The ideal gas law states the PV=nRT, where P=pressure, V=volume, n=number of moles of gas, R=the gas constant, and T=temperature. Most gasses act very closely to prediction.
Fill in any three of the inputs, and the result of the fourth quantity will show up in the relevant output box. Ignore boxes that show NaN, 0 or Infinity.
CalcTool's unit menus allow you to input and output in whatever units are convenient for you.