Tweezers holding a shimmering diamond

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Diamond Size Guide

When buying a brilliant vintage diamond engagement ring, the diamond shape and size is a crucial consideration. In this blog, we will help you develop an understanding of how diamond sizing works, so you can be confident when choosing the perfect gemstone for your partner.

To understand how the size of a diamond is determined, we will define what a carat is, peruse a diamond carat size comparison chart, and guide you through some simple formulae in our diamond weight calculator section so you can find the classic engagement ring of your dreams.

What is a carat?

A carat is an ancient measurement unit of mass used within the diamond industry, and one of the main indicators of a diamond’s value. The term carat derives from the carob tree – the seeds of which were used to weigh gold, and from the early 16th Century, diamonds. Diamond traders used carats as a unit of measuring diamonds, believing that all the seeds were identical in size and weight. They were wrong.

Carats were standardised at the General Conference on Weights and Measures in 1907 and have since become the official metric used when buying diamonds and other gemstones, meaning Art Deco diamond rings, for example, are all consistent in sizing. Generally, larger diamonds are given a higher carat rating.

On a metric scale, one carat is equal to 200 milligrams, or 0.2 grams. Bear in mind that carat weight is not the same as size or volume. For instance, different gemstones have different densities, and a 1 carat diamond will not necessarily equal another 1ct gem’s physical dimensions.

Does carat size matter?

Despite carat size being a relatively simple concept, it can cause confusion. This is caused by two factors:

  • People tend to disregard the marked effect the other 3Cs (colour, clarity, and cut) and the overall diamond quality of craftsmanship (in well-cut diamonds) have on a diamond’s price.
  • When we talk about a diamond’s ‘size’, what we are discussing is its weight, as measured in carats – not its physical measurements. A heavier diamond will be considered a larger diamond, whether this is visibly the case or not.

Essentially, all diamonds – regardless of the cut and colour – are almost identical in their physical make-up, and it is the combination of the four C’s (carat, colour, clarity, and cut) that determine the diamond’s grade. As a general rule of thumb, the finer the stone, the higher the price, with varying values amongst different size diamonds.

Diamond size chart

Two diamonds that are equal in carat weight may look completely different due to the diamond shape. For example, a 1ct marquise diamond will usually appear larger than a 1ct round diamond.

The diamond carat size chart below illustrates the visual difference in a 1 carat diamond of different shapes. Keep in mind that while these measurements are typical, every diamond is unique.

Size Round Princess Oval Marquise Pear Cushion Emerald Asscher Radiant Heart
Length x Width (mm) 6.50






























Crown (mm2) 33.2 30.2 33.3 36.9 34.0 29.2 31.8 28.7 32.1 28.1

How to measure a diamond

Diamond weight calculator

To calculate a diamond’s weight in carats, start by weighing the stone and recording its weight in grams. Then divide that number by 0.2 For example, if a diamond weighs 0.2 grams, dividing this number by 0.2 equals 1, therefore, the stone weighs 1 carat.

How to estimate diamond carat weight using measurements

If you do not have a scale when diamond buying, you can estimate a diamond’s carat weight by taking its measurements. Keep in mind that the below formulas will give you approximate estimates, and you should use a scale to ensure pinpoint accuracy.

Estimating the carat weight of round diamonds

For round or oval diamonds, use the following formula to estimate their carat weight:

Weight in carats = Diameter in mm x Diameter in mm x Depth in mm x 0.006 The diameter is measured as a straight line through the centre of the diamond and connects two points on the diamond’s girdle (outer edge). The depth refers to the vertical distance from the diamond’s top facet to its bottom point.

For example, if your round diamond has a 6mm girdle diameter and a 4mm depth, then the carat weight can be estimated to be around 4.5ct, making this diamond a 4.5 carat stone.

Estimating the carat weight of square diamonds

Use the following formula to estimate the carat weight of square and rectangular cuts, such as the Asscher cut, the emerald cut, the radiant cut, and the princess cut:

Weight in carats = Length x Width x Depth x Coefficient

Measure the diamond’s length and width along its girdle – the depth is defined the same way as for round and oval diamonds. The value of the coefficient depends on the length-to-width ratio of the diamond. These are the possible values:

  • L/W Ratio 1.25: Coefficient = 0.0080
  • L/W Ratio 1.50: Coefficient = 0.0090
  • L/W Ratio 2.00: Coefficient = 0.0100
  • L/W Ratio 2.50: Coefficient = 0.0105

Do not worry if your diamond’s length-to-width ratio is not exactly equal to one of the values above – use the closest value.

For example, if you have an emerald-cut diamond that is 2.5mm deep, 5mm long and 3.5mm wide, its L/W ratio is 5/3.5 = 1.43.

By inputting these values into our formula, we yield a carat weight of 0.315ct (4 x 3.5 x 2.5 x 0.009).