What depth percentage and table percentage are
Depth percentage and table percentage are two of the proportions printed on a grading report for a round brilliant diamond. Each one is a measurement divided by the stone’s width, so that a small diamond and a large one can be compared on the same scale.
GIA defines them in its article "Diamond Cut: Anatomy of a Round Brilliant". The total depth percentage is the table-to-culet depth expressed as a percentage of the average girdle diameter, reported to the nearest tenth of a percent. The table percentage is the average of four table measurements, each taken from bezel point to bezel point, expressed as a percentage of the average girdle diameter and reported to the nearest whole percent. This calculator applies exactly those definitions to the figures you enter, and rounds the result the same way.
Why the average girdle diameter
No round diamond is perfectly round. GIA measures the diameter in several places, records the smallest and the largest, and uses their average: (minimum + maximum) ÷ 2. On a report the measurements line shows the two diameters and the depth, for example 6.42 – 6.46 × 3.98 mm, which is the input this tool expects.
The difference between the two diameters is worth looking at in its own right. The tool shows it as "out of round": (maximum − minimum) ÷ average. A stone of 6.42 – 6.46 mm is 0.6% out of round. That is a measurement too, and the tool attaches no judgement to it.
A worked example
Take a round brilliant measuring 6.42 – 6.46 × 3.98 mm, with table readings of 3.60 mm. The average girdle diameter is (6.42 + 6.46) ÷ 2 = 6.44 mm. The total depth is 3.98 ÷ 6.44 × 100 = 61.80%, reported as 61.8%. The table is 3.60 ÷ 6.44 × 100 = 55.90%, reported as 56%.
Small measurement differences move these figures noticeably. A depth reading 0.05 mm higher, 4.03 mm, gives 62.6%. That is why a percentage worked out from a home caliper or a seller’s rounded figures may not match the report to the decimal, and why the report’s own figure, taken with GIA’s instruments, is the one to rely on where there is one.
Why this tool shows no "ideal" range
Many websites print an "ideal" depth and table band beside a calculator like this one. This page does not, deliberately. GIA’s cut grade for a round brilliant is assessed from a combination of proportions, including crown angle, pavilion angle, girdle thickness, polish and symmetry, together with how the stone looks. A given depth can belong to an Excellent stone with one set of angles and a lower grade with another. GIA publishes no single acceptable range for depth or table on its own.
The "ideal" charts that do exist come from retailers and differ from each other, sometimes by several percentage points. Presenting one as a standard would be presenting a sales convention as a measurement. The honest position is the one GIA’s own system takes: depth and table are inputs to a cut grade, not a grade.
Fancy shapes
The GIA definitions quoted here are for the round brilliant. Ovals, pears, emeralds and other fancy shapes are not round, so they have no single girdle diameter, and their reports describe proportions in their own way. For a fancy shape, use the depth and table figures printed on the report. The length-to-width ratio calculator on this site handles the measurement fancy shapes are most often compared by.