When you compare diamonds online or in a shop, the millimetre size is easy to see but the carat weight drives the price. Two stones with the same face-up size can differ by 10–20% in weight because one is cut deeper or has a thicker girdle, and that hidden weight costs money without making the diamond look bigger. Converting millimetres to carats helps you judge whether a stone “spreads” well for its weight.
This calculator uses the factors appraisers apply for twelve shapes. For step cuts like the emerald shown in the example, the factor rises as the stone gets longer, because a long rectangular stone fills more of its bounding box. The tool reports the length-to-width ratio – 1.30–1.50 is the classic emerald outline, 1.00–1.05 a square Asscher – and the depth percentage, which should usually sit around 60–70% for fancy shapes and 59–63% for rounds.
Enter a price per carat from a dealer list or offer and the calculator multiplies it by the estimated weight. Remember that the price per carat itself jumps at weight thresholds and varies with colour, clarity and cut.
How to use the Diamond Carat 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.
- Price per carat (optional) — Use any currency.
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Pick the shape, enter length, width (or diameter) and depth, choose girdle and shape corrections and, optionally, a price per carat. The tool computes the ratio, interpolates the shape factor and multiplies out weight and value.
Worked example
An emerald cut measures 7.0 × 5.0 mm and is 3.4 mm deep. Its ratio is 1.40, between the 1:1 (0.0080) and 1.5:1 (0.0092) factors, so k = 0.0080 + 0.4/0.5 × 0.0012 = 0.00896. Weight = 7 × 5 × 3.4 × 0.00896 = 1.066 ct; depth = 3.4/5 = 68%. At $6,000 per carat the stone is worth about $6,397.
Typical ratios by shape
| Shape | Popular length-to-width ratio |
|---|---|
| Emerald | 1.30–1.50 |
| Oval | 1.30–1.50 |
| Pear | 1.45–1.75 |
| Marquise | 1.75–2.25 |
| Radiant | 1.00–1.05 (square) or 1.20–1.35 |
| Cushion | 1.00–1.10 (square) or 1.15–1.25 |
| Princess, Asscher | 1.00–1.05 |
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. |
| Price per carat (optional) | Amount in $. Use any currency. |
What each result means
- Weight in grams = [Estimated weight]×0.2
- Estimated value = [Estimated weight]×[Price per carat (optional)]
- Weight in points = [Estimated weight]×100
Formula
Carats = L × W × depth × k(shape, L/W) × corrections (round: D² × depth × 0.0061)
Estimated value = carats × price per carat
Worked example
Inputs — Diamond shape: Emerald; Length: 7 mm; Width: 5 mm; Depth (table to culet): 3.4 mm; Girdle thickness: Very thin to medium (+0%); Shape correction (wings, bulge, culet): 0 %; Price per carat (optional): $6,000.
Results — Length-to-width ratio = 1.4; Shape factor used = 0.00896; Correction multiplier = 1; Estimated weight = 1.066 ct; Depth percentage = 68 %; Weight in grams = 0.2132 g; Estimated value = $6,397; Weight in points = 107 pt.
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 many carats is a 7 × 5 mm emerald-cut diamond?
About 1.07 ct at 3.4 mm depth: the 1.4:1 ratio gives a factor of 0.00896, and 7 × 5 × 3.4 × 0.00896 = 1.066 ct.
What is a good length-to-width ratio for an emerald cut?
Traditional emerald cuts run 1.30–1.50; below 1.10 they look square (closer to an Asscher), above 1.60 noticeably long.
How do I check if a diamond spreads well?
Compare its millimetre size with a well-cut stone of the same weight: a 1 ct round should be about 6.4–6.5 mm. A smaller diameter means hidden weight in depth or girdle.
Does the price scale linearly with carats?
No. Price per carat itself rises with size, so a 2 ct stone often costs three to four times a 1 ct stone of the same quality.
What depth percentage is ideal?
Rounds look brightest around 59–63%; emerald cuts typically 61–68%, ovals and pears 58–66%.
Why use the average diameter for ovals?
Squaring the mean of length and width approximates an oval's area better for typical ratios; it is the convention in appraisal formulas.
Can I estimate weight of a diamond in a ring?
Yes – measure the visible length and width with a gauge; depth needs a special gauge or an estimate from typical depth percentages.