Hydrogen Deficiency Index (HDI) Calculator

Input method

Molecular formula

Element counts

Molecular formula Parentheses and hydrates are supported. Elements such as O and S do not change the index; halogens count as H.

Carbon (C)

Hydrogen (H)

Nitrogen (N)

Fluorine (F)

Chlorine (Cl)

Bromine (Br)

Iodine (I)

Astatine (At)

Halogens (F, Cl, Br, I, At) are counted with H in the formula.

O, S, Se, and other group 16 elements do not change the index.
Each halogen behaves like a hydrogen in the formula (subtract in the numerator).
Nitrogen increases the numerator by +1 for each N atom.
Supports hydrates like CuSO4·5H2O and parentheses such as (CH3)3CCl.
Radicals/ions may yield fractional values.

FAQs

It is the degree of unsaturation: the total number of rings and π bonds in a neutral organic formula, computed from C, H, N, and halogens.

They typically form two bonds and do not change the hydrogen requirement for saturation, so they drop out of the formula by cancellation.

Each halogen (F, Cl, Br, I, At) effectively replaces one H in the formula, so add them to H when calculating the numerator.

Fractions can occur for radicals or ionic formulas. For typical closed-shell neutrals, the value should be an integer.

No. HDI only counts rings/π bonds. Aromaticity requires additional criteria (planarity, conjugation, Hückel rule) and spectroscopic confirmation.

Charged species can shift hydrogen counts; this basic calculator assumes neutral molecules. Treat results for ions with caution.

Yes. Parentheses and hydrate dots are supported, such as (CH3)3CCl or CuSO4·5H2O. The parser multiplies groupings and sums parts.

For educational use; verify with course or lab requirements.