How many mm is a 1 carat topaz?

About 6.4 mm for a round at typical depth, likely 6.2 to 6.5 mm. A 1 ct oval topaz is about 7.7 x 5.5 mm.

How big is a 2 carat topaz?

A 2 ct round topaz is about 8.0 mm, and a 2 ct oval topaz with a 1.4 ratio about 9.7 x 6.9 mm. At 3 ct, a round topaz reaches about 9.2 mm.

Is a 1 carat topaz bigger than a 1 carat diamond?

They are practically the same size, because topaz (SG 3.53) has almost the same density as diamond (SG 3.52).

How much does a topaz the size of a 1 carat diamond weigh?

A topaz cut to that size weighs about 1.00 ct, because weight at the same size follows density (SG 3.53 for topaz against 3.52 for diamond).

Is a 1 carat topaz the same size as a 1 carat diamond?

Very nearly. Their densities differ by well under 1%, so the difference in size is too small to see.

Why does topaz need careful setting?

Perfect cleavage means a sharp blow in the wrong direction can cause a clean break, even though topaz is hard.

Topaz Carat to mm Size Chart

Topaz size by carat weight

Topaz has nearly the same density as diamond. That makes it an easy stone to size: a topaz of a given weight and cut is almost exactly the size of a diamond of the same weight.

Blue topaz is the most common color in jewelry. Imperial topaz in orange and pink tones is rarer. Colorless topaz is sometimes used as a diamond alternative. All share the same density range.

Topaz face-up size by carat weight (typical proportions)
Carats	Round	Oval (1.4 ratio)	Cushion	Diamond round, same weight
0.25 ct	4.0 mm	4.9 x 3.5 mm	3.8 mm	4.1 mm
0.50 ct	5.1 mm	6.1 x 4.4 mm	4.8 mm	5.1 mm
0.75 ct	5.8 mm	7.0 x 5.0 mm	5.5 mm	5.9 mm
1.00 ct	6.4 mm	7.7 x 5.5 mm	6.1 mm	6.5 mm
1.50 ct	7.3 mm	8.8 x 6.3 mm	7.0 mm	7.4 mm
2.00 ct	8.0 mm	9.7 x 6.9 mm	7.7 mm	8.1 mm
3.00 ct	9.2 mm	11.1 x 7.9 mm	8.8 mm	9.3 mm
4.00 ct	10.1 mm	12.2 x 8.7 mm	9.7 mm	10.2 mm
5.00 ct	10.9 mm	13.2 x 9.4 mm	10.4 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 topaz density (3.49 to 3.57).

Topaz compared with diamond

Topaz has a specific gravity of 3.53, almost identical to diamond's 3.52, so a topaz and a diamond of the same weight and cut are effectively the same size. A diamond size chart is a good guide for topaz.

Use the diamond equivalent weight calculator to compare topaz with diamond, moissanite and other gems side by side, or the mm to carat calculator if you have a stone to measure.

Common topaz shapes

Ovals, cushions, rounds, emerald cuts and fancy shapes are all common. Large topaz is available, so cocktail rings with stones of 10 ct or more are not unusual.

Buying notes

Most blue topaz gets its color from treatment. That is standard, but it should be disclosed.
Topaz is 8 on the Mohs scale but has perfect cleavage, so a hard knock in one direction can split it. Protective settings help.
Because topaz and diamond have nearly the same density, a diamond size chart gives a good first guess for topaz.
For large stones, check depth. A deep topaz can carry a lot of hidden weight.

Topaz at a glance: specific gravity 3.53 (range 3.49 to 3.57), Mohs hardness 8. Almost the same density as diamond.

Other gemstone size charts

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.