Degree of Unsaturation Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
The degree of unsaturation is calculated with DoU = (2C + 2 + N − H) ÷ 2, from the number of carbon, hydrogen, and nitrogen atoms in the molecular formula. Benzene (C₆H₆) has a degree of unsaturation of 4, corresponding to its single ring and three double bonds.
Explanation
The degree of unsaturation (also called the index of hydrogen deficiency) indicates how many rings and multiple bonds (double or triple) an organic molecule contains, using only its molecular formula, even before knowing its exact structure — somewhat like our molar concentration calculator derives quantitative information from basic data without requiring a full analysis of the solution. The principle rests on an implicit comparison with the equivalent saturated molecule (an alkane with the same number of carbons, with no ring or multiple bond): each closed ring or double bond 'costs' exactly two fewer hydrogen atoms than this saturated reference, and each triple bond costs four (the equivalent of two double bonds). The result obtained therefore represents the total number of rings and multiple bonds combined, without specifying their exact distribution: a degree of unsaturation of 4, for example, could correspond to one ring with three double bonds (like benzene), or to four separate double bonds with no ring at all, or even to two triple bonds — only further structural analysis (spectroscopy, for example) can decide between these possibilities. This calculation is a very useful first step in structural organic chemistry: even before proposing a plausible molecular structure for an unknown molecular formula, the degree of unsaturation immediately gives a strong constraint on the number of rings and multiple bonds that structure must necessarily contain.
Example: benzene (C₆H₆)
Inputs
Carbon: 6. Hydrogen: 6. Nitrogen: 0.
Calculation
DoU = (2×6 + 2 + 0 − 6) ÷ 2 = (12 + 2 − 6) ÷ 2 = 8 ÷ 2 = 4.
Result
Benzene has a degree of unsaturation of 4, exactly its single ring plus its three double bonds.
Frequently asked questions
Why does a triple bond count for 2 in the degree of unsaturation?
Because a triple bond 'saves' two more hydrogen atoms than a single bond, exactly as two separate double bonds would together: chemically, a triple bond is equivalent to two degrees of unsaturation combined in the same part of the molecule. This is why the total degree of unsaturation alone cannot distinguish a molecule with two double bonds from a molecule with a single triple bond: both give the same DoU value.
Why doesn't oxygen appear in this formula?
Because oxygen generally forms two bonds (as in an alcohol or ether group), which does not affect the number of hydrogen atoms needed to saturate the molecule the same way an extra carbon or nitrogen atom would: adding an oxygen atom to a carbon chain changes neither the number of possible multiple bonds nor the number of rings, so it does not appear in the calculation.
How are halogens (chlorine, bromine...) accounted for?
In the full formula, each halogen atom (fluorine, chlorine, bromine, iodine) counts the same way as a hydrogen atom, since a halogen also forms only a single covalent bond. This calculator deliberately limits itself to molecular formulas without halogens, the simplest and most common case for teaching purposes; for a halogenated molecule, it would be enough to add the number of halogens to the number of hydrogens before using the formula. Once the structure is identified, its synthesis yield can then be assessed with our reaction yield calculator.