The partial pressure of oxygen in the atmosphere at sea level is approximately 160 millimeters of mercury, or 160 torr. The calculation comes from the ratio of oxygen in the atmosphere, or roughly 21 percent of 760 millimeters of mercury of air pressure at sea level.
Partial pressures of gases follow Dalton's law of partial pressures that states the pressure of a gas mixture is equal to the sum of partial pressures of the individual components. This expression is derived from the ideal gas law, which explains the pressure of gases changes based upon temperature, volume and density in the air. Partial pressures describe the distribution of certain gases in a mixture.