Table percentage is the ratio of the table width, compared to the total width (or diameter) of the diamond.
*For rounds, the largest diameter is used. For fancies, the smallest diameter (width as opposed to length) is used.
One thing to note, the trend toward smaller table percentages is quite a recent phenomenon. Until the last decade or so, the preferred choice was a slightly larger table as it added width to the stone without sacrificing brilliance and a small bit of fire. Presently, “perfection” is creating a great demand for diamonds that are cut to “ideal” proportions, increasing the price for Ideal Cut diamonds.