Gem Carat Calculator

Gemstone and Carat Calculators

Quick answer

Enter a stone's measurements to estimate its carat weight, or a weight to see its size. A 6.5 x 4.0 mm round diamond is about 1.03 ct, and a 1 ct round diamond about 6.5 mm. Every result shows a range and the formula behind it.

Estimates from published formulas, not appraisals. Data verified .

mm to carat: estimate weight from size

Shape
Units
Measure across the girdle, edge to edge.
Measure again at 90 degrees. The two are averaged.
Depth entered as
From the flat top to the point or keel underneath.
More options: measured SG and weight corrections
Only if you have weighed the stone in water or have a lab figure.

Weight corrections

Tick only what you can see. Each one shifts the estimate and widens the range.

Results update as you type.

Estimated weight

1.03 ct

Likely range: 0.98 to 1.12 ct

0.206 g, 206 mg, 103 points

A round diamond of this size probably weighs about 1.03 carats, most likely between 0.98 and 1.12 ct. Only a scale gives the true weight.

Assumptions used
Material
Diamond (natural)
Specific gravity
3.52 (typical range 3.50 to 3.53)
Method
Diamond trade formula, diamond trade constant 0.0061
Other published factors
colored-stone factor 0.0018 x SG 3.52
Depth
62% of width
Length to width
1.00 to 1
Corrections
none
Proportion allowance
plus or minus 4%
How this was calculated

carats = (average diameter)² x depth x 0.0061

The range combines the other published factors for this shape, the SG range for the material, a 4% allowance for proportions the formula cannot see, and any corrections you ticked. Full method on the How we calculate page.

All calculators

Why these calculators are different

Most size charts online are built for one material, usually diamond, and give a single number as if it were exact. Gem weight does not work like that. A carat is a unit of weight, and how big a stone of a given weight looks depends on its density, shape and cut. A 1 ct sapphire is smaller than a 1 ct diamond, and a 1 ct emerald is bigger.

So every tool here covers 29 materials and 15 cuts, shows its working, and gives an honest range instead of false precision. The formulas come from published gemological and trade references, and every chart on the site is generated from the same engine as the calculators, so the numbers always agree.

Size charts

Gemstone carat to mm charts

Guides

Who this site is for

Shoppers comparing diamond, lab-grown diamond, moissanite and colored stones. Hobbyists who want to know what a stone in a family ring weighs. And small jewellery sellers on marketplaces who need to describe stones accurately, with a weight they can stand behind. Read about the site and our editorial policy.