How do you calculate the degree of unsaturation?

You convert the molecular formula to its equivalent alkane formula. Then you compare the number of H atoms to those in the alkane with the number of C atoms.
The degree of unsaturation or U-value gives the total number of rings and double bonds in a molecule. Here’s a way to calculate it.
The formula for an alkane is ##C_nH_(2n+2)##. Hexane is CH.
The formulas for hexene and cyclohexane are CH. Thus the introduction of a double bond or a ring removes 2 H atoms. Each ring or double bond counts as a degree of unsaturation. A triple bond counts as two double bonds.
We could write this as
U = (H atoms in alkane H atoms in compound).
If we have a compound with formula CH the saturated alkane would be CH. Then U = (8 4) = 2.
This tells that the molecule must have 2 double bonds 1 ring + 1 double bond 2 rings or a triple bond. It must be either propa-12-diene cyclopropene or propyne.
If there are heteroatoms X we make the following substitutions:
Thus
CHN CH(CH) = CH; U = (10 10) = 0. The compound is saturated.
CHO CH6; U = (6 6) = 0
For CH6 (benzene) U = (14 6) = 4. Any time you get U 4 you should suspect the presence of an aromatic ring.
CHNO CH(CH) = CH. The corresponding alkane is CH. U = (14 6) = 4. Aromatic?
CHO CH; U = (14 12) = 1.
CHNBr CH(CH)H = CH; U = (18 16) = 1.