What crown height and pavilion depth measure
A round brilliant diamond has three vertical zones. The crown is the part above the girdle, rising to the flat table on top. The girdle is the thin band around the widest point. The pavilion is the part below the girdle, narrowing to the culet at the bottom. Crown height and pavilion depth are the heights of the two outer zones, each written as a percentage of the stone’s width so that stones of any size can be compared on one scale.
GIA defines both in its article "Diamond Cut: Anatomy of a Round Brilliant". Crown height is measured from the table to the intersection of the bezel facet and the girdle, expressed as a percentage of the average girdle diameter. Pavilion depth is measured from the culet to the intersection of the pavilion main facet and the girdle, again as a percentage of the average girdle diameter. GIA reports both to the nearest half of a percent, and this calculator rounds the same way while also showing the unrounded figure.
The average girdle diameter, again
Both percentages divide by the same number: the average girdle diameter, which GIA calculates as the smallest measured diameter plus the largest, halved. This is the same denominator used for total depth and table size, which is why all four proportions on a report sit on a common scale. The depth and table calculator on this site uses the identical first step.
The measurement points matter. Crown height stops where the bezel facet meets the girdle, and pavilion depth stops where the pavilion main facet meets the girdle. Those are specific points on the girdle outline, not its top and bottom edges in general, and they are the points GIA names. A measurement taken somewhere else on the girdle will give a different figure.
A worked example
Take a round brilliant measuring 6.42 – 6.46 mm across, with a crown height of 0.97 mm and a pavilion depth of 2.78 mm. The average girdle diameter is (6.42 + 6.46) ÷ 2 = 6.44 mm. Crown height is 0.97 ÷ 6.44 × 100 = 15.06%, reported to the nearest half percent as 15.0%. Pavilion depth is 2.78 ÷ 6.44 × 100 = 43.17%, reported as 43.0%.
Small differences in the readings move these figures. On a 6.44 mm stone, 0.03 mm of height is very close to half a percent, which is the full width of GIA’s rounding step. A crown height read as 1.00 mm instead of 0.97 mm gives 15.53%, reported as 15.5%. That is why a figure worked out from a hand gauge, or from a seller’s rounded measurements, can differ from a report by a step or more, and why a report’s own figure, taken with GIA’s instruments, is the one to rely on where there is one.
Crown, pavilion and the angles
Heights and angles describe the same facets in two ways. GIA defines the crown angle as the angle between the bezel facet plane and the table plane, reported as the average of eight measurements to the nearest half degree, and the pavilion angle as the angle between the pavilion main facet plane and the table plane, averaged over eight measurements and reported to the nearest 0.2 degrees.
This calculator does not derive angles from heights. Doing so needs further measurements and assumes perfectly symmetrical facets, which real stones do not have, so any angle it printed would be an approximation dressed as a measurement. Where a report gives the angles, read them from the report.
Why no "ideal" range is shown
Many proportion calculators print an "ideal" crown height and pavilion depth next to the result. This one does not. GIA’s cut grade for a round brilliant is assessed from a combination of proportions together with polish, symmetry and the stone’s appearance, so a given crown height can belong to stones in different grades depending on everything else. GIA publishes no single acceptable range for either figure on its own.
The ideal charts that do circulate come from retailers and differ from each other. Presenting one of them here would present a sales convention as a standard. The page reports the arithmetic of your measurements and leaves the grade to a grading report.