You can use a number of different units to measure gas pressure just as you can use different units to measure length (the kilometre, the mile, etc.). In Science, the most common units to measure pressure are millimetres of mercury (mm Hg), atmospheres (atm), and kilopascals (kPa). The S.I. unit for gas pressure is Pascals, however the other units are used just as often.
Here is how the units relate: