One Carat Diamond Ring Buying Guide

Updated November 6th, 2024.

A one carat diamond engagement ring holds a special allure for those seeking a timeless and exquisite gemstone to symbolise a proposal. This guide is designed to help you navigate the intricate journey of choosing the ideal 1ct engagement ring. From understanding the 4Cs – carat weight, cut, colour, and clarity – to exploring key questions and what to consider before you buy, this guide has you covered.

We will provide you with the knowledge to make an informed decision and find the perfect one carat diamond engagement ring that matches your preferences and budget.

Historically, diamonds were measured with the number of carob seeds that equate to the same weight; a one carat diamond and a typical carob seed each equate to 0.20 grams. The metric has since evolved to become the highly accurate measurement known as ‘carat’ weight, measured and recorded by diamond grading laboratories like the GIA.

When considering the size of a one carat diamond, it's important to understand that it is often measured in millimetres, so how it looks on your hand will differ depending on the cut and shape.

Diamond shape can greatly affect how big the 1ct diamond appears; elongated shapes, such as pear or oval diamond ring can look larger than round diamonds of the same carat weight. So when it comes to questioning how big a one carat diamond is, there’s no one, straight answer.

A well cut, round brilliant diamond could look a different size to a poorly cut diamond with the same carat weight. Learn about cut grades and how this can affect the size your diamond appears.

In the table below we’ve collected some data on widths for one carat diamonds across a range of diamond shapes.

As you can see, a round one carat diamond size is usually around 6.34mm, while a cushion diamond is slightly narrower at 5.7mm. These are not uniform widths for all one carat diamonds but they are a helpful guideline for their size.

The table below details the width in mm of 1 carat diamond for different shapes.

'Carat' is a measurement of weight, rather than size, equating to 0.20 grams. Technically speaking, not all one carat diamonds will appear the same size. A well cut, round brilliant diamond could look a different size to a poorly cut diamond with the same carat weight. Learn about cut grades and how this can affect the size your diamond appears.

On average, one carat natural diamond ring costs between £6,200 and £9,000, with the optimum grades of beauty and value for money – or around £2,300 for a 1ct engagement ring with a lab diamond.

When setting a budget for your engagement ring, consider the costs of the various ring designs that interest you. The simpler the design, the more budget you can dedicate to the centre diamond of your ring! If you can’t find anything quite to your taste, we have an easy process to create your own engagement ring.

Here are some typical prices for different ring design styles, before the cost of the centre diamond is added:

• Solitaire from £850

• Diamond Band from £1,350

• Halo from £1,650

• Trilogy from £1,375

The table below shows the price of natural and lab diamonds for some common carat weights around 1ct.

There are several variables, other than carat weight, that can determine a diamond’s value. If you were to compromise greatly on the cut, clarity and colour grade of a natural one carat diamond, you could find options as low as £4,000. This, however, would directly impact the beauty of the stone; finding the right balance between the quality of the 4Cs is crucial for finding a diamond with brilliance, sparkle and lustre.

Keep the following grade ranges in mind:

• Triple Excellent cut – meaning Excellent cut, Excellent polish, and Excellent symmetry, or at the very least Very Good. Only round diamonds receive a cut grade, so look at polish and symmetry for other diamond shapes.

• D to F colour, or down to G or H if choosing a yellow gold ring.

• VS2 to SI1 clarity, or slightly higher for step-cut diamonds, like emerald cuts.

These grade boundaries will find you the most aesthetically beautiful diamonds - bright and rich in brilliance, whilst ensuring value for your money. Don’t get caught out by qualities that sound great, but in reality will cost you more for negligible benefit: for example, a higher clarity grade than VS2 will cost more money but will not be noticeable to the naked eye.

Connect with a Queensmith gemmologist – let them know your rough budget, and they’ll prepare a selection of the best diamonds or gemstones for your ring ahead of your appointment. No complex decisions, just great value for money.

When shopping for diamonds, consider those just under 1 carat, such as 0.97ct, 0.98ct, or 0.99ct. These diamonds can offer substantial savings while looking almost identical to a 1ct diamond. In most cases, you won't notice the slight difference in size.

The visible difference between a 1ct diamond and a 0.90ct diamond is minimal – but the price difference can be significant. Many engagement ring buyers will prefer to dedicate their budget to a high quality, beautiful diamond between 0.90ct to 0.99ct, rather than aiming to hit the 1ct but compromising on the quality of the stone.

Remember, the cut quality of the diamond has the largest impact on the stone's beauty, closely followed by the colour grade. A slightly smaller, but brighter, whiter and sparklier diamond will likely give you more joy than a larger stone that's dim, dull or yellow in tone.

These two photos show the same diamond ring. We’ve manipulated one of the images to make the stone slightly larger. One image reflects the size of a 0.90ct diamond, and the other a 1ct diamond. How noticeable is the 0.10ct difference to you?

The average carat size for engagement ring buyers is 1.25ct at Queensmith, as detailed in our annual ring trend report. The average carat size for UK engagement ring buyers is closer to 1ct, and you'll notice this varies by location, with hubs like London seeing larger carat weights purchased. The rise in popularity of lab diamonds, which have a lower cost-per-carat than natural diamonds, means the average carat weight is increasing.

You shouldn’t put too much pressure on buying a one carat diamond; as there are lots of alternatives that will make for a beautiful engagement ring.

Gemstones like sapphires, rubies and emeralds retail at a significantly lower cost than diamonds.

Lab grown diamonds can cost typically 60%, or even up to 80% less than a natural, mined diamond. Unlike diamond simulants like moissanite, lab diamonds are 100% real diamonds and are visually and physically identical to natural diamonds, so are a great way to ensure a large diamond at an exceptionally reasonable price.