Ruby Size Chart: Carat to mm
Quick answer
A 1 carat round ruby 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, ruby looks smaller than a diamond of the same carat weight.
Typical depth and outline assumed; deep-cut stones face up smaller. Data verified .
Ruby carat to mm calculator
Ruby size by carat weight
Ruby is red corundum. It has the same density as sapphire, so at any carat weight a ruby and a sapphire of the same cut are the same size. Both are smaller than a diamond of equal weight.
Fine rubies above a carat are scarce, which is why so many ruby rings use small stones, often well under half a carat. Knowing what those small weights look like in millimetres helps you judge a listing quickly.
| 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 ruby density (3.95 to 4.05).
Ruby compared with diamond
Ruby has a specific gravity of 4.00 against 3.52 for diamond. At the same carat weight, a ruby is smaller than a diamond, by about 4% in diameter. Put the other way round, a ruby cut to the size of a 1 ct diamond weighs about 1.14 ct.
Use the diamond equivalent weight calculator to compare ruby with diamond, moissanite and other gems side by side, or the mm to carat calculator if you have a stone to measure.
Common ruby shapes
Ovals and cushions dominate, with rounds, pears and hearts in smaller numbers. Small calibrated rubies (for example 3 mm rounds or 5 x 3 mm ovals) are common in accent and channel settings.
Buying notes
- Ask about treatment. Heat is common. Some low-priced rubies are filled with lead glass, which is much less durable and must be disclosed.
- A lab report is worth having for any ruby priced as a fine natural stone.
- If a ring lists ruby weight as total weight, split it up with the total carat weight calculator before comparing.
- Ruby is a 9 on the Mohs scale, so it suits everyday rings when the stone itself is not heavily fractured or filled.
Ruby at a glance: specific gravity 4.00 (range 3.95 to 4.05), Mohs hardness 9.
Other gemstone size charts
- Sapphire 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 ruby?
About 6.1 mm for a round at typical depth, likely 6.0 to 6.3 mm. A 1 ct oval ruby is about 7.4 x 5.3 mm.
How big is a 2 carat ruby?
A 2 ct round ruby is about 7.7 mm, and a 2 ct oval ruby with a 1.4 ratio about 9.3 x 6.6 mm. At 3 ct, a round ruby reaches about 8.8 mm.
Is a 1 carat ruby bigger than a 1 carat diamond?
A 1 ct ruby 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 ruby the size of a 1 carat diamond weigh?
A ruby cut to that size weighs about 1.14 ct, because weight at the same size follows density (SG 4.00 for ruby against 3.52 for diamond).
Does glass-filled ruby weigh the same?
Not exactly. The glass in fractures has a different density from corundum, so weight estimates for heavily filled stones are less reliable. The chart assumes solid corundum.
Is a 1 carat ruby big?
In face-up size, a 1 ct ruby is a little smaller than a 1 ct diamond. In rarity terms, a fine 1 ct ruby is a significant stone.
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.