DoU = (2C + 2 + N − H − X) / 2
Degree of unsaturation (DoU), also called the Index of Hydrogen Deficiency (IHD), tells you the total number of rings plus double bonds in a molecular formula. The formula is DoU = (2C + 2 + N − H − X) / 2, where C = carbons, H = hydrogens, N = nitrogens, X = halogens. Oxygen and sulfur do not appear in the formula because they are divalent and don't change the hydrogen count. Each double bond contributes 1 DoU, each triple bond contributes 2, and each ring contributes 1. A benzene ring = 4 DoU (3 double bonds + 1 ring). This calculation is invaluable in organic chemistry for narrowing down possible structures from a molecular formula, especially when combined with spectroscopic data (IR, NMR, MS).