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 0C), which translates to 32 degrees Fahrenheit (32 0F) or 273.15 degrees kelvin (273.15 0K). This is essentially the freezing point of pure water at sea level, in air at standard pressure.
Standard pressure supports 760 millimeters in a mercurial barometer (760 mmHg). This is about 29.9 inches of mercury, and represents approximately 14.7 pounds per inch (14.7 lb/in2). Imagine a column of air measuring one inch square, extending straight up into space beyond the atmosphere. The air in such a column would weigh about 14.7 pounds.
The density of air at STP is approximately 1.29 kilogram per meter cubed (1.29 kg/m3). This fact comes as a surprise to many people; a cubic meter of air weighs nearly three pounds!
See also: SI (International System of Units), specific gravity.