Take a fixed mass of gas and keep its temperature constant. If you squeeze it into a smaller volume its pressure rises, and if you let it expand its pressure goes down. The two change in opposite directions, so pressure and volume are inversely proportional. Halve the volume and the pressure doubles. The product of pressure and volume stays the same: P1 × V1 = P2 × V2. The subscript 1 means the starting values and the subscript 2 means the final values.
Use any units you like for pressure (pascals, Pa, or kilopascals, kPa) and for volume (m3 or cm3), as long as both pressures use the same unit and both volumes use the same unit. Then no conversions are needed. Remember that 1 kPa = 1000 Pa.
Worked example. A gas at 120 kPa has a volume of 300 cm3. It is compressed to 200 cm3 at constant temperature. Rearrange to P2 = (P1 × V1) ÷ V2 = (120 × 300) ÷ 200 = 180 kPa. The volume fell and the pressure rose, which is what you expect, so the answer is sensible. To find a volume instead, rearrange to V2 = (P1 × V1) ÷ P2.
The equation only works when the mass of gas is fixed, so no gas can leak out or be pumped in, and when the temperature is constant. If the gas is compressed slowly, energy can escape to the surroundings and the temperature stays the same.