Degree of unsaturation calculator
Type a molecular formula such as C8H10N4O2 or C6H5Cl. The calculator counts carbon, hydrogen, nitrogen and halogen atoms and gives the degree of unsaturation — the number of rings plus double bonds (a triple bond counts twice) the structure must contain.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Type the molecular formula, e.g. C6H6, C8H10N4O2 or CH3CH2OH (parentheses and repeated elements are added up).
- Read the degree of unsaturation and its meaning.
- Check the line with your counts to see how it was calculated.
DoU = (2C + 2 + N − H − X) ÷ 2
- C — carbon (and silicon) atoms, N — nitrogen (and phosphorus), H — hydrogen, X — F, Cl, Br, I
- Oxygen and sulfur are ignored: they add bonds in pairs
- Each ring = 1, each double bond = 1, each triple bond = 2
Worked example
Caffeine, C₈H₁₀N₄O₂: (2 × 8 + 2 + 4 − 10) ÷ 2 = 12 ÷ 2 = 6. Caffeine has two rings and four double bonds (two C=O, one C=C, one C=N): 2 + 4 = 6.
Benzene, C₆H₆: (12 + 2 − 6) ÷ 2 = 4 — one ring and three double bonds. Chloroform, CHCl₃: (2 + 2 − 1 − 3) ÷ 2 = 0.
Tips
- Use it before drawing structures. From NMR or mass spectra you get a formula; the degree of unsaturation tells you how many rings and π bonds to fit in.
- Count C=O too. Carbonyl groups count as double bonds, so esters and ketones add 1.
- Ions. For a charged species, add or remove hydrogens to reach the neutral parent before calculating.
Frequently asked questions
What is the degree of unsaturation?
Also called the index of hydrogen deficiency or double bond equivalents, it is the number of hydrogen pairs a molecule lacks compared with the saturated open-chain compound of the same carbon count. Each ring or double bond removes one pair, a triple bond two.
What is the formula?
DoU = (2C + 2 + N − H − X) ÷ 2, where C, N, H and X are the numbers of carbon, nitrogen, hydrogen and halogen atoms. Oxygen and sulfur do not change it. The calculator also counts silicon like carbon and phosphorus like nitrogen.
Why are halogens counted like hydrogen?
A halogen forms one bond, just like hydrogen, so swapping H for Cl does not change the number of rings or double bonds: CH₄ and CHCl₃ both have a degree of unsaturation of 0.
What does a value of 4 tell me?
That a benzene ring may be present: a six-membered ring with three double bonds adds up to exactly 4. Most compounds with a DoU of 4 or more and roughly as many hydrogens as carbons are aromatic, but four separate rings or double bonds are also possible — the number limits the structures, it does not pick one.
Why do I get a half number?
A neutral molecule made of these elements always gives a whole number. A half means an odd count of H, N or halogens that does not fit — usually a typo, an ion or a radical.