Standard temperature and pressure, abbreviated STP, refers to nominal conditions in the atmosphere at sea level. This value is important to physicists, chemists, engineers, and pilots and navigators.
Standard temperature is defined as zero degrees Celsius (0 °C), which translates to 32 degrees Fahrenheit (32 °F) or 273.15 degrees kelvin (273.15 K). This is essentially the freezing point of pure water at sea level, in air at standard pressure.
Standard pressure (exactly 100 kPa since 1982) supports 750 millimeters in a mercurial barometer (750 mmHg). This is about 29.5 inches of mercury, and represents approximately 14.5 pound-forces per square inch (14.5 PSI).
At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of 1.28 kg/m³. This fact comes as a surprise to many people; a cubic meter of air weighs nearly three pounds!
