What size (carats) diamond should I buy?

We analyzed the 70000+ engagement rings in James Allen’s diamond inspiration gallery to determine what size diamonds people buy in practice.

Natural Diamonds

The table below shows the carats for each percentile by diamond shape. For example, for round natural diamonds, the median carat weight (50th percentile) is 0.88ct, while the 75th percentile is 1.09ct.

Natural Diamond Carat Percentile Summary
Shape 5% 10% 25% 50% 75% 90% 95%
Round 0.41 0.5 0.7 0.88 1.09 1.5 1.74
Cushion 0.52 0.7 0.85 1.01 1.21 1.7 2.01
Radiant 0.51 0.64 0.81 1.01 1.3 1.76 2.02
Princess 0.45 0.51 0.71 0.92 1.05 1.5 1.71
Oval 0.5 0.6 0.76 1 1.15 1.51 1.8
Pear 0.5 0.51 0.71 0.9 1.03 1.32 1.51
Marquise 0.43 0.5 0.61 0.77 1 1.11 1.22
Heart 0.42 0.5 0.7 0.82 1.01 1.15 1.51
Emerald 0.51 0.69 0.82 1.01 1.32 1.74 2.02
Asscher 0.7 0.7 0.84 1.01 1.2 1.6 2

The table below summarizes the data from the other direction – what is the percentile for milestone carat weights? For example, for round natural diamonds, a 1ct diamond is the 66th percentile and a 1.5ct diamond is the 90th percentile.

Natural Diamond Percentiles for Milestone Carat Weights
Shape 0.25ct 0.5ct 0.75ct 1ct 1.25ct 1.5ct 1.75ct 2ct 2.25ct 2.5ct
Round 0.01 0.13 0.38 0.66 0.82 0.9 0.95 0.97 0.99 0.99
Cushion 0 0.03 0.17 0.46 0.78 0.86 0.92 0.95 0.99 0.99
Radiant 0 0.05 0.21 0.42 0.72 0.83 0.9 0.93 0.98 0.98
Princess 0 0.09 0.32 0.6 0.85 0.91 0.96 0.97 0.99 0.99
Oval 0 0.05 0.25 0.55 0.82 0.88 0.95 0.97 0.99 0.99
Pear 0 0.08 0.33 0.64 0.88 0.94 0.97 0.98 1 1
Marquise 0 0.14 0.49 0.76 0.96 0.97 0.99 1 1 1
Heart 0 0.13 0.39 0.74 0.92 0.94 0.98 0.99 1 1
Emerald 0 0.04 0.19 0.46 0.71 0.82 0.9 0.93 0.98 0.98
Asscher 0 0.01 0.19 0.44 0.78 0.85 0.93 0.96 0.99 0.99

Tables are easy to understand, but provide a limited summary of the data. The empirical cumulative distribution function of natural diamond carat weight is shown below. You can estimate the percentile for any carat weight from these graphs.

Lab-Created Diamonds

The table below shows the carats for each percentile by diamond shape. For example, for round lab diamonds, the median carat weight (50th percentile) is 1.13ct, while the 75th percentile is 1.36ct. The carat weights for lab diamonds are higher than for natural diamonds – people are buying larger lab diamonds due to their lower prices.

Lab Diamond Carat Percentile Summary
Shape 5% 10% 25% 50% 75% 90% 95%
Round 0.71 0.8 0.92 1.13 1.36 1.65 1.9
Cushion 0.93 0.97 1.01 1.01 1.1 1.48 2.06
Radiant 0.85 0.9 1.01 1.02 1.03 1.04 1.04
Princess 0.72 0.84 1.01 1.52 1.73 1.99 2.05
Oval 0.53 0.63 0.81 0.95 1.05 1.14 1.17
Pear 0.59 0.7 0.72 0.77 1.01 1.04 1.17
Emerald 0.79 0.82 1 1.05 1.5 1.53 1.56
Asscher 1.06 1.07 1.08 1.29 1.5 1.51 1.51

The table below summarizes the percentiles for milestone carat weights. For example, for round lab diamonds, a 1ct diamond is the 32nd percentile and a 1.5ct diamond is the 81st percentile.

Lab Diamond Percentiles for Milestone Carat Weights
Shape 0.25ct 0.5ct 0.75ct 1ct 1.25ct 1.5ct 1.75ct 2ct 2.25ct 2.5ct
Round 0 0 0.08 0.32 0.66 0.81 0.92 0.96 0.98 0.99
Cushion 0 0 0.05 0.24 0.88 0.88 0.93 0.93 1 1
Radiant 0 0 0 0.17 1 1 1 1 1 1
Princess 0 0 0.08 0.24 0.44 0.48 0.76 0.88 1 1
Oval 0 0.03 0.21 0.58 0.96 0.96 0.96 0.96 1 1
Pear 0 0.04 0.43 0.7 1 1 1 1 1 1
Emerald 0 0 0.04 0.26 0.72 0.74 1 1 1 1
Asscher 0 0 0 0 0.5 0.75 1 1 1 1

The full empirical cumulative distribution function of lab diamond carat weight is shown below. James Allen only started selling lab-created diamonds in the second half of 2019. The sample size of the fancy shaped diamonds is relatively small, leading to a more discontinuous CDF.