Degree of Unsaturation Calculator
The degree of unsaturation — how many rings and double bonds a molecular formula must contain — from its atom counts.
A saturated acyclic molecule has 2C + 2 hydrogens (plus one per nitrogen, minus one per halogen; oxygen changes nothing).
How the degree of unsaturation calculator works
A saturated acyclic molecule has 2C + 2 hydrogens (plus one per nitrogen, minus one per halogen; oxygen changes nothing). Every ring or π bond removes two hydrogens, so the count of rings plus π bonds is (2C + 2 + N − H − X) ÷ 2. Benzene, C₆H₆, comes out at 4: one ring and three double bonds. A triple bond counts as two.
Formula: DoU = (2C + 2 + N − H − X) ÷ 2
Worked examples
| Inputs | Degree of unsaturation | Note |
|---|---|---|
| Benzene C₆H₆ | 4 | 4 |
| Caffeine C₈H₁₀N₄O₂ | 6 | 6 |
| Ethanol C₂H₆O | 0 | 0 — saturated |
FAQFrequently asked questions
What does the number tell me?
How many rings plus π bonds the structure must contain in total, not which. A 1 could be a ring or a double bond; 4 is classically a benzene ring — one ring and three double bonds.
Why do halogens count like hydrogen, and oxygen not at all?
A halogen takes a bonding position hydrogen would otherwize fill, so it is treated as hydrogen. Oxygen inserts between two atoms without changing the hydrogen count, so it drops out.
How do nitrogen atoms change it?
Each nitrogen is trivalent and adds one hydrogen to the saturated count, so it adds one inside the bracket. Phosphorus behaves the same way.
What if I get a half?
A neutral molecule cannot have a half. It means the formula is for an ion or a radical, or the hydrogen count has a typo.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.