Degree of Unsaturation (DU) / Index of Hydrogen Deficiency (IHD) Formula

The Degree of Unsaturation (DU) tells you the total number of rings and π (pi) bonds present in an organic compound.

General Formula

Where:

• C = Number of carbon atoms
• H = Number of hydrogen atoms
• N = Number of nitrogen atoms
• X = Number of halogen atoms (F, Cl, Br, I)
• O, S, Se = Ignored (they do not affect DU)

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Shortcut Formula

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Effect of Different Atoms

Atom	Effect on DU Calculation
C	Count normally
H	Subtract
N	Add 1 H equivalent (+1)
F, Cl, Br, I	Treat as H
O, S	Ignore

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Meaning of DU

DU	Possible Structure
0	Saturated compound (no ring, no π bond)
1	One ring or one double bond
2	Two double bonds, or one triple bond, or one ring + one double bond
3	Three π bonds/rings in any combination
4	Benzene ring (1 ring + 3 double bonds = 4 DU)