# What is the formula of partial pressure?

For an ideal gas, the partial pressure relationship is called Henry's Law. It's just the total pressure ${P}_{\text{tot}}$ multiplied by the mole fraction ${x}_{j}$ of the mixture that is occupied by component $j$.
${P}_{j} = {x}_{j} {P}_{\text{tot}}$
You're really just multiplying the total pressure by the percentage corresponding to one component. So if the total pressure was $1$ atm, then if the partial pressure of component $j$ is $0.8$ atm, its mole fraction is $\frac{0.8 \cancel{a t m}}{1 \cancel{a t m}} = 0.8$.