Find Your Ring Size at Home – A Step-by-Step Guide

Black Jack® Sizing Guide

Find Your Ring Size Anywhere in the World.

Measure in millimetres first. Then use the Black Jack global chart to convert your measurement to the closest US, UK, European, Indian or Japanese size.

For custom, engraved, Memory Diamond and difficult-to-resize rings, confirm your measurement before production begins.

Finger Circumference Existing Ring Diameter Global Conversion Chart Wide-Band Guidance
Two Numbers Matter Most Inside Diameter + Finger Circumference INSIDE DIAMETER measure edge-to-edge FINGER CIRCUMFERENCE wrap snugly, not tightly

3 Ways to Measure

Use one method—or compare two for better confidence.

mark overlap wrap at the base of the intended finger
Idea 1 · Finger Method

Paper Strip or Non-Stretch Thread

Measure the finger circumference directly.

  • Use a strip about 8–10 mm wide
  • Wrap snugly around the finger base
  • Mark the exact overlap point
  • Measure the marked length in millimetres
INSIDE ONLY do not include metal thickness
Idea 2 · Existing Ring

Measure Inside Diameter

Best for gifts if you can borrow a ring that fits the same finger.

  • Lay the ring flat
  • Measure through the exact centre
  • Measure inside edge to inside edge
  • Record the diameter in millimetres
professional ring gauges + mandrel
Idea 3 · Best Confirmation

Jeweler’s Ring Sizer

Best choice for expensive, wide, custom or difficult-to-resize rings.

  • Ask a jeweler to size the intended finger
  • Use a gauge similar in width to your ring
  • Check knuckle clearance
  • Send Black Jack the measured size + mm result

3 Quick Examples

How a millimetre measurement becomes a ring size.

Example 01 · Circumference 54.4 mm

A finger measures 54.4 mm around. This is a common mid-range size.

US 7 UK/AU O France 54.5 India 14 Japan 14
Example 02 · Existing Ring 18.1 mm diameter

An existing ring measures 18.1 mm inside edge-to-edge.

US 8 UK/AU Q France 57 India 17 Japan 17
Example 03 · Larger Fit 62.1 mm

A measured circumference of 62.1 mm maps to a larger international size.

US 10 UK/AU T 1/2 France 62.5 India 22 Japan 21

Instant Size Converter

Enter your millimetre measurement.

Choose circumference or inside diameter. The tool finds the nearest reference size in the Black Jack conversion table.

For custom production, use the converter as guidance and confirm any borderline measurement with Black Jack or a professional jeweler.

Enter your measurement above. Millimetres are the most reliable cross-country reference.

Global Conversion Chart

Major ring-size systems in one table.

Use inside diameter or circumference as your master measurement. Country/brand labels can differ slightly, so do not rely on the label alone for a custom ring.

Inside Diameter Circumference US / Canada UK / Australia France / EU Italy Spain India* Japan
14.1 mm 44.2 mm 3 F 44 4 4.25 5 4
14.5 mm 45.5 mm 3.5 G 1/2 45.5 5.5 5.5 6 6
14.9 mm 46.8 mm 4 H 1/2 47 6.75 6.75 7 7
15.3 mm 48.0 mm 4.5 I 1/2 48 8 8 8 8
15.7 mm 49.3 mm 5 J 1/2 49.5 9.25 9.25 9 9
16.1 mm 50.6 mm 5.5 L 50.5 10.5 10.5 10 10
16.5 mm 51.9 mm 6 M 52 12 12 12 11
16.9 mm 53.1 mm 6.5 N 53 13 13 13 13
17.3 mm 54.4 mm 7 O 54.5 14.5 14.5 14 14
17.7 mm 55.7 mm 7.5 P 56 16 16 16 16
18.1 mm 57.0 mm 8 Q 57 17 17 17 17
18.5 mm 58.3 mm 8.5 R 58.5 18.5 18.5 18 18
18.9 mm 59.5 mm 9 S 59.5 19.75 19.75 19 19
19.4 mm 60.8 mm 9.5 T 61 21 21 21 20
19.8 mm 62.1 mm 10 T 1/2 62.5 22.25 22.25 22 21
20.2 mm 63.4 mm 10.5 U 1/2 63.5 23.5 23.5 24 23
20.6 mm 64.6 mm 11 V 1/2 65 25 25 25 24
21.0 mm 65.9 mm 11.5 W 3/4 66 26 26 26 —
21.4 mm 67.2 mm 12 Y 67.5 27.5 27.5 27 —
21.8 mm 68.5 mm 12.5 Z 69 28.75 28.75 29 26
22.2 mm 69.7 mm 13 Z+1 70 30 30 29 27
Important conversion note: International ring-size systems are not perfectly interchangeable across every jewelry brand or ring design. US/UK/Australia/France/Italy/Spain/Japan references above follow current Tiffany regional charts; India is matched to the nearest Tanishq Indian size. Use the millimetre diameter/circumference as the final common reference.

Fit Rules That Matter

The same numeric size can feel different on different rings.

Wide Bands

Wider bands generally feel tighter than slim bands. A larger size may be appropriate, but confirm before custom production instead of automatically adding a full size.

Knuckle Clearance

If the knuckle is much wider than the finger base, measure both. The ring must pass the knuckle without becoming loose at the base.

Temperature

Do not measure very cold fingers or immediately after exercise/heat. Measure when hands are at a normal comfortable temperature.

Between Sizes

If a measurement sits between two sizes, the larger size is often more comfortable—but band width and design should be considered before finalizing.

Black Jack custom-order recommendation: send both the international size you normally wear and either the exact inside diameter (mm) or finger circumference (mm). This gives the production team a measurable reference instead of relying only on a country-size label.

Need More Help?

Use the guide before your custom ring enters production.

Contact Black Jack Support→ Cancellation Policy→ Returns & Refunds→ Memory Diamond Engagement Rings→

Ring Size Support

Send us your measurement before production.

Email your Order ID, ring style, band width if known, normal country size, and either your inside diameter or finger circumference in millimetres.

For custom and Memory Diamond rings, confirming size before production can help avoid unnecessary resizing or remake work.

Email Ring Size Support →