Sapphire Carat to mm Size Chart
Quick answer
A 1 carat round sapphire is about 6.1 mm across and a 2 carat about 7.7 mm. A 1 ct oval is about 7.4 x 5.3 mm. With a specific gravity of 4.00, sapphire looks smaller than a diamond of the same carat weight.
Typical depth and outline assumed; deep-cut stones face up smaller. Data verified .
Sapphire carat to mm calculator
Sapphire size by carat weight
Sapphire is corundum, the same mineral as ruby, and it is noticeably denser than diamond. That single fact explains most of the confusion when shoppers compare a sapphire with a diamond of the "same" weight: the sapphire is smaller.
Sapphire comes in almost every color. Blue is the classic, but pink, yellow, green, purple, orange and colorless (often sold as white sapphire) are all common, and all share the same density. The chart below therefore works for any sapphire color, natural or lab-grown.
| Carats | Round | Oval (1.4 ratio) | Cushion | Diamond round, same weight |
|---|---|---|---|---|
| 0.25 ct | 3.9 mm | 4.7 x 3.3 mm | 3.7 mm | 4.1 mm |
| 0.50 ct | 4.9 mm | 5.9 x 4.2 mm | 4.6 mm | 5.1 mm |
| 0.75 ct | 5.6 mm | 6.7 x 4.8 mm | 5.3 mm | 5.9 mm |
| 1.00 ct | 6.1 mm | 7.4 x 5.3 mm | 5.8 mm | 6.5 mm |
| 1.50 ct | 7.0 mm | 8.5 x 6.0 mm | 6.7 mm | 7.4 mm |
| 2.00 ct | 7.7 mm | 9.3 x 6.6 mm | 7.4 mm | 8.1 mm |
| 3.00 ct | 8.8 mm | 10.6 x 7.6 mm | 8.4 mm | 9.3 mm |
| 4.00 ct | 9.7 mm | 11.7 x 8.4 mm | 9.3 mm | 10.2 mm |
| 5.00 ct | 10.4 mm | 12.6 x 9.0 mm | 10.0 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 2% that comes from the spread in published factors and in sapphire density (3.95 to 4.05).
Sapphire compared with diamond
Sapphire has a specific gravity of 4.00 against 3.52 for diamond. At the same carat weight, a sapphire is smaller than a diamond, by about 4% in diameter. Put the other way round, a sapphire cut to the size of a 1 ct diamond weighs about 1.14 ct.
Use the diamond equivalent weight calculator to compare sapphire with diamond, moissanite and other gems side by side, or the mm to carat calculator if you have a stone to measure.
Common sapphire shapes
Most sapphires are cut as ovals and cushions, because those outlines keep more weight from the rough crystal. Rounds are less common in larger sizes. Many sapphires are cut with a deep belly underneath, which hides weight and makes them face up smaller than the chart.
Buying notes
- Compare millimetres, not carats, when choosing between a sapphire and a diamond for the same setting.
- Heat treatment is common and widely accepted for sapphire. Ask the seller to disclose any treatment in writing.
- A deep-bellied stone can look small for its weight. Ask for face-up measurements as well as carat weight.
- At 9 on the Mohs scale, sapphire is hard enough for daily wear in rings, which makes it a popular engagement stone.
Sapphire at a glance: specific gravity 4.00 (range 3.95 to 4.05), Mohs hardness 9. Includes blue, pink, yellow and white sapphire.
Other gemstone size charts
- 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
- Spinel 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 sapphire?
About 6.1 mm for a round at typical depth, likely 6.0 to 6.3 mm. A 1 ct oval sapphire is about 7.4 x 5.3 mm.
How big is a 2 carat sapphire?
A 2 ct round sapphire is about 7.7 mm, and a 2 ct oval sapphire with a 1.4 ratio about 9.3 x 6.6 mm. At 3 ct, a round sapphire reaches about 8.8 mm.
Is a 1 carat sapphire bigger than a 1 carat diamond?
A 1 ct sapphire is smaller than a 1 ct diamond: 6.1 mm against 6.5 mm round, about 4% smaller across. In volume it matches a 0.88 ct diamond.
How much does a sapphire the size of a 1 carat diamond weigh?
A sapphire cut to that size weighs about 1.14 ct, because weight at the same size follows density (SG 4.00 for sapphire against 3.52 for diamond).
Are lab-grown sapphires the same size per carat?
Yes. Lab-grown sapphire is the same mineral with the same density, so this chart applies to it too.
Why does my sapphire look smaller than the chart?
Probably a deep pavilion. Colored stones are often cut to keep weight rather than to maximise face-up size. Measure it and run the mm to carat calculator to see.
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.