I'm trying to calculate the pressure of outer space given the denisty of helium is of the order 1 atom per cm^3, temprature 3K. 1 atom per cm^3 => 10^6 per m^3. It's a low pressure so I guess I can use the ideal gas equation. I also know the mass of a helium atom is 6.68x10^-27 kg. What's the best route forward? Best Regards, James.