How big is a 5 carat ruby?

Quick answerUpdated September 2026

A 5 carat round ruby is roughly 10 to 10.5 mm across, about the width of a fingernail. Ruby is denser than diamond (about 4.0 against 3.5), so a ruby looks about 4 percent smaller across than a diamond of the same carat weight. For big lab rubies, buy by millimetres: 8 mm is about 2 carats, 12 mm about 7 to 8, and 18 mm well over 20.

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 is weight, not size. Two stones of the same weight can look different sizes depending on how deep they are cut, and budget listings often estimate weight. The face-up size in millimetres is what you actually see.

Ruby vs diamond

Ruby (corundum) has a specific gravity of about 4.0, against about 3.5 for diamond and 3.2 for moissanite. So one carat of ruby is a smaller stone than one carat of diamond, and much smaller than one carat of moissanite.

Round ruby, approximate millimetres to carats (typical cut depth; real stones vary)
Diameter	Approx. weight	Shop that size
6 mm	about 1 ct	6 mm lab ruby
8 mm	about 2.2 ct	8 mm lab ruby
10 mm	about 4.5 ct	10 mm lab ruby
12 mm	about 7.5 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. Mined rubies over a carat or two with good colour are rare; a 5 carat ruby is a centre stone that dominates a ring. As a lab ruby it is inexpensive.

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