How big is a 5 carat ruby?

Quick answerUpdated November 1, 2026

A 5 carat round ruby is roughly 10 to 10.5 mm across. Ruby () is denser than diamond, with a specific gravity of about 4.0 against 3.52, so a ruby is about 4 percent narrower than a diamond of the same carat weight and cut. For big lab rubies, buy by millimetres: 8 mm is about 2.3 carats, 12 mm about 7.5 to 8, and 18 mm well over 20 (estimates from the International Gem Society’s weight formula).

Illustration: how big is a 5 carat ruby? — Illustration (AI render) of this kind of piece, not a photo of any listing.

Why millimetres beat carats

Carat measures weight, not size (GIA): a metric carat is 200 milligrams. Two stones of the same weight can look different sizes depending on how deep they are cut. The face-up size in millimetres is what you see.

Ruby vs diamond

Ruby is corundum, with a specific gravity of about 4.0, against about 3.52 for diamond (16 CFR 23.12) and 3.22 for moissanite (GIA). So one carat of ruby is a smaller stone than one carat of diamond, and smaller still than one carat of moissanite.

How the chart was worked out

The International Gem Society’s formula for a well-proportioned round stone is diameter × diameter × depth × specific gravity × 0.0018, with measurements in millimetres and the result in carats. The chart uses a specific gravity of 4.0 and a depth of about 62% of the diameter. A deeper stone weighs more for the same width, so treat every figure as approximate.

Round ruby, approximate millimetres to carats (estimated at about 62% depth; real stones vary)
Diameter	Approx. weight	Shop that size
6 mm	about 1 ct	6 mm lab ruby
8 mm	about 2.3 ct	8 mm lab ruby
10 mm	about 4.5 ct	10 mm lab ruby
12 mm	about 7.7 ct	12 mm lab ruby
14 mm	about 12 ct	14 mm lab ruby
16 mm	about 18 ct	16 mm lab ruby
18 mm	about 26 ct	18 mm lab ruby

Frequently Asked Questions

Is a 5 carat ruby big?

Yes. At about 10 to 10.5 mm across it is far wider than a 1 carat diamond (about 6.5 mm). GIA says fine ruby can command high per-carat prices; as a lab ruby the same size is inexpensive, since rough lab corundum can cost a few cents a carat (International Gem Society).

Sources

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