The carat is the gem trade’s unit of mass, fixed internationally at exactly 200 milligrams since the early 20th century. Earlier, carats varied from city to city because they were based on the seeds of the carob tree. Today, a stone’s carat weight is the single number that most affects its price, and prices jump at “magic sizes” such as 0.50, 0.75, 1.00 and 2.00 ct.
When a stone is set in jewellery, its weight has to be estimated from measurements. This calculator uses the standard gemological formulas: each shape has a factor that converts length, width (or diameter) and depth into carats, assuming the density of diamond. The default example is an oval, for which the average of length and width is squared – an approach that slightly overestimates elongated ovals and is standard practice in appraisal.
Because buyers and sellers use different units, the result is shown in carats, grams, milligrams, ounces, pearl grains and points at once. That makes it easy to compare a jeweller’s description in points with a lab report in carats, or to see how small a “5-carat” stone really is in grams.
How to use the Carat Weight Calculator
- Diamond shape
- Diameter (average girdle diameter)
- Length
- Width
- Depth (table to culet)
- Girdle thickness
- Custom girdle correction
- Shape correction (wings, bulge, culet) — Optional: up to +10% for wide wings or high shoulders on pears, ovals, marquises and hearts; up to +18% for a bulging step-cut pavilion.
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Select the cut, enter the measurements in millimetres, inches or centimetres, choose the girdle thickness and any shape correction. The estimated carats are multiplied out into every other weight unit.
Worked example
An oval 8 × 6 mm with 3.7 mm depth and a slightly thick girdle: average diameter (8 + 6)/2 = 7 mm, so 7² × 3.7 × 0.0062 = 1.124 ct; with +3% for the girdle the estimate is 1.158 ct. That is 0.232 g, 231.6 mg, 0.00817 oz, 4.63 grains or 116 points, and a round brilliant of the same weight would be about 6.76 mm across.
Weight units at a glance
| Unit | In carats | In grams |
|---|---|---|
| 1 carat (ct) | 1 | 0.2 |
| 1 point (pt) | 0.01 | 0.002 |
| 1 grain (gem) | 0.25 | 0.05 |
| 1 gram | 5 | 1 |
| 1 ounce (avoirdupois) | 141.75 | 28.35 |
| 1 troy ounce | 155.52 | 31.10 |
Inputs explained
| Field | What to enter |
|---|---|
| Diamond shape | Choose one: Round brilliant, Oval, Princess, Cushion … |
| Diameter (average girdle diameter) | Units: mm, cm, in |
| Length | Units: mm, cm, in |
| Width | Units: mm, cm, in |
| Depth (table to culet) | Units: mm, cm, in |
| Girdle thickness | Choose one: Very thin to medium (+0%), Slightly thick (+3%), Thick (+5%), Very thick (+8%) … |
| Custom girdle correction | In % |
| Shape correction (wings, bulge, culet) | In %. Optional: up to +10% for wide wings or high shoulders on pears, ovals, marquises and hearts; up to +18% for a bulging step-cut pavilion. |
What each result means
- Weight in grams = [Estimated weight]×0.2
- Weight in milligrams = [Estimated weight]×200
- Weight in ounces = [Estimated weight]×0.2÷28.349523125
- Weight in grains = [Estimated weight]×4
- Weight in points = [Estimated weight]×100
- Equivalent round brilliant diameter (61% depth) = cbrt([Estimated weight]÷(0.0061×0.61))
Formula
Weight (ct) = dimensions (mm) × shape factor × girdle and shape corrections
1 ct = 0.2 g = 200 mg = 100 points = 4 grains · 1 oz (avoirdupois) = 141.75 ct
Worked example
Inputs — Diamond shape: Oval; Length: 8 mm; Width: 6 mm; Depth (table to culet): 3.7 mm; Girdle thickness: Slightly thick (+3%); Shape correction (wings, bulge, culet): 0 %.
Results — Length-to-width ratio = 1.33; Shape factor used = 0.0062; Correction multiplier = 1.03; Estimated weight = 1.158 ct; Depth percentage = 61.7 %; Weight in grams = 0.2316 g; Weight in milligrams = 231.6 mg; Weight in ounces = 0.00817 oz; Weight in grains = 4.63 gr; Weight in points = 116 pt; Equivalent round brilliant diameter (61% depth) = 6.78 mm.
Key terms
- Carat (ct)
- Unit of mass for gems, exactly 200 milligrams; 1 ct = 100 points.
- Shape factor
- Empirical constant that turns length × width × depth (in mm) into carats for one cut; it reflects the stone's volume fraction and diamond's density (3.52 g/cm³).
- Girdle
- The thin rim around the widest part of the stone; a thick girdle adds weight that the dimensions alone do not show.
- Length-to-width ratio
- Length ÷ width of a fancy shape; elongated emeralds, pears and marquises use a ratio-dependent factor.
- Depth percentage
- Total depth ÷ width (or diameter) × 100; about 59–63% for a well-cut round brilliant.
Frequently asked questions
How much is 1 carat in grams?
Exactly 0.2 g (200 mg). So 2 ct = 0.4 g and 5 ct = 1 g.
What is a point in diamond weight?
One hundredth of a carat: a 0.25 ct stone is a "25-pointer".
What is a grain?
In the pearl and diamond trade a grain is 0.25 ct (50 mg) – a "4-grainer" is about 1 carat.
How many carats are in an ounce?
One avoirdupois ounce (28.35 g) equals 141.75 carats.
How big is a 1-carat oval?
Roughly 8 × 6 mm with a depth near 3.5–3.7 mm; this calculator gives 1.12 ct for 8 × 6 × 3.7 mm before girdle correction.
Is carat the same as karat?
No. Carat (ct) is gem mass; karat (K) is gold purity, 24 K being pure gold.
Why do estimates differ from the certificate?
Formulas assume typical proportions; uneven girdles, bulging pavilions or unusual crowns can shift real weight by 5–10%.