What Is STP in Chemistry?

Understanding Standard Temperature and Pressure

chemistry thermometer
STP in chemistry is at 1 atmosphere and 0 degrees Celsius. Marga Buschbell Steeger / Getty Images

STP in chemistry is the abbreviation for Standard Temperature and Pressure. STP most commonly is used when performing calculations on gases, such as gas density. The standard temperature is 273 K (0° Celsius or 32° Fahrenheit) and the standard pressure is 1 atm pressure. This is the freezing point of pure water at sea level atmospheric pressure. At STP, one mole of gas occupies 22.4 L of volume (molar volume).

Note the International Union of Pure and Applied Chemistry (IUPAC) applies a more stringent standard of STP as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 100,000 Pa (1 bar, 14.5 psi, 0.98692 atm). This is a change from their earlier standard (changed in 1982) of 0 °C and 101.325 kPa (1 atm).

Uses of STP

Standard reference conditions are important for expressions of fluid flow rate and the volumes of liquids and gases, which are highly dependent on temperature and pressure. STP commonly is used when standard state conditions are applied to calculations. Standard state conditions, which include standard temperature and pressure, may be recognized in calculations by the superscript circle. For example, ΔS° refers to the change in entropy at STP.

Other Forms of STP

Because laboratory conditions rarely involve STP, a common standard is standard ambient temperature and pressure or SATP, which is a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of exactly 1 atm (101,325 Pa, 1.01325 bar).

The International Standard Atmosphere or ISA and the U.S. Standard Atmosphere are standards used in the fields of fluid dynamics and aeronautics to specify temperature, pressure, density, and the speed of sound for a range of altitudes at the mid-latitudes. The two sets of standards are the same at altitudes up to 65,000 feet above sea level.

The National Institute of Standards and Technology (NIST) uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm) for STP. The Russian State Standard GOST 2939-63 uses the standard conditions of 20 °C (293.15 K), 760 mmHg (101325 N/m2) and zero humidity. The International Standard Metric Conditions for natural gas are 288.15 K (15.00 °C; 59.00 °F) and 101.325 kPa. The International Organization for Standardization (ISO) and the United States Environmental Protection Agency (US EPA) both set their own standards, too.

Correct Use of the Term STP

Even though STP is defined, you can see the precise definition depends on the committee that set the standard! Therefore, rather than citing a measurement as performed at STP or standard conditions, it's always best to explicitly state the temperature and pressure reference conditions. This avoids confusion. In addition, it is important to state the temperature and pressure for the molar volume of a gas, rather than citing STP as the conditions.