How to Measure Ring Size in MM

Use millimetres to measure ring circumference or inside diameter, then match the result to the correct Australian size letter, US size and EU size with a simple chart or calculator.

📏 Diameter in mm 🔄 Circumference in mm 🧮 Instant Calculator 🇦🇺 AU Size Chart 🏠 Home Methods

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Diameter

Measure inside edge to inside edge

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Circumference

Wrap paper or string around finger

÷ 3.14

Convert circumference to diameter

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Size Up

Safer if measurement is between sizes

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Millimetres Give the Cleanest Ring Measurement

Ring sizing charts commonly work from inside diameter in mm or finger circumference in mm. If you measure circumference with paper or string, divide that number by 3.14 to estimate the inside diameter, then compare it with the nearest chart value.

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How to Measure in MM

Method 1 — Finger Circumference in mm

Wrap paper or string around the finger

Use a strip of paper, floss, or non-stretch string and wrap it around the base of the finger where the ring will sit.

Mark the overlap point

Mark where the strip or string meets itself without pulling too tight.

Measure with a ruler in millimetres

The measured length is your finger circumference in mm.

Match to the closest chart value

You can use the circumference directly or divide by 3.14 to estimate the diameter in mm.

Method 2 — Ring Diameter in mm

Place an existing ring on a flat surface

Use a ring that already fits the correct finger comfortably.

Measure the inside diameter only

Measure straight across the inner circle from one inside edge to the opposite inside edge in mm.

Ignore the outer band thickness

The ring size is based on the inside opening, not the outside width.

Use the nearest diameter on the chart

If the ring sits between two values, the larger one is usually more comfortable.

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MM Conversion Formulas

Circumference to diameter

Diameter (mm) = Circumference (mm) ÷ 3.14

Example: 53.2 mm ÷ 3.14 ≈ 16.9 mm

Diameter to circumference

Circumference (mm) = Diameter (mm) × 3.14

Example: 16.9 mm × 3.14 ≈ 53.1 mm

Quick rule

Use circumference if measuring the finger directly

Paper and string methods usually give circumference first.

Quick rule

Use diameter if measuring an existing ring

Rings are easiest to compare by inside diameter in mm.

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Ring Size Calculator in MM

Enter either circumference or diameter in millimetres

Closest Match

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Ring Size Chart in MM

AU Size	Diameter (mm)	Circumference (mm)	US Size	EU Size	Typical Fit
A	12.0	37.8	—	38	Child
B	12.3	38.5	—	39	Child
C	12.5	39.3	1	40	Child
D	12.8	40.0	2	41.5	Child
E	13.0	40.8	2½	43	Child
F	13.2	41.5	3	44	Child / Small
G	13.5	42.3	3½	45	Small Women
H	13.7	43.0	3¾	46	Small Women
I	13.9	43.8	4¼	47	Women
J	14.2	44.5	4¾	48	Women
K	14.4	45.3	5¼	50	Women
L	14.6	46.0	5¾	51	Women
M	14.9	46.8	6¼	52.5	Popular ♀
N ⭐	15.1	47.5	6¾	54	Most Common ♀
O	15.3	48.3	7¼	55	Popular ♀
P	15.6	49.0	7¾	56	Women / Men
Q	15.8	49.8	8¼	57.5	Women / Men
R	16.1	50.5	8¾	59	Men
S	16.3	51.3	9¼	60	Men
T ⭐	16.6	52.0	9¾	62	Most Common ♂
U	16.8	52.8	10¼	63	Popular ♂
V	17.0	53.5	10¾	64	Men
W	17.3	54.3	11¼	65	Men
X	17.5	55.0	11¾	66.5	Large Men
Y	17.8	55.8	12¼	68	Large Men
Z	18.0	56.5	12¾	69	Large Men

⭐ N = common women’s AU size

⭐ T = common men’s AU size

📐 Measure in mm for best chart matching

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Tips for Accurate MM Measuring

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Measure later in the day. Fingers can be smaller early in the morning and larger after heat or activity.

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Measure three times. Use the average if your readings vary slightly.

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Use a mm ruler. Millimetre markings are much more useful than inches for ring size charts.

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Wider bands feel tighter. If the ring is wide, a slightly larger size may feel better.

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Choose the larger size if between values. A slightly loose ring is usually safer than a tight one.

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Common MM Mistakes

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A Small MM Error Can Change the Size

Even a small reading difference can move you into another ring size, so careful measuring matters.

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Measuring the outer diameter. Always measure the inside opening only.

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Using a stretchy string. This can distort the circumference measurement.

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Pulling too tight. The strip should feel snug, not compressed into the skin.

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Ignoring the knuckle. The ring must still pass over the knuckle comfortably.

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Rounding down too aggressively. If your reading sits between two chart values, the larger option is often better.

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FAQ

Is ring size measured by diameter or circumference in mm? ▾

Both are used. If you measure the finger with paper or string, you usually get circumference in mm first. If you measure an existing ring, you usually get inside diameter in mm first.

How do I convert circumference in mm to diameter in mm? ▾

Divide the circumference by 3.14. For example, 53.2 mm circumference becomes about 16.9 mm diameter.

What if my mm measurement is between two ring sizes? ▾

Choosing the larger size is usually more comfortable, especially for wider bands.

Can I use a normal ruler for ring size in mm? ▾

Yes, as long as the ruler has clear millimetre markings and you measure carefully.

What mm size is a common women’s ring? ▾

A commonly referenced women’s size is around AU N, which is about 15.1 mm inside diameter or 47.5 mm circumference.