Diamond Carat Weight Calculator, Size Chart & Guide
Diamond carat weight calculator and guide to estimate stone weight from millimetre measurements for modern and old cut diamonds, including rose cuts. Exact carat weight requires an unmounted stone on a calibrated gem scale.
Diamond Carat Weight Estimator
For round shapes, measure the diameter in two directions across the widest points. If you only have one diameter, repeat the same number for length and width.
If you leave depth blank, the calculator will assume a typical depth percentage for the chosen shape. Old cuts and rose cuts can vary more, so results are always approximate.
Diamond carat weight explained
Carat is the unit used to describe diamond weight. One carat equals 0.2 grams and is divided into 100 points, so a 0.25 carat stone is 25 points. Carat is often confused with size, but what you see on the hand depends on millimetre measurements and how the stone is cut, not only on carat weight.
Two diamonds of the same carat can face up differently. A deeper stone can hide more weight in the pavilion and look smaller from above, while a shallower stone may look larger but can leak light. Old cut and rose cut diamonds often have very different proportions from modern rounds, which is why a simple size chart is not always enough.
How this calculator estimates carat weight
The calculator uses three main measurements in millimetres:
- Length across the longest point of the stone.
- Width at the widest point at a right angle to the length.
- Depth from table to culet. If depth is not known, the tool assumes a typical depth percentage for the chosen shape.
For each shape and cut style the calculator applies a standard gem trade formula of the form Length × Width × Depth × shape factor. The shape factor and depth percentage are based on common proportions for round brilliants, ovals, emerald cuts, princess cuts, old cuts and rose cuts.
Results are given as an estimated carat weight plus a range. This reflects the fact that real stones vary in depth, girdle thickness and facet style. Old European, old mine and rose cuts in particular can sit well above or below the average, so the calculator is best used as a guide rather than a precise grading tool. For exact carat weight the diamond needs to be unset and weighed loose on a calibrated gem scale. These formulas are widely used in the trade but are still approximations; they cannot replace a professional valuation.
Round brilliant vs old cuts vs rose cuts
A well cut modern 1 carat round diamond usually measures around 6.4 to 6.5 mm across. Old European cuts of a similar weight can be slightly deeper with a smaller face up size, but their broader facets and open culets give a different type of presence on the hand. Old mine or cushion old cuts can be even more individual, sometimes squarer with chunky corners or cushion outlines.
Rose cuts are usually much shallower than modern round brilliants, often with a domed top and a flat base. They can look larger for their carat weight when viewed face up, but the actual weight for a given diameter is often lower than a comparable modern stone. This is why the calculator allows for more variation in estimates for rose cuts.
When you are comparing antique and modern rings it can be helpful to think in terms of both carat weight and visible size in millimetres. Carat weight is still the reference for value, but millimetres and cut style tell you how the diamond will actually look in a setting.
Quick round brilliant reference chart
The table below shows typical face up sizes for well cut round brilliant diamonds. These are averages and individual stones can be slightly larger or smaller for the same weight.
| Carat weight | Approx. diameter (mm) |
|---|---|
| 0.25 ct | ≈ 4.0 mm |
| 0.50 ct | ≈ 5.1 mm |
| 0.75 ct | ≈ 5.8 mm |
| 1.00 ct | ≈ 6.4 to 6.5 mm |
| 1.50 ct | ≈ 7.3 mm |
| 2.00 ct | ≈ 8.1 mm |
| 3.00 ct | ≈ 9.3 mm |
These measurements assume modern round brilliant proportions. Antique old cuts of the same carat weight can measure differently because of deeper pavilions, open culets and different crown heights. The calculator takes this into account when you choose an old cut shape.
Diamond carat and size FAQs
Is carat weight the same as diamond size?
No. Carat is weight in grams, while size is how large the diamond appears in millimetres. A deeper stone can carry more weight below the girdle and look smaller face up, while a shallower stone may look larger for the same carat.
How accurate is this diamond carat weight calculator?
It is designed as a guide based on common formulas and typical proportions for each shape. Results are estimates only and shown with a range. The only way to know the exact carat weight is to have the diamond unset and weighed loose on a calibrated gem scale, or to rely on a trusted laboratory report that has already done this work.
Why does my old cut diamond look larger or smaller than a modern diamond with the same carat?
Old European, old mine and rose cuts often have different depth, crown height and facet patterns compared with modern round brilliants. More weight can be held in the pavilion or the crown, which changes how much of the stone is visible from above. This is normal and part of the character of old cut diamonds.
Should I measure a diamond in or out of the setting?
Measurements are always easier and more reliable on loose stones, but many antique and vintage pieces cannot be unset. If you measure in the setting, take care to measure across the widest visible points without including the metal claws. Small errors in millimetres can change the estimate, so treat the result as a guide.
Can I use this calculator for sapphires, emeralds or other gemstones?
Diamond formulas are based on the density of diamond. Other gemstones have different densities, so the same measurements would give a different carat weight. You can still use the result as a very rough guide, but gemstone specific charts are better for accurate estimates.
Does diamond shape affect how big it looks for the carat weight?
Yes. Long shapes like ovals, marquise and pear cuts can look larger on the finger for the same carat weight because the length covers more of the finger. Round and square stones show their size more evenly in all directions. Old cuts and rose cuts have their own distinct presence as well.
What is the best way to know the exact carat weight of my diamond?
The most reliable method is to have the diamond unset and weigh the loose stone on a calibrated gem scale. If the stone has a laboratory report from a recognised gemmological lab, the report will state the carat weight. Calculators and charts are helpful when you only have millimetre measurements, but they cannot replace proper weighing and grading.
How does this relate to ring sizing?
Diamond carat weight and ring size are separate measurements. Carat tells you how much the stone weighs, while ring size tells you the inner diameter or circumference of the band. If you need help with both, you can also use the Ring Size Calculator and Chart on the dedicated page or contact for guidance.