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Diamond Carat

While most people have heard of a 1 carat diamond, how many people actually know what a carat is or how big a 1 carat diamond should be?

A quick way to estimate the carat weight of an ideal modern round brilliant cut diamond is to use this formula:

Length(mm) x Width(mm) x Height(mm) x 61/10000

Using carat to describe a diamond can be a little misleading. Carat refers to the weight of a diamond and one carat actually equals to 0.2 grams. But when you look at a diamond, what you actually see is the diamond’s diameter and not its weight. One quick way to tell whether a diamond has potential to be well-cut is to see whether it has the correct diameter for its weight. For example, a well-cut 1ct diamond should be approximately 6.5mm in diameter.

Diamond Size Chart

Here is a diamond size chart for you to quickly reference.

carat weight
carat weight 2

*Please note that these numbers are for diamonds with a thin to medium girdle (2-3%) and a total depth of 60%.

You can also use the chart below to find out how big a diamond will look with your finger size.


If you don’t know her ring size, you need to find out now. If you’re early on in your search as you should be then you should be able to do this without raising suspicion. There are ways that you can guess her ring size like using her other rings or asking a friend. What I did was very straight-forward, I found out at the beginning of our relationship and kept note of it until I needed it.

The Right Carat

It is possible that a 1ct diamond has a physically smaller diameter than a 0.8ct diamond, it all depends on how well the diamond is cut. The unfortunate truth is that on top of the fact poorly cut diamonds have a chance of being physically smaller, they are also more likely to be poorer performing diamonds as well. Poor performing diamonds often look smaller than they should be because of their optics.

When looking for an ideal cut diamond, not only are you trying to find a diamond that has the correct size for its weight, you should also ensure that the proportions are within the range that is recommended in the "How to pick a diamond" tutorial.

Price per Carat

When you search for diamonds, you need to look at the price per carat (ppc). The ppc price bands are split into carat ranges like 0 – 0.99ct, 1ct – 1.49ct, etc. and the ppc typically increases with carat size. However, even within a band such as 1ct – 1.49ct, there is a 5-10% jump in ppc around 1.25ct. There are many of these price jumps in every price band and they are often referred to as magic numbers.

The price jumps are understandable, as many more people demand a 1ct diamond than a 0.99 ct diamond. One interesting fact you might want to know about carat weighting is that a diamond will only be rounded up to 1ct if it is 0.999cts. Can you imagine what would happen to the cutter if he ended up with a diamond, which was 0.998cts?

Although you will probably not find a diamond that is 0.998cts, this demonstrates that it is possible to save some money by avoiding these price jumps by purchasing a diamond just under these magic numbers. It is true that you can save money by using these magic numbers. In essence you are exploiting the inefficiencies of the diamond market.

However, as the demand for diamonds that are just shy of these magic numbers increase, so have the market forces that work to balance out the prices. That means you probably should not worry too much about using this strategy to save money. Also, if you don’t know enough about the diamond you could very likely get burned. The problem is that are so many factors affecting the ppc of diamonds that you probably will not be able to notice these jumps when looking at price lists.

So should you optimise for size rather than cut?

Many people,  recommend optimising for cut. But if you optimise for cut, there is a good chance you may end up paying a 20-30% premium over a diamond optimised for size. One thing you can do is to go out to a jewellery store and compare the diamond you want with ones that are 20-30% larger, assuming you stay within the same price band. If you get more wow from the bigger diamond, then maybe you can consider optimising for size.

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