How to Measure a Diamond

How do you measure a diamond?

Diamonds are measured with a precision diamond instrument. We use a Presidium dial gauge for this purpose. This enables us to measure the diameter and depth of loose diamonds. In detail, both measurements are taken in millimetres. (mm).

Don’t confuse diamond carat weight with measurement

Don’t confuse the size of a diamond with the weight of the diamond. Although both are related, they are different. Size is determined by measuring the physical size of the stone. Conversely, diamond weight is measured in Carats using a set of scales.

The Presidium gauge gives accurate readings for the diameter and depth of your diamond. The diameter gives information to help select the correct size of setting for your diamond. If you own your own diamond, we require this information to make a ring mount.

The Presidium diamond gauge

Buy the Presidium Gauge in our shop

Diamond measurement Presidium gauge

The best instrument for measuring your diamond is the Presidium gauge. We suggest using the dial version. Other variations exist. For example, some gauges use a digital display instead of a diamond.

This diamond measurement instrument gives an accurate reading of the millimetre size of your diamond.

Using the diamond measurement gauge

How to find out the diamond size without using a gauge

It is impossible to find an accurate diamond size without an instrument. Provided you have a diamond size chart, it is possible to estimate the size. Diamond charts (see below) feature most diamond shapes with their carat weight and approximate sizes.

View the chart online

Diamond Size Chart - Millimetre Sizes for Diamonds

Finding the size of a diamond from a diamond certificate

Some diamonds are laboratory certified. As a result, they include a report. To explain, this report includes key information. For example, the size, diamond colour and clarity. Diamond size appears on the report as shown below. Look under ‘measurements’ for the size. Here we see the following measurements.

Measurements………….4.97 – 4.98 x 3.01mm

But why are there three different measurements? To clarify, round diamonds vary fractionally in diameter. Depending on where you measure the width, most diamonds vary fractionally. As a result, the smallest and largest diamond reading appears. In addition, we find a record of the depth.

Replacement GIA certificate

How to measure small diamonds

Small diamonds come in regular millimetre sizes. We measure small diamonds to ensure consistency. Some items of jewellery feature many small diamonds. For example, cluster rings. As a result, we sieve and measure tiny diamonds. In addition to colour and clarity, we match diamonds in size. Interestingly, we buy diamonds in mixed sizes. Our team sorts diamonds using diamond sieves. Metal plates separate certain sizes.

Diamonds for eternity rings are just one example of accurately measured diamonds.

Diamond sizes for an eternity ring

What is the smallest size for a diamond?

In general, the smallest diamond size is 1mm in diameter. Diamonds are single-cut or fully cut in this size and arrive in parcels varying in quantity. 

How to measure a diamond. A guide to measuring the size of your diamond

We help many clients and jewellers with diamond measurements. Our knowledge and expertise come after decades of working with diamonds. As a result, we have answers for most diamond measurement questions. For example, how large specific shapes are. In reality, most fancy diamond shapes cause confusion. Many feature less regular measurements. For example, a Marquise can be short and fat. By comparison, some are long and skinny by comparison.

Why measure a diamond?

There are different reasons for finding out the measurement of a diamond. Firstly, it gives us information on what setting size to make.

We supply finished ring mounts for clients with existing diamonds. Therefore, we need the full measurements of the diamond to make the setting. For example, a diamond weighing 0.50cts. We would expect this diamond to measure 5mm in diameter. Interestingly, diameter is the most important measurement for a round brilliant cut diamond. Most ring designs come in a range of setting sizes. Each size holds a different diamond size. In this instance, we would create a 5mm setting for the diamond.

Big diamond in measuring gauge

Estimating carat weight by measuring a diamond

Sometimes we need to take the measurement of a diamond to estimate carat weight. When we create a jewellery valuation, we cannot always weigh the diamond. By taking measurements, we can estimate the diamond’s weight. Furthermore, we work with various formulas to calculate the weight.

Which measurements to note for your diamond

There are 3 x measurements to consider for most diamonds. We take the diameter for round diamonds. For elongated diamonds, we take length x width measurements. Finally, we take a recording of the depth.

The depth will be critical in some situations. For example, thick diamonds can be too deep for a setting. Measuring the depth helps to check this.

Formulae for calculating diamond carat weight require a depth measurement. (see the main photo for measuring the depth on the gauge.)

Round Brilliant Cut DiamondsDiameter – Diameter (this is the subtle difference from smallest to largest reading) x depth from table to the culet.

Princess Cut DiamondsLength x Width (most princess cuts are not perfectly square, but if the diamond is, length and width will be equal x depth from table to culet.)

Emerald Cut / Baguette Cut Diamonds  – Length x width x depth

Trilliant Cut DiamondsTypically measured point to point x depth.

About Mark Johnson

Mark is the founder and owner of Serendipity Diamonds which he founded in 2007 after more than a decade of working with polished diamonds. Today, he works as part of an expert team of fine jewellery experts based in their luxury Ryde showroom on the Isle of Wight in the UK.

Mark Johnson

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