Degrees of Unsaturation Calculator
Calculate double-bond equivalents from a neutral CHNO or halogen-containing formula, with atom counts, assumptions and clear input checks.
Degrees of unsaturation, also called double-bond equivalents or the index of hydrogen deficiency, help narrow the structures consistent with a molecular formula. This calculator handles a single neutral, connected organic species containing carbon, hydrogen, nitrogen, oxygen and the halogens F, Cl, Br or I under ordinary valence assumptions.
How to use this tool
- Enter the complete molecular formula, not its empirical ratio.
- Confirm that the species is neutral, connected and closed-shell.
- Check the carbon, hydrogen, nitrogen and halogen counts.
- Use the result alongside structural or spectroscopic evidence.
Formula
DBE = (2C + 2 + N − H − X) / 2
C, H and N are atom counts; X is the combined count of F, Cl, Br and I. The model uses tetravalent carbon, trivalent nitrogen, divalent oxygen and monovalent halogens. Oxygen therefore does not appear in the equation.
Example
Pyridine has formula C5H5N. Its DBE is (2 × 5 + 2 + 1 − 5) / 2 = 4. Its structure contains one ring and three double bonds, accounting for all four units.
Frequently asked questions
How much does each bond or ring contribute?
One independent ring or one double bond contributes one unit. A triple bond contributes two. A formula alone cannot tell you which combination of rings and multiple bonds is present.
Does DBE 4 prove a benzene ring?
No. Several unrelated structures can have the same total. DBE is a constraint for interpreting other evidence, not a structure identification or aromaticity test.
Why are some formulas rejected?
A negative or half-integer result is outside this neutral closed-shell model. Charged species, radicals, salts, separate components and other elements are excluded rather than given a potentially misleading interpretation.
Can a valid-looking formula still be unsuitable?
Yes. Syntax and a nonnegative integer do not establish connectivity, valence or chemical existence. Nitrogen in unusual bonding states can also violate the assumptions. Parentheses only group atom counts; they do not describe a bond graph.
Sources
Last updated: 2026-10-30