: Measure Diameter in MM

Choose a ring that fits the correct finger and hand well

Select a ring worn on the same finger and hand as the new ring will be worn on. Sizes differ between fingers on the same hand and between left and right hands — never use a ring from a different finger as a proxy. Confirm the ring fits correctly under normal temperature conditions.

Measure the internal diameter in millimetres

Lay the ring flat on a table. Measure straight across the inside from inner wall to inner wall in mm. Digital calipers give the most precise result to 0.1 mm. A fine ruler also works. Do not measure the outside diameter — the band width will produce an inaccurately large result.

Enter into the MM calculator above and read all conversions

Select "Diameter (mm)" from the dropdown, enter your measurement, and click the button. Your full size in US, UK/AU, EU, French, German, Swiss, and Japanese systems will display instantly — the most complete mm ring size conversion available online.

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International Ring Size Systems Explained in MM

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Every Ring Size System is Based on MM — Here's How Each Works

Every international ring sizing system ultimately derives from millimetre measurements of the finger. The key difference between systems is what they display — some show the circumference in mm directly (EU/French), some use letters (UK/AU), some use arbitrary numbers (US, Japanese). Knowing your finger diameter or circumference in mm lets you convert to any system instantly using the chart above. The EU sizing system is the most transparent — the EU number IS the finger circumference in mm (EU 54 = 54 mm circumference).

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US / Canada — Number Scale

US sizes run from 3 (14.0 mm diameter / 44 mm circ) to 13½ (22.6 mm / 71 mm circ) in ¼ or ½ increments. US size 7 = 17.32 mm diameter = the most common women's size.

US 7 = 17.32 mm ⌀

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UK / AU / NZ — Letter Scale

UK and AU sizes are identical — letters F through Z+. Each half-letter step = approximately 0.4 mm diameter change. UK N½ = 17.32 mm diameter = the most common women's size in the UK, Australia, and New Zealand.

UK N½ = 17.32 mm ⌀

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EU / Denmark — Circumference in MM

The EU number IS the finger circumference in mm — the most direct mm system. EU 54 = 54 mm circumference = 17.19 mm diameter. Used across most of continental Europe and Scandinavia.

EU 54 = 54 mm circ

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France — Circumference in MM

French sizing also uses circumference in mm, very similar to the EU system but with some ¼-step increments. French size 54 = approximately 54.4 mm circumference = US 7 / UK N½. Used in France, Belgium, and parts of Southern Europe.

FR 54 ≈ 54 mm circ

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Germany — Diameter in MM

German ring sizes are the inside diameter in mm — the most direct diameter-based system. German size 17 = 17 mm diameter = approximately US 6½ / UK M½. Used in Germany, Austria, and the Netherlands.

DE 17 = 17 mm ⌀

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Japan — Number Scale

Japanese sizes run from 1 to 30 — the numbers are not in mm but are a separate arbitrary scale. Japanese size 14 = US 7 = UK N½ = 17.32 mm diameter. Japanese sizes are used across Japan, China, and parts of South-East Asia.

JP 14 = 17.32 mm ⌀

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Tips for Accurate MM Ring Size Measurement

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Measure at room temperature — never in extreme heat or cold. Finger circumference changes measurably with temperature. In heat (above 30°C), fingers swell by up to 1–2 mm circumference — adding up to a full ring size. In cold (below 10°C), fingers contract by a similar amount. Always measure indoors at normal room temperature (18–24°C) for an accurate, year-round mm ring size.
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Measure in the afternoon — not first thing in the morning. Fingers are at their smallest first thing in the morning and largest in the late afternoon and evening. The difference can be up to 1.5 mm in circumference — nearly half a ring size. For the most practical year-round fit, measure between 2pm and 6pm when your fingers are at their typical daily size.
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Use a metric ruler — not an imperial tape measure — for precision. Ring sizes change by as little as 0.4 mm per half-size step. An imperial ruler measuring in 16ths of an inch gives only approximately 1.6 mm precision — too coarse for accurate ring sizing. Always use a metric ruler marked in millimetres and read to the nearest 0.5 mm for reliable results.
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The EU number is your circumference in mm — the simplest conversion. The EU ring size number equals the finger circumference in mm exactly — so if you measure 54 mm around your finger, your EU size is 54, your US size is 7, and your UK/AU size is N½. For anyone wanting the simplest possible ring sizing approach, measuring circumference in mm and using the EU system requires no conversion at all.
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When between sizes — always go up. If your circumference measurement falls between two chart entries, select the larger size. A ring that is very slightly roomy can be worn comfortably — a ring even 0.3 mm too tight can feel restrictive by the end of a warm day. This is especially important for wide-band rings, which need an extra 0.4–0.8 mm (½ US/UK size) added to accommodate the wider band grip.
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Knuckle larger than finger base? Size for the knuckle. If your knuckle is noticeably wider than the base of your finger — common on index and middle fingers — measure both the knuckle and the base circumference in mm, add the two together and divide by 2, then round up. This ensures the ring can pass over the knuckle while not being too loose at the base.
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Wide band rings need +0.4 to +0.8 mm added to diameter. Wide, flat band rings (such as men's wedding bands) grip the finger more tightly than slim rings with a round profile, because more surface area is in contact with the skin. Add approximately 0.4 mm to your measured diameter (about ½ a UK/US size) for wide bands 6 mm or wider. Enter the adjusted diameter into the calculator above for the correct wide-band size.
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Take three readings and average for reliability. Even with careful technique, finger measurements can vary by 0.3–0.5 mm between attempts due to slight posture or pressure differences. Take three separate readings during the same measuring session, add them together, divide by three, and round to the nearest 0.5 mm. Enter this averaged value into the mm calculator for the most statistically reliable ring size result.

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Frequently Asked Questions — Ring Sizes in MM

What is the average women's ring size in mm? ▾

The average women's ring size worldwide is approximately 17.32 mm diameter / 54.40 mm circumference — equivalent to US size 7, UK/AU size N½, EU size 54, and Japanese size 14. The common women's range is 16.1 mm to 18.3 mm diameter (50.6 mm to 57.6 mm circumference), corresponding to US 5½–8¼ and UK K½–Q. If buying a surprise ring for a woman without knowing her size, 17.32 mm diameter (54.40 mm circumference) is the best default estimate.

What is the average men's ring size in mm? ▾

The average men's ring size is approximately 19.76 mm diameter / 62.10 mm circumference — equivalent to US size 10, UK/AU size T½, EU size 62, and Japanese size 20. The common men's range is 19.0 mm to 21.4 mm diameter (59.5 mm to 67.2 mm circumference), corresponding to US 9–12 and UK R½–X½. For a surprise gift for a man, 19.76 mm diameter (62.10 mm circumference) is the safest default, with most retailers offering free resizes within 30 days.

How do I measure my ring size in mm at home? ▾

There are two methods: (1) Paper strip circumference method — cut a thin non-stretchy strip of paper, wrap it around the base of your finger, mark where the ends overlap, measure the overlap length in mm with a metric ruler. This is your circumference in mm. Enter it into the MM calculator on this page with "Circumference (mm)" selected. (2) Existing ring diameter method — lay a well-fitting ring flat and measure straight across the inside from inner wall to inner wall in mm. Enter this as "Diameter (mm)" in the calculator. Both measurements instantly convert to US, UK/AU, EU, French, German, and Japanese ring sizes.

What is the difference between ring size diameter mm and circumference mm? ▾

The diameter (mm) is the distance straight across the inside of a ring from one inner wall to the other — the widest point through the centre. The circumference (mm) is the total distance around the inside of the ring — the measurement you get when you wrap a strip of paper around your finger. The two are related by the formula: Circumference = Diameter × π (3.14159). So a ring with a 17.32 mm diameter has an inside circumference of 17.32 × 3.14159 = 54.40 mm. Both measurements appear in the chart above and are accepted by all jewellers worldwide.

Is the EU ring size the same as the circumference in mm? ▾

Yes — exactly. The European (EU) ring size number equals the finger circumference in mm directly. EU size 54 means the finger circumference is 54 mm. EU size 62 means 62 mm circumference. This makes the EU system the most transparent and directly mm-based ring size system. To convert from EU to diameter, simply divide by π: EU 54 ÷ 3.14159 = 17.19 mm diameter. To convert from any measured circumference in mm to EU size, the number is already your EU size — no conversion needed.

How much does ring size change with temperature in mm? ▾

Finger circumference can change by 1–2 mm with significant temperature changes. In hot conditions (above 35°C), fingers swell — adding up to 1.5 mm circumference, which is about one US/UK ring size. In cold conditions (below 5–10°C), fingers contract by a similar amount. Even daily fluctuation from morning to evening can account for 1–1.5 mm. This is why jewellers recommend measuring at room temperature (18–24°C), in the afternoon, after your hands have been at a stable indoor temperature for at least 15–20 minutes.

How many mm is one ring size in US / UK? ▾

One full US ring size equals approximately 0.8 mm in diameter or approximately 2.55 mm in circumference. One half US size = approximately 0.4 mm diameter / 1.27 mm circumference. One UK/AU half-letter step (e.g., N to N½) = approximately 0.2 mm diameter / 0.63 mm circumference. One EU size step = exactly 1 mm circumference = approximately 0.32 mm diameter. This means the EU system has the finest gradations of any ring size scale, while the US ½-size step is the coarsest at 0.4 mm diameter per step.

Do UK and Australian ring sizes use the same mm values? ▾

Yes — UK and Australian ring sizes are completely identical. UK size N½ and AU size N½ both correspond to exactly 17.32 mm diameter and 54.40 mm circumference (US 7, EU 54). There is no conversion needed between UK and AU letter sizes — they are one and the same sizing system. New Zealand also uses the same UK/AU letter system. All mm values in this ring size chart in mm apply equally to UK, Australian, and New Zealand jewellers.