不饱和度(unsaturated number,UN)，又称缺氢指数，是有机物分子不饱和程度的量化标志，通常用希腊字母 表示。此概念在推断有机化合物结构时很有用。

从有机物结构计算不饱和度的方法：

例子：丙烯的不饱和度为1，乙炔的不饱和度为2，环己酮的不饱和度为2。

The degree of saturation (also known as the index of hydrogen deficiency (IHD) or rings plus double bonds) formula is used in organic chemistry to help draw chemical structures. The formula lets the user determine how many rings, double bonds, and triple bonds are present in the compound to be drawn. It does not give the exact number of rings or double or triple bonds, but rather the sum of the number of rings and double bonds plus twice the number of triple bonds. The final structure is verified with use of NMR, mass spectrometry and IR spectroscopy, as well as qualitative inspection.