Stone & Ring Size Guide: Millimetres, Carats & Ring Fit
ZGems education · Updated
This combined guide covers two important measurements: gemstone size in millimetres and carats, and ring fit using Australian, US and European sizing. Compare stone dimensions first, then check the ring size before ordering or commissioning custom jewellery.
Part 1 — Stone size, dimensions & carat weight
Carat measures weight, not visible diameter. Use a gemstone's measurements in millimetres when choosing a stone or checking an existing setting.
Read the measurements before the carat number
- Round stones: compare diameter, or minimum and maximum diameter if both are given, plus depth.
- Other shapes: compare length × width × depth in millimetres. An oval listed as 8 × 6 × 4 mm is 8 mm long, 6 mm wide and 4 mm deep.
- Carat weight: 1 ct equals 0.20 g. Two gems can have the same weight and different dimensions because their material density and cutting differ.
See GIA’s explanation of the metric carat and GIA’s explanation of weight versus visible size. Ask for the selected stone’s measurements rather than inferring them from a carat option.
Compare circular diameters
This is an enlarged relative illustration, not an actual-size ruler. It shows how changing a circular outline from 6 mm to 12 mm changes its visible footprint. Do not place a ring or gemstone against the screen to measure it.
On a small screen, scroll across the table to see every column.
| Diameter (mm) | Diameter (inches) |
|---|---|
| 4 | 0.157 |
| 5 | 0.197 |
| 6 | 0.236 |
| 6.5 | 0.256 |
| 7 | 0.276 |
| 8 | 0.315 |
| 9 | 0.354 |
| 10 | 0.394 |
| 11 | 0.433 |
| 12 | 0.472 |
Inches are rounded to three decimal places. Jewellery measurements are usually compared in millimetres; use the exact millimetre dimensions when discussing a setting.
Approximate round diamond size reference
For round brilliant diamonds only. These approximate diameter references are educational examples, not ZGems product specifications. They must not be used as a conversion chart for sapphires, rubies, emeralds, tourmalines or every diamond shape.
On a small screen, scroll across the table to see every column.
| Diamond weight (ct) | Approximate diameter (mm) | Approximate diameter (inches) |
|---|---|---|
| 0.50 | 5.2 | 0.205 |
| 0.70 | 5.8 | 0.228 |
| 0.90 | 6.3 | 0.248 |
| 1.00 | 6.5 | 0.256 |
| 1.25 | 6.9 | 0.272 |
| 1.50 | 7.4 | 0.291 |
| 1.75 | 7.8 | 0.307 |
| 2.00 | 8.2 | 0.323 |
| 2.50 | 8.8 | 0.346 |
| 3.00 | 9.4 | 0.370 |
Millimetre references: IGI’s Diamond Grading Chart. Actual measurements vary with cutting and proportions. IGI illustrates different visible sizes at the same carat weight; ask for the individual stone’s dimensions and report details when available.
Compare shapes by length and width
On a small screen, scroll across the table to see every column.
| Outline example | Example face-up dimensions | What to compare |
|---|---|---|
| Round | 6.5 × 6.5 mm | Diameter and depth. |
| Oval | 8 × 6 mm | Length, width and how broad the outline looks. |
| Cushion | 7 × 7 mm | Outline, corners and actual proportions. |
| Emerald or radiant outline | 8 × 6 mm | Length, width and the actual faceting. |
| Pear | 9 × 6 mm | The pointed end and its setting protection. |
| Marquise | 10 × 5 mm | Both points and the elongated outline. |
These dimensions are examples chosen to explain measurement, not sizes offered for a particular listing. An 8 × 6 mm outline has a length-to-width ratio of about 1.33; a 9 × 6 mm outline has a ratio of 1.50. Depth and shape still affect the finished setting.
Coloured gems need their own measurements
There is no single reliable “1 carat equals this many millimetres” rule for every gemstone. Material density, outline, depth and cutting all affect how weight is distributed. A diamond chart cannot establish the size of a sapphire, ruby, emerald or tourmaline.
For ZGems’ 1–5 ct gemstone options, confirm the selected stone’s length, width and depth. Do not assume that every material or shape uses the same dimensions. If matching an existing piece, share the measurements and the matching requirement before ordering.
Read the colour guide and clarity guide alongside size rather than choosing from carat weight alone.
Before choosing or reusing a setting
- Send the jeweller the exact stone dimensions and outline, including depth.
- For an existing mount, ask the jeweller to assess the seat and setting; the outer opening is not enough to confirm fit.
- Discuss setting height, stone protection and the intended jewellery type.
- For a ring, confirm finger size separately. A gemstone’s diameter does not determine ring size.
- For jewellery with several stones, establish whether a quoted carat figure is the centre stone’s weight or total carat weight.
Use our US, Australian and European ring size guide for the finger fit, or send the product link and measurements through a custom jewellery enquiry.
Stone sizing questions
Is a two-carat stone twice as wide as a one-carat stone?
No. Carat is weight, and a stone has three dimensions. Compare the actual millimetre dimensions rather than multiplying its visible width by the carat difference.
Is a carat different in the US, Australia or Europe?
No. A metric carat is 0.20 g. Ring size labels differ between regions, but the metric carat does not.
Do the circles show the actual physical size?
No. They are enlarged relative illustrations. Device size, zoom and printing can change a displayed scale; use a ruler or the individual stone’s measured dimensions.
Will a stone fit a setting if the carat weight matches?
Not necessarily. Shape, length, width, depth and setting construction must be checked by the jeweller.
Explore stones and related guides
Explore gemstones · Explore lab-grown diamonds · Cuts and shapes guide · gemstone education hub
Part 2 — Ring size, conversion & comfortable fit
Ring size is independent of the gemstone's carat weight. Use the chart to compare Australian letter sizes, US numerical sizes and European circumference-based sizes, then confirm the actual fit with a ring sizer or jeweller.
US, Australian and European ring sizing
- US: a numerical scale, commonly including half and quarter sizes.
- Australia: a letter scale, including half-letter sizes. Always tell us which country or sizing system your measurement comes from.
- Europe: the circumference-based EU reference uses the ring’s inside circumference in millimetres. For example, EU 54 refers to approximately 54 mm around the inside. Some European retailers use other national numbering systems, so check the stated system.
Diameter is the distance straight across the opening. Circumference is the distance around that opening. They are different measurements: a 17.3 mm diameter is approximately a 54.3 mm circumference, not a size 17.3 in the EU circumference system.
International ring size comparison chart
Use this as an approximate comparison, then confirm the fit. The US and Australian letter references below follow GIA’s published chart. Other jewellers’ gauges and charts can differ slightly. The EU column shows the corresponding inside circumference; a retailer may offer whole or half numbers rather than every decimal shown.
On a small screen, scroll across the table to see every column.
| US size | Australia letter reference | EU / inside circumference (mm) | Inside diameter (mm) |
|---|---|---|---|
| 3 | F | 44.3 | 14.1 |
| 3½ | G | 45.6 | 14.5 |
| 4 | H | 46.8 | 14.9 |
| 4½ | I | 48.1 | 15.3 |
| 5 | J | 49.3 | 15.7 |
| 5½ | K | 50.6 | 16.1 |
| 6 | L | 51.8 | 16.5 |
| 6½ | M | 53.1 | 16.9 |
| 7 | N | 54.3 | 17.3 |
| 7½ | O | 55.6 | 17.7 |
| 8 | P | 56.9 | 18.1 |
| 8½ | Q | 58.1 | 18.5 |
| 9 | R | 59.7 | 19.0 |
| 9½ | S | 60.9 | 19.4 |
| 10 | T | 62.2 | 19.8 |
| 10½ | U | 63.5 | 20.2 |
| 11 | V | 64.7 | 20.6 |
| 11½ | W | 66.0 | 21.0 |
| 12 | X | 67.2 | 21.4 |
| 12½ | Y | 68.5 | 21.8 |
| 12¾ | Z | 69.1 | 22.0 |
References: GIA’s international ring size chart and Cartier’s European circumference-based sizing reference. These are general conversions, not a guarantee that a particular ring will fit or that every size is available from ZGems.
Measure a ring that already fits
- Choose a round ring that fits the exact finger and hand where the new ring will be worn. A ring from another finger is only a rough clue.
- Place it flat and measure straight across the centre of the opening, from inside edge to inside edge, in millimetres. Exclude the metal band.
- Record the inside diameter and compare the millimetre measurement with the chart. If the opening is distorted, do not rely on this method.
- For a custom or important purchase, take the existing ring to a jeweller for measurement and confirm the size for the proposed design.
The inside-diameter method is illustrated in Michael Hill’s measuring guide. A photograph, screen circle or outside diameter cannot replace the inside measurement.
For the most reliable fit, use a ring sizer
Ask a jeweller to measure the intended finger with fitting rings, taking the band width and profile into account. The ring must pass over the knuckle and sit comfortably once in place. Tell the jeweller whether you plan to wear several rings together.
Paper or string can stretch, crease or tighten differently from a ring. Treat home measurements as estimates rather than the final basis for an expensive custom order. Avoid measuring while your hands are unusually hot or cold; repeat a measurement if it seems inconsistent.
Professional fitting guidance: GIA’s ring sizing advice.
Band width, half sizes and ordering
- Wide bands and stacks: they can feel different from a fine band. Confirm the fit using a comparable width instead of adding an automatic size allowance.
- Between sizes: ask for fitting advice for that design and tell us the measurements you obtained. Do not guess which country’s chart an unlabelled number belongs to.
- Surprise gifts: an existing well-fitting ring can help. Confirm which finger it is normally worn on.
- Resizing: feasibility depends on the metal, stone layout and construction. Ask about the particular item before ordering.
See Michael Hill’s ring fitting tips for discussion of hand differences, knuckles and wide shanks. For ZGems, confirm available sizes, sizing arrangements and any cost or timing for the selected ring; this guide does not promise a resizing service.
Ring sizing questions
Is a US size 7 the same as an EU size 7?
No. The same numeral can refer to completely different systems. A US 7 is approximately 54.3 mm inside circumference in the reference chart above; EU circumference-based sizing is around that millimetre value.
Should I measure the outside of my ring?
No. Measure the opening from inside edge to inside edge. Band thickness makes the outside diameter unsuitable for this comparison.
Can I measure my finger using a circle on my phone?
Screen size and zoom can change the displayed scale. Use millimetre measurements or a physical ring sizer; this web page is not a calibrated measuring instrument.
What should I send with a ZGems enquiry?
Send the ring link or design idea, intended finger, your measured size and its country/system, any inside diameter or circumference measurement, and the planned band width through our custom jewellery enquiry.
Choose a stone and plan your ring
Stone size reference above · Cuts and shapes guide · Explore jewellery · gemstone education hub