Diamond size charts usually list a single millimetre value for each carat weight, but the relationship depends on the shape and on how deep the stone is cut. Because weight grows with the cube of size, doubling the carat weight increases the diameter by only about 26%: a 0.5 ct round is about 5.1 mm, a 1 ct round about 6.45 mm and a 2 ct round about 8.1 mm.
This converter works in both directions. In mm → carat mode you enter the measured diameter (or length and width) and depth, and it applies the standard shape factor. In carat → mm mode you enter a target weight, a depth percentage and, for fancy shapes, a length-to-width ratio; the calculator solves the formula for the width or diameter, then derives length and depth. That is useful for checking a seller’s claim, planning a setting or comparing what different weights look like on the hand.
The results assume typical proportions with your chosen girdle and shape corrections. Stones cut deep or with thick girdles will measure smaller for the same weight than the table suggests.
How to use the MM to Carat Conversion Calculator
- I want to
- Diamond shape
- Diameter (average girdle diameter)
- Length
- Width
- Depth (table to culet)
- Target weight
- Depth percentage — Depth ÷ width × 100. About 61% for a round brilliant, 60–70% for fancy shapes.
- Length-to-width ratio
- 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
Choose the direction. For mm → ct, enter the dimensions and depth; for ct → mm, enter the weight, depth percentage and ratio. Both directions use the same shape factors, girdle and shape corrections.
Worked example
mm → carat: a round stone 5.2 mm across and 3.2 mm deep weighs 5.2² × 3.2 × 0.0061 = 0.528 ct. Carat → mm: for exactly 1 ct with 61% depth, D = ∛(1 ÷ (0.0061 × 0.61)) = 6.45 mm and the depth is 6.45 × 0.61 = 3.94 mm. For a 1 ct oval with a 1.4 ratio and 62% depth, W = ∛(1 ÷ (1.2² × 0.62 × 0.0062)) = 5.65 mm and L = 7.91 mm.
Round diamond mm to carat chart
| Weight | Diameter (round, 61% depth) | Depth |
|---|---|---|
| 0.25 ct | 4.07 mm | 2.48 mm |
| 0.5 ct | 5.12 mm | 3.12 mm |
| 0.75 ct | 5.86 mm | 3.58 mm |
| 1 ct | 6.45 mm | 3.94 mm |
| 1.25 ct | 6.95 mm | 4.24 mm |
| 1.5 ct | 7.39 mm | 4.51 mm |
| 2 ct | 8.13 mm | 4.96 mm |
| 2.5 ct | 8.76 mm | 5.34 mm |
| 3 ct | 9.31 mm | 5.68 mm |
| 4 ct | 10.24 mm | 6.25 mm |
| 5 ct | 11.03 mm | 6.73 mm |
Inputs explained
| Field | What to enter |
|---|---|
| I want to | Choose one: Convert millimetres to carats, Find the size (mm) for a carat weight |
| 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 |
| Target weight | In ct |
| Depth percentage | In %. Depth ÷ width × 100. About 61% for a round brilliant, 60–70% for fancy shapes. |
| Length-to-width ratio | A number |
| 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
- Length needed = [Width ÷ diameter needed]×[Length-to-width ratio]
- Depth needed = [Width ÷ diameter needed]×[Depth percentage]÷100
- Weight in grams = [Estimated weight]×0.2
- Weight in points = [Estimated weight]×100
Formula
mm → ct: ct = D² × depth × 0.0061 (round), L × W × depth × k (fancy shapes) × corrections
ct → mm (round): D = ∛(ct ÷ (0.0061 × depth% ÷ 100 × corrections)), depth = D × depth%
Fancy: W = ∛(ct ÷ (ratio × depth% × k × corrections)), L = ratio × W
Worked example
Inputs — I want to: Convert millimetres to carats; Diamond shape: Round brilliant; Diameter (average girdle diameter): 5.2 mm; Depth (table to culet): 3.2 mm; Girdle thickness: Very thin to medium (+0%).
Results — Shape factor used = 0.0061; Correction multiplier = 1; Estimated weight = 0.528 ct; Depth percentage = 61.5 %; Weight in grams = 0.1056 g; Weight in points = 53 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 mm is a 1 carat diamond?
A well-cut round brilliant of 1 ct is about 6.45 mm in diameter and 3.94 mm deep at 61% depth.
How many carats is a 5 mm diamond?
About 0.47 ct for a round brilliant with 61% depth (5² × 3.05 × 0.0061).
What size is a 2 carat round diamond?
About 8.1 mm across – only 26% wider than a 1 ct stone, because weight grows with the cube of size.
How do I convert mm to carats for an oval?
Square the average of length and width, multiply by depth and 0.0062. An 8 × 6 × 3.6 mm oval is about 1.09 ct.
What size oval is 1 carat?
With a 1.4:1 ratio and 62% depth: about 7.9 × 5.65 mm, 3.5 mm deep.
Why does my diamond look smaller than the chart?
It may be cut deep or have a thick girdle – weight hidden below the girdle does not add face-up size.
Does this work for lab-grown diamonds?
Yes – lab-grown and natural diamonds have the same density, so the same factors apply.