DBE = (2C + 2 + N − H − X) / 2
Double Bond Equivalents (DBE), also called Rings Plus Double Bonds (RPDB) or Index of Hydrogen Deficiency, is critical in structure elucidation from mass spectrometry data. After determining a molecular formula from high-resolution MS, DBE immediately constrains possible structures. The formula DBE = (2C + 2 + N − H − X) / 2 ignores oxygen and sulfur (divalent atoms don't change saturation). Each double bond and each ring contributes 1 DBE; a triple bond contributes 2. Caffeine (C₈H₁₀N₄O₂) has DBE = (16+2+4−10)/2 = 6, corresponding to two fused rings plus four C=O/C=N double bonds. In proteomics and metabolomics, DBE helps identify lipid classes, distinguish isomers, and classify unknown metabolites.