A technique is proposed for estimating the surface of the color gamut of an output device, in 3-D colorimetric space. The method employs a modified convex hull algorithm. This approach is shown to be more general, and more accurate, than existing techniques. Simple numerical metrics are derived from this surface description: namely the gamut volume in 3-D space; and the percentage of colors from the Pantone Matching System which fall within the gamut.