Index of hydrogen deficiency

The index of hydrogen deficiency (IHD, or degree of unsaturation) is a quantitative metric we assign to organic compounds. It calculates the number of rings and pi bonds in a structure, ignoring sulphur and oxygen atoms.

Where $X$ is a halogen:

$$\text{IHD}=\frac{1}{2}(2C+2-H-X+N)$$

We also have a handy key:

• Double bond is degree 1.
• Triple bond is degree 2.
• Rings are degree 1.
• Aromatic rings are degree 4.