How to Measure Ring Size in CM

How to Measure Ring Size in CM – UK Guide 2026

The complete guide to measuring ring size in centimetres — paper strip, string, existing ring & diameter methods, plus a full CM to UK ring size chart for men and women

🇬🇧 UK Ring Sizes 📏 CM & MM Methods 📜 Paper Strip Method 🧵 String Method 💍 Existing Ring Method ✅ 2026 Updated

How to Measure Ring Size in CM – Key Facts

To measure ring size in CM, wrap a strip of paper or string around your finger, mark the overlap, then measure the length in centimetres with a ruler. That length is your finger circumference in CM. Divide by π (3.14159) to get the diameter in CM. The average UK women's ring size M has a circumference of 5.19 cm and the average men's size T is 6.14 cm.

📏 Measure in CM ✖️ ÷ π = Diameter ⭐ Women's avg: 5.19 cm ⭐ Men's avg: 6.14 cm

5.19cm

💍 Average Women's
Circumference (UK M)

6.14cm

💍 Average Men's
Circumference (UK T)

÷π

🔣 CM Circumference
→ CM Diameter

×10

📐 Convert CM
to MM (×10)

ℹ️

How to Measure Ring Size in CM – Overview 2026

Measuring ring size in centimetres (CM) is straightforward — you measure the inner circumference of the finger in CM using a paper strip, string, or flexible tape measure. UK ring sizes (letters A–Z+2) are based on the inner circumference in millimetres, so once you have your CM measurement you simply multiply by 10 to get mm, then look up the corresponding UK letter in the chart. This guide covers every measurement method, conversion formulas, and a complete CM-to-UK ring size chart for 2026.

🧮 How to Measure Ring Size in CM – Calculator

Enter your CM measurement to find your UK ring size instantly

Your UK Ring Size

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🔧 4 Ways to Measure Ring Size in CM at Home

📜

Method 1: Paper Strip

Cut a thin strip of paper (~10 cm long). Wrap snugly around the base of the finger, mark the overlap, then measure the strip length in CM. This is your circumference in CM.

🧵

Method 2: String or Thread

Wrap a non-stretchy piece of string around the finger base, mark the overlap, then lay flat against a CM ruler. Ideal if you don't have paper — works to the same accuracy.

💍

Method 3: Existing Ring

Place a well-fitting ring on a CM ruler and measure the inner diameter in CM. Multiply by π (3.14159) to get the circumference, then look up the UK size in the chart below.

📐

Method 4: Tape Measure

Use a flexible sewing tape measure wrapped directly around the base of the finger. Read off the CM value at the overlap point — the most direct measurement method of all.

📏

Method 5: Digital Calliper

Measure the finger's diameter directly in CM using a digital calliper for maximum precision. Convert to circumference by multiplying by π, or look up the diameter directly in the chart.

🏪

Method 6: Jeweller Sizing

For engagement rings or wedding bands, visit a UK jeweller for free professional sizing using metal ring gauges. This is the most accurate method — recommended for any ring over £100.

📜 Step-by-Step: Measure Ring Size in CM Using Paper Strip

Cut a thin strip of paper (approx. 10 cm long × 0.5 cm wide)

Use plain printer paper or any non-stretchy paper. The strip should be long enough to wrap fully around your finger with some overlap. Avoid tissue paper — it stretches and gives inaccurate results.

Wrap the strip snugly around the base of the correct finger

Wrap at the base of the finger — the narrowest part just above the palm. In the UK, engagement rings and wedding bands are traditionally worn on the left ring finger. The strip should be snug but not tight — you should be able to slide it slightly.

Mark exactly where the paper overlaps

Use a pen or pencil to mark the strip precisely where it overlaps. A clear, sharp mark gives you the most accurate measurement. If unsure, mark and re-wrap twice to confirm.

Lay the strip flat and measure in CM with a ruler

Lay the marked strip flat on a hard surface. Place a CM ruler from the end of the strip to your pencil mark. Read the measurement in centimetres — this is your finger circumference in CM.

Look up your UK ring size in the CM chart below

Find your CM circumference in the Circ. (CM) column of the chart in the next section. The UK letter size in the first column is your ring size. If between two sizes, always go to the larger UK letter.

Measure 2–3 times and take the average

Fingers change size throughout the day — they are smallest in the morning and largest in the evening. Measure at least twice at room temperature in the evening, take the average, and use that as your final CM measurement.

💍 Step-by-Step: Measure Ring Size in CM Using an Existing Ring

Choose a ring that fits the correct finger well

Select a ring that fits comfortably on the same finger and hand you intend to wear the new ring on. A ring that is slightly loose or tight will give an inaccurate result.

Place the ring flat on a CM ruler

Lay the ring face-up on a flat surface next to a CM ruler. Make sure the ruler has both CM and MM markings for maximum precision.

Measure the inner diameter in CM

Measure the diameter of the inside of the ring only — the part that touches the finger. Do not measure the outer diameter. Read the measurement in CM (e.g. 1.65 cm).

Convert diameter to circumference (optional)

Multiply the diameter in CM by π (3.14159) to get the circumference in CM. Example: 1.65 cm × 3.14159 = 5.18 cm circumference = approximately UK size M.

Match to the UK ring size chart below

Find your diameter or circumference in CM in the chart below. Use the Diam. (CM) or Circ. (CM) column to find your UK letter size directly — no conversion to MM needed.

🔣 Ring Size in CM – Conversion Formulas

CM Circumference → MM (for UK chart lookup)

MM = CM × 10

Example: 5.19 cm × 10 = 51.9 mm = UK size M (women's average)

CM Diameter → CM Circumference

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

Example: 1.65 cm × 3.14159 = 5.18 cm circumference = UK M

CM Circumference → CM Diameter

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

Example: 6.14 cm ÷ 3.14159 = 1.955 cm diameter = UK T (men's average)

Inches → CM Circumference

CM = Inches × 2.54

Example: 2.04 inches × 2.54 = 5.18 cm = UK M

📋 Ring Size Chart UK in CM – Full A to Z+2 Reference 2026

UK Size	Circ. (CM)	Diam. (CM)	Circ. (MM)	Diam. (MM)	🇺🇸 US	🇪🇺 EU	Best For
F	4.37 cm	1.39 cm	43.7 mm	13.9 mm	3	44	XS Women
G	4.49 cm	1.43 cm	44.9 mm	14.3 mm	3.5	45	Small Women
H	4.62 cm	1.47 cm	46.2 mm	14.7 mm	4	46	Small Women
I	4.71 cm	1.50 cm	47.1 mm	15.0 mm	4.5	47	Small Women
J	4.80 cm	1.53 cm	48.0 mm	15.3 mm	5	48	Women
J½	4.87 cm	1.55 cm	48.7 mm	15.5 mm	5.25	49	Women
K	4.93 cm	1.57 cm	49.3 mm	15.7 mm	5.5	49	Women
K½	4.99 cm	1.59 cm	49.9 mm	15.9 mm	5.75	50	Women
L ⭐	5.06 cm	1.61 cm	50.6 mm	16.1 mm	6	51	Women Popular
L½	5.12 cm	1.63 cm	51.2 mm	16.3 mm	6.25	51	Women Popular
M ⭐	5.19 cm	1.65 cm	51.9 mm	16.5 mm	6.5	52	Most Common ⭐
M½	5.25 cm	1.67 cm	52.5 mm	16.7 mm	6.75	53	Women Popular
N ⭐	5.31 cm	1.69 cm	53.1 mm	16.9 mm	7	53	Most Common ⭐
N½	5.38 cm	1.71 cm	53.8 mm	17.1 mm	7.25	54	Women
O	5.44 cm	1.73 cm	54.4 mm	17.3 mm	7.5	54	Unisex
O½	5.50 cm	1.75 cm	55.0 mm	17.5 mm	7.75	55	Unisex
P	5.59 cm	1.78 cm	55.9 mm	17.8 mm	8	56	Unisex
Q	5.72 cm	1.82 cm	57.2 mm	18.2 mm	8.5	57	Slim Men
R	5.85 cm	1.86 cm	58.5 mm	18.6 mm	9	59	Men
S	5.97 cm	1.90 cm	59.7 mm	19.0 mm	9.5	60	Men
T ⭐	6.14 cm	1.956 cm	61.4 mm	19.56 mm	10	61	Men Most Common ⭐
U ⭐	6.27 cm	1.996 cm	62.7 mm	19.96 mm	10.5	63	Men Popular
V ⭐	6.40 cm	2.037 cm	64.0 mm	20.37 mm	11	64	Men Popular
W	6.53 cm	2.078 cm	65.3 mm	20.78 mm	11.5	65	Men
X	6.66 cm	2.118 cm	66.6 mm	21.18 mm	12	67	Large Men
Y	6.78 cm	2.159 cm	67.8 mm	21.59 mm	12.5	68	Large Men
Z	6.91 cm	2.199 cm	69.1 mm	21.99 mm	13	69	XL Men
Z+1	6.97 cm	2.220 cm	69.7 mm	22.20 mm	13.5	70	XL / Custom
Z+2	7.23 cm	2.301 cm	72.3 mm	23.01 mm	14	72	Custom

⭐ UK M (5.19 cm) = most common women's size

⭐ UK T (6.14 cm) = most common men's size

📏 Multiply CM × 10 to get MM

🔣 Divide CM circumference by π to get CM diameter

💡 Tips for Accurate Ring Size Measurement in CM

🌡️ Measure in the evening at room temperature. Fingers are at their largest towards the end of the day. Avoid measuring first thing in the morning or in cold weather — fingers can shrink by up to a full UK letter size.
📜 Use non-stretchy paper or string. Tissue paper, elastic thread, or anything stretchy will give a smaller-than-actual measurement. Use printer paper strips or cotton thread for accuracy.
🔁 Measure at least twice. Always re-wrap and re-measure to confirm. Take the average of two or three readings in CM for the most reliable result.
📏 Check over the knuckle. If your knuckle is wider than the base of your finger, wrap the strip around the knuckle too. The ring must fit over it — size for the knuckle if it is significantly wider.
💍 Wide bands need a size up. If buying a ring 6 mm wide or more, add approximately 0.05–0.06 cm to your circumference measurement before looking up the chart to account for the wider band.
✅ When between sizes, always choose larger. If your CM measurement falls between two rows in the chart, always select the larger UK letter. A slightly loose ring is far easier and cheaper to adjust than a ring that is too tight.

📏 CM vs MM: Which Should I Use for UK Ring Sizing?

UK ring size charts typically list measurements in millimetres (MM), but measuring in CM and multiplying by 10 is equally accurate. If your ruler only shows CM, simply note your measurement in CM (e.g. 5.19 cm) and use the Circ. (CM) column in the chart above — no conversion needed. For measurements near a half-size boundary, using the MM column (51.9 mm) gives slightly more precision, as MM values are easier to read exactly on a standard ruler.

🔗 Related UK Ring Size Guides

📌 More Pages on This Site

📋 Full UK Ring Size Chart A–Z+2 💍 Men's Engagement Ring Size Guide UK ♀️ Women's Average Ring Size UK 🔄 UK to US Ring Size Conversion

🌐 Trusted UK Jeweller Resources

📐 Beaverbrooks – Ring Size Guide 📐 Goldsmiths – Know Your Ring Size 📐 H.Samuel – Ring Size Chart

❓ How to Measure Ring Size in CM – FAQs

How do I measure my ring size in CM at home? ▼

Cut a thin strip of non-stretchy paper or use a piece of string. Wrap it snugly around the base of the finger you want to wear the ring on. Mark where the strip overlaps, then lay it flat against a ruler and read the length in centimetres (CM). This is your finger circumference in CM. Look up your CM value in the Circ. (CM) column of the chart above to find your UK ring size letter.

What is the average UK women's ring size in CM? ▼

The average UK women's ring size is M, which has a circumference of 5.19 cm (51.9 mm) and a diameter of 1.65 cm (16.5 mm). Most UK women fall between sizes L and N, corresponding to circumferences of 5.06 cm to 5.31 cm. If you measure approximately 5.1–5.3 cm around your ring finger, you are in the most common women's range.

What is the average UK men's ring size in CM? ▼

The average UK men's ring size is T, which has a circumference of 6.14 cm (61.4 mm) and a diameter of 1.956 cm (19.56 mm). Most UK men fall between sizes T and V, corresponding to circumferences of 6.14 cm to 6.40 cm. If your measurement is approximately 6.1–6.4 cm, you are in the most common men's UK ring size range.

How do I convert CM to a UK ring size? ▼

To convert a CM circumference measurement to a UK ring size, multiply the CM value by 10 to get millimetres, then look up the MM value in a standard UK ring size chart. Alternatively, use the Circ. (CM) column in the chart on this page to find your UK letter size directly — no conversion needed. For example, 5.19 cm × 10 = 51.9 mm = UK size M.

How accurate is measuring ring size in CM with paper? ▼

Measuring with a paper strip in CM is accurate to within approximately ±0.05 cm (0.5 mm), which typically corresponds to about half a UK ring size. This is accurate enough for fashion rings and everyday jewellery. For engagement rings, eternity rings, or any ring over £100 in value, always get professionally sized at a UK jeweller with metal ring gauges for the most precise result.

What if my CM measurement is between two UK ring sizes? ▼

If your CM circumference falls between two rows in the chart, always choose the larger UK ring size. A slightly loose ring is more comfortable to wear, easier to put on and take off, and can be adjusted at any UK jeweller for a small fee. A ring that is too tight is uncomfortable and can restrict circulation — and is more expensive to resize upwards than downwards.

Can I measure ring size in CM using an existing ring? ▼

Yes — place a well-fitting ring flat on a CM ruler and measure the inner diameter in CM. Multiply that number by π (3.14159) to get the inner circumference in CM. For example, if the inner diameter is 1.65 cm: 1.65 × 3.14159 = 5.18 cm circumference = approximately UK size M. You can then match this to the Circ. (CM) column in the chart above.