Spinel Carat to mm Size Chart
Quick answer
A 1 carat round spinel is about 6.3 mm across and a 2 carat about 8.0 mm. A 1 ct oval is about 7.6 x 5.5 mm. With a specific gravity of 3.60, spinel looks smaller than a diamond of the same carat weight.
Typical depth and outline assumed; deep-cut stones face up smaller. Data verified .
Spinel carat to mm calculator
Spinel size by carat weight
Spinel is a little denser than diamond and lighter than sapphire, so it sits between them on the size chart. A spinel and a diamond of the same weight look very similar in size.
Spinel comes in red, pink, purple, blue and gray, and has a long history of being mistaken for ruby and sapphire. Today it is valued in its own right.
| Carats | Round | Oval (1.4 ratio) | Cushion | Diamond round, same weight |
|---|---|---|---|---|
| 0.25 ct | 4.0 mm | 4.8 x 3.4 mm | 3.8 mm | 4.1 mm |
| 0.50 ct | 5.0 mm | 6.1 x 4.3 mm | 4.8 mm | 5.1 mm |
| 0.75 ct | 5.8 mm | 6.9 x 5.0 mm | 5.5 mm | 5.9 mm |
| 1.00 ct | 6.3 mm | 7.6 x 5.5 mm | 6.0 mm | 6.5 mm |
| 1.50 ct | 7.2 mm | 8.8 x 6.3 mm | 6.9 mm | 7.4 mm |
| 2.00 ct | 8.0 mm | 9.6 x 6.9 mm | 7.6 mm | 8.1 mm |
| 3.00 ct | 9.1 mm | 11.0 x 7.9 mm | 8.7 mm | 9.3 mm |
| 4.00 ct | 10.0 mm | 12.1 x 8.7 mm | 9.6 mm | 10.2 mm |
| 5.00 ct | 10.8 mm | 13.1 x 9.3 mm | 10.3 mm | 11.0 mm |
Generated by our formula engine at typical depth: 61% of width for rounds and ovals, 70% for cushions. Round sizes carry a range of about plus or minus 3% that comes from the spread in published factors and in spinel density (3.57 to 3.70).
Spinel compared with diamond
Spinel has a specific gravity of 3.60 against 3.52 for diamond. At the same carat weight, a spinel is smaller than a diamond, by about 1% in diameter. Put the other way round, a spinel cut to the size of a 1 ct diamond weighs about 1.02 ct.
Use the diamond equivalent weight calculator to compare spinel with diamond, moissanite and other gems side by side, or the mm to carat calculator if you have a stone to measure.
Common spinel shapes
Ovals and cushions are most common, with rounds and octagons. Many spinels are cut to keep weight, with deep pavilions, so measure before trusting a size chart.
Buying notes
- Spinel is often sold without treatment. Ask anyway, and get a report for valuable stones.
- Iron or zinc rich spinel can be denser than the default, up to about 3.90, which makes it smaller per carat.
- Lab-grown spinel exists and has been used as an imitation of other gems for a long time.
- At 8 on the Mohs scale, spinel is a durable choice for everyday rings.
Spinel at a glance: specific gravity 3.60 (range 3.57 to 3.70), Mohs hardness 8. Iron or zinc rich spinel can be denser, up to about 3.90.
Other gemstone size charts
- Sapphire Carat to mm Chart
- Ruby Carat to mm Chart
- Emerald Carat to mm Chart
- Amethyst Carat to mm Chart
- Topaz Carat to mm Chart
- Garnet Carat to mm Chart
- Aquamarine Carat to mm Chart
- Cubic Zirconia Carat to mm Chart
- Diamond size chart in mm
- Gemstone specific gravity chart
Related questions
How many mm is a 1 carat spinel?
About 6.3 mm for a round at typical depth, likely 6.2 to 6.5 mm. A 1 ct oval spinel is about 7.6 x 5.5 mm.
How big is a 2 carat spinel?
A 2 ct round spinel is about 8.0 mm, and a 2 ct oval spinel with a 1.4 ratio about 9.6 x 6.9 mm. At 3 ct, a round spinel reaches about 9.1 mm.
Is a 1 carat spinel bigger than a 1 carat diamond?
A 1 ct spinel is smaller than a 1 ct diamond: 6.3 mm against 6.5 mm round, about 1% smaller across. In volume it matches a 0.98 ct diamond.
How much does a spinel the size of a 1 carat diamond weigh?
A spinel cut to that size weighs about 1.02 ct, because weight at the same size follows density (SG 3.60 for spinel against 3.52 for diamond).
Is spinel bigger or smaller than sapphire per carat?
Bigger. Spinel is less dense than sapphire, so the same weight takes up more space.
Why does spinel sometimes weigh more than expected?
Some spinels contain more iron or zinc, which raises density. If you can measure SG, enter it in the calculator as a custom value.
Sources and method
Weight estimates use the colored-stone shape factors published by the International Gem Society and the Dendritics standard formula, and the diamond trade constants and length to width ratio tables published in jewelry trade references (Esslinger, Omni, Octonus). Specific gravity values are cross-checked against the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect.
Every number on this page is generated by the same formula engine as the calculators. Read the full method, constants and limits and the editorial policy. Data last verified 2026-11-04.
Estimates only. This is not an appraisal, grading report or valuation. A calibrated scale or a lab report is the only real weight.