How to Calculate a Diamond’s Carat Weight

Knowing a diamond’s carat weight is essential when buying, selling, or valuing diamond jewelry. While professional jewelers use precision scales and lab equipment, you can still estimate a diamond’s carat weight at home using simple measurements or an accurate scale.

Below, we’ll explain exactly how carat weight works and show you two reliable ways to calculate it.

What Is a Carat?

A carat (ct) is a unit of weight, not size.

• 1 carat = 200 milligrams

• 1 carat = 0.2 grams

Because carat measures weight, two diamonds with the same carat weight may look different in size depending on their cut and proportions. This is especially true when comparing different gemstone types—diamonds, rubies, emeralds, and sapphires all have different densities.

Method 1: Weighing a Diamond (Most Accurate)

If you have access to a precision digital scale calibrated in grams or milligrams, this is the most accurate method.

Step-by-Step:

1. Weigh only the loose diamond (not the setting).

2. Record the weight in grams.

3. Divide the gram weight by 0.2 to convert to carats.

Example:

• Diamond weight: 0.30 grams

• Calculation:
  0.30 ÷ 0.2 = 1.50 carats

✔️ Result: The diamond weighs 1.5 carats

Tip: Jewelry scales must be extremely precise. Household kitchen scales are usually not accurate enough.

Method 2: Measuring a Diamond (Estimation)

If you don’t have a scale, you can estimate carat weight by measuring the diamond in millimeters. This method provides a close approximation and works best for round and oval diamonds.

Measuring a Round Diamond

You’ll need:

• A ruler or caliper marked in millimeters

Step-by-Step:

1. Measure the diameter (across the top).

2. Measure the depth (top to bottom).

3. Use this formula:

Diameter × Diameter × Depth × 0.006

Example:

• Diameter: 6 mm

• Depth: 3 mm

Calculation:
6 × 6 × 3 × 0.006 = 0.648 carats

✔️ Approximate weight: 0.65 carats

Measuring an Oval Diamond

1. Measure the length and width

2. Average the two measurements:

   • (Length + Width) ÷ 2

3. Measure the depth

4. Use the same formula

Example:

• Length: 10 mm

• Width: 6 mm

• Average diameter: 8 mm

• Depth: 4 mm

Calculation:
8 × 8 × 4 × 0.006 = 1.536 carats

✔️ Approximate weight: 1.54 carats

Important Notes About Carat Estimation

• Measurement formulas provide estimates, not exact values

• Cut proportions can affect results

• Mounted stones are harder to measure accurately

• Jewelers and labs use calibrated equipment for official grading

Final Thoughts

Carat weight plays a major role in a diamond’s value, but it’s only one part of the equation—cut, clarity, and color matter just as much.

Whether you’re estimating at home or verifying a purchase, understanding how carat weight works gives you confidence, knowledge, and yes—maybe a little jewelry bragging rights too.