How big is a 15CT diamond?

The strict answer is that that a 15 carat stone weighs 15 carats. You probably want to know how big the stone looks.

Use the link, below, to get a visual idea of about how large a stone might look depending on its carat weight, and its cut.

If the 15 carat stone is a brilliant cut, the stone would be about 16.3 MM across at the girdle.