Millimeters to Carats: The EK|EE 1913 Diamond Size Conversion Guide

When you're investing in a luxury diamond, carat weight is only part of the equation. At EK|EE 1913, we believe that understanding how your diamond will actually look and wear on the hand is just as important as its specs on paper. That’s why we created this easy-to-follow GIA-based mm to carat conversion chart - so you can make informed decisions when designing your engagement ring, bridal set, or heirloom piece.

Unlike fast-fashion jewelry, fine diamonds are measured and sold by weight, but what you see is the surface spread in millimeters. That means a 1-carat round brilliant diamond may visually look larger or smaller than a 1-carat emerald cut, simply due to the shape’s proportions.

Whether you're browsing through our Bridal & Engagement Design Studio or customizing a stone on your own, this guide is here to give you clarity and confidence.

GIA Diamond Size Chart (Millimeters to Carats)

Below is a curated reference guide using GIA-proportioned standards for five of the most popular diamond cuts. All measurements are approximate but provide a reliable baseline when choosing stones by size rather than just carat.

Round Brilliant Cut

4.1 mm – 0.25 ct
5.2 mm – 0.50 ct
5.8 mm – 0.75 ct
6.5 mm – 1.00 ct
7.4 mm – 1.50 ct
8.2 mm – 2.00 ct
8.9 mm – 2.50 ct
9.3 mm – 3.00 ct
10.0 mm – 4.00 ct
10.9 mm – 5.00 ct

Princess Cut

3.5 mm – 0.25 ct
4.4 mm – 0.50 ct
5.0 mm – 0.75 ct
5.5 mm – 1.00 ct
6.5 mm – 1.50 ct
7.0 mm – 2.00 ct
7.5 mm – 2.50 ct
8.0 mm – 3.00 ct
8.5 mm – 4.00 ct
9.0 mm – 5.00 ct

Cushion Cut

4.0 mm – 0.25 ct
4.5 mm – 0.50 ct
5.0 mm – 0.75 ct
5.5 mm – 1.00 ct
6.5 mm – 1.50 ct
7.0 mm – 2.00 ct
7.5 mm – 2.50 ct
8.0 mm – 3.00 ct
8.5 mm – 4.00 ct
9.0 mm – 5.00 ct

Oval Cut

5.0 x 3.5 mm – 0.25 ct
6.0 x 4.0 mm – 0.50 ct
7.0 x 5.0 mm – 0.75 ct
7.7 x 5.7 mm – 1.00 ct
8.5 x 6.0 mm – 1.50 ct
9.0 x 6.5 mm – 2.00 ct
10.0 x 7.0 mm – 2.50 ct
10.5 x 7.5 mm – 3.00 ct
11.5 x 8.0 mm – 4.00 ct
12.0 x 8.5 mm – 5.00 ct

Emerald Cut

5.0 x 3.0 mm – 0.25 ct
6.0 x 4.0 mm – 0.50 ct
6.5 x 4.5 mm – 0.75 ct
7.0 x 5.0 mm – 1.00 ct
8.0 x 6.0 mm – 1.50 ct
9.0 x 6.5 mm – 2.00 ct
10.0 x 7.0 mm – 2.50 ct
10.5 x 7.5 mm – 3.00 ct
11.0 x 8.0 mm – 4.00 ct
12.0 x 8.5 mm – 5.00 ct

Why Millimeter Sizing Matters

Choosing a diamond based on carat alone can be misleading. Some cuts carry their weight deep, making them look smaller from above, while others spread wider. That’s why top jewelers and GIA-trained professionals always reference millimeter measurements to match your setting and design style.

When using our Bridal & Engagement Design Studio, stone sizes are displayed in millimeters. Refer to this chart anytime you’re selecting your stone, and if you need help converting, our team is always available.

Need Help Choosing the Right Size?

Reach out to us at admin@emeryelise.com. Whether you’re customizing a lab-grown solitaire or selecting a 3-stone platinum masterpiece, we’re here to help you find the perfect fit.

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