Calculate the index of hydrogen deficiency (IHD) from a molecular formula — tells you how many rings and/or double bonds a molecule has.
Enter the number of each atom in your molecular formula, or type the formula directly (e.g., "C6H6" or "C8H10N4O2"). The calculator instantly determines the degree of unsaturation, which tells you the total number of rings plus double bonds in the molecule. Oxygen and sulfur don't appear in the formula — they don't affect the calculation.
The degree of unsaturation (DoU), also called the index of hydrogen deficiency (IHD) or double bond equivalents (DBE), is calculated as: DoU = (2C + 2 + N − H − X) / 2, where C = carbons, H = hydrogens, N = nitrogens, and X = halogens (F, Cl, Br, I). Oxygen and sulfur are divalent and don't change the count.
Interpreting the result: Each double bond contributes 1 DoU. Each ring contributes 1 DoU. A triple bond contributes 2 DoU. A benzene ring contributes 4 DoU (3 double bonds + 1 ring). So benzene (C₆H₆) has DoU = 4, and caffeine (C₈H₁₀N₄O₂) has DoU = 6.
Worked example: Aspirin is C₉H₈O₄. DoU = (2×9 + 2 − 8) / 2 = (18 + 2 − 8) / 2 = 6. This accounts for the benzene ring (4 DoU) + 2 C=O double bonds (2 DoU).