Diamond Size Calculator
Choose the shape and enter the length, width and depth in millimetres, as written on a grading report or measured with a gauge. The calculator estimates the carat weight and the size of a round stone of the same weight.
- Carat from mm
- Five shapes
- Round equivalent
For a round stone, the width is ignored and the length is the diameter.
Carat is weight, not size, but the two are closely linked. Jewellers estimate the weight of a mounted stone from its measurements and a factor for the shape. A well-cut 1 carat round diamond is about 6.4 mm across and 4 mm deep.
Diamond size chart
Round brilliant, ideal 61.5% depth. Search for a carat weight.
| Carat | Diameter (mm) | Depth (mm) |
|---|---|---|
| 0.25 | 4.1 | 2.5 |
| 0.5 | 5.1 | 3.1 |
| 0.75 | 5.8 | 3.6 |
| 1 | 6.4 | 4 |
| 1.5 | 7.4 | 4.5 |
| 2 | 8.1 | 5 |
| 3 | 9.3 | 5.7 |
Diameters are typical for a good cut. A shallow cut looks bigger face up for the same weight.
Carat and diameter
Round brilliant. Slide the carat weight.
The top view and side profile are drawn to the same scale, so you can see how depth grows with weight.
How to estimate carat
Measure and multiply. Step through it.
Measure
Length, width, depth in mm.
One carat is 0.2 grams, so a 1 carat diamond weighs the same as a fifth of a paper clip.
Round, oval or princess
Shapes. Compare them.
The most sparkle.
Looks bigger for its weight.
Square with sharp corners.
Elongated shapes like oval and emerald look larger face up than a round of the same weight.
Before buying
Size checks. Tick each item.
Tick each item as you go.
Ask for the grading report. It lists the exact measurements and weight.
What you get
Diamond sizing. Each tile explains one result.
Estimated weight.
Carat × 0.2.
Cut proportion.
For fancy shapes.
Same weight, round.
The estimate is for loose or mounted stones you cannot weigh. A grading lab weighs to 0.01 ct.
Frequently Asked Questions
No question matches that search.
How big is a 1 carat diamond?
About 6.4 mm across for a round brilliant.
How do I work out carat from mm?
For a round, diameter² × depth × 0.0061.
How big is a half carat diamond?
About 5.1 mm across for a round.
Does a bigger carat mean a bigger look?
Yes, but only slowly. Twice the weight is about 26% wider.
How much does a carat weigh?
0.2 grams.
Why does my diamond measure differently?
Cut depth changes the width. A deep cut hides weight.
Which shape looks biggest?
Oval, marquise and pear look largest for their weight.
Size up any diamond
Enter the measurements and get a carat estimate in seconds. Use it to compare stones fairly and spot one that is cut deep to add weight.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.