Degree of Unsaturation Calculator
Find the degree of unsaturation (index of hydrogen deficiency) of a molecular formula to reveal how many rings and π bonds it contains.
How it works
DoU = ( 2·C + 2 + N − H − X ) / 2 (oxygen and sulfur do not count)
The degree of unsaturation — also called the index of hydrogen deficiency — counts how many hydrogen pairs a molecule is missing compared with the fully saturated, open-chain formula. Each unit stands for one ring or one π bond, so a value of 4 signals, for example, a benzene ring (three double bonds plus one ring). Oxygen and divalent sulfur are ignored because inserting them does not change the hydrogen count, while each nitrogen adds one and each halogen removes one.
Worked example
Benzene is C₆H₆: DoU = (2×6 + 2 + 0 − 6 − 0) / 2 = (12 + 2 − 6) / 2 = 8 / 2 = 4. That matches its three C=C double bonds plus the ring — four degrees of unsaturation in total.
Sources: Degree of unsaturation — Wikipedia
Frequently asked questions
What does one degree of unsaturation represent?
Exactly one ring or one π bond. A double bond is one degree, a triple bond is two (two π bonds), and each ring adds one more. The calculator gives the total; the structure decides how it is split.
Why are oxygen and sulfur left out?
Adding an –O– or –S– between two atoms does not remove any hydrogens, so divalent oxygen and sulfur have no effect on the hydrogen deficiency and drop out of the formula entirely.
What if I get a non-integer answer?
A fractional degree of unsaturation means the atom counts do not describe a valid neutral molecule — usually a typo in the hydrogen or nitrogen count. The value must be a whole number for a real closed-shell compound.
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This calculator is for educational purposes. Double-check important results before acting on them.