Standard temperature and pressure

Standard temperature and pressure (STP), also known as standard conditions for temperature and pressure, refers to defined sets of reference conditions used for scientific and industrial measurements. These conditions enable reliable comparison of experimental data, particularly relating to gases whose volumes vary extensively with temperature and pressure. Although the concept is universal, no single authority defines STP for all applications; consequently, different organisations maintain distinct standard conditions, each tailored to particular scientific, commercial or regulatory needs.

Purpose and Scope

The expression of gas volumes, volumetric flow rates and related parameters often requires a common reference point so that values reported by different laboratories or industries can be meaningfully compared. Terms such as “standard cubic metre per second” or “normal cubic metre per second” depend critically on the temperature and pressure taken as standard. Because the actual volume of a gas changes significantly with both parameters, failure to specify the reference conditions can lead to confusion or error. In some technical publications, “standard conditions” or the older expression “normal conditions” are used without proper definition, making clear specification of reference values essential for good practice.

Definitions in Chemistry and Physics

The International Union of Pure and Applied Chemistry (IUPAC) originally defined STP as 0 °C (273.15 K) and 1 atmosphere (101.325 kPa). In 1982 IUPAC revised this definition to 0 °C and exactly 100 kPa (1 bar). The change aligned the standard pressure with a value closer to the global mean atmospheric pressure at typical altitudes of human habitation.
The National Institute of Standards and Technology (NIST) uses several standard conditions depending on context. For general reference NIST commonly employs a temperature of 20 °C (293.15 K) and a pressure of 1 atmosphere (101.325 kPa). For thermodynamic measurements a temperature of 25 °C (298.15 K) and a pressure of 1 bar are widely used. NIST also specifies 15 °C (288.15 K) in the context of temperature compensation for petroleum products. These multiple definitions reflect differing conventions across scientific and industrial domains.
The ISO 13443 standard establishes conditions of 15 °C and 101.325 kPa for natural gas and related fluids, defining the basis for the standard cubic metre used widely in gas commerce. By contrast, the American Petroleum Institute adopts 60 °F (approximately 15.56 °C) and 14.696 psi (1 atmosphere).

Historical Standards

Before 1918, users of the metric system typically defined standard conditions as 0 °C and 1 atmosphere. In the same period, users of imperial and U.S. customary units commonly employed 60 °F and 14.696 psi, largely owing to prevailing practices in the global oil and gas industries. Although these earlier standards persist in some specialised contexts, they are no longer the most widely used.

Current International Usage

Many scientific, governmental and industrial bodies maintain their own definitions of standard conditions. These may differ in temperature, pressure or both. IUPAC’s current standard (0 °C and 100 kPa) contrasts with its earlier definition, while gas companies in Europe, Australia and South America commonly use 15 °C and 101.325 kPa. The International Organization for Standardization (ISO), the United States Environmental Protection Agency (EPA) and NIST each specify multiple sets of reference conditions in their respective regulations and guidelines.
Common abbreviations include:

• NTP (Normal Temperature and Pressure)
• SATP (Standard Ambient Temperature and Pressure)
• SCF (Standard Cubic Foot)
• EGIA (Electricity and Gas Inspection Act conditions in Canada)

Because such terms may refer to different values in different contexts, explicit definition is essential.

International Standard Atmosphere

In aeronautics and fluid dynamics the International Standard Atmosphere (ISA) provides a model of atmospheric properties as a function of altitude. At mean sea level it specifies a temperature of 15 °C, a pressure of 101.325 kPa and a density of roughly 1.225 kg/m³. The model assumes a temperature lapse rate of 6.5 °C per kilometre up to the tropopause. The ISA represents typical mid-latitude conditions and is widely employed in aircraft performance calculations. The United States Standard Atmosphere mirrors the ISA up to 65,000 feet above sea level.

Standard Laboratory Conditions

Because many definitions of STP prescribe temperatures substantially lower than typical laboratory environments, the term standard laboratory conditions is used in educational and experimental settings. These conditions vary geographically due to differences in climate, altitude and the extent of heating or cooling in buildings. For example, schools in New South Wales, Australia often adopt 25 °C and 100 kPa. Standards bodies such as ASTM International provide detailed specifications for conditioning materials under controlled conditions, although these differ from the conventional STP definitions.

Molar Volume of a Gas

The molar volume of an ideal gas depends directly on the temperature and pressure used as reference conditions. Consequently, specifying a molar volume without reference conditions is ambiguous. Using the ideal gas law, standard molar volumes at various commonly used conditions can be calculated:

• At 0 °C and 101.325 kPa: approximately 22.414 dm³/mol
• At 0 °C and 100 kPa: approximately 22.711 dm³/mol
• At 15 °C and 101.325 kPa: approximately 23.645 dm³/mol
• At 25 °C and 101.325 kPa: approximately 24.466 dm³/mol
• At 25 °C and 100 kPa: approximately 24.790 dm³/mol

For imperial units, values such as 0.8366 ft³ per gram-mole at 60 °F and 14.696 psi are typical. Care must be taken to distinguish between the universal gas constant RRR and the specific gas constant Rs=R/mR_s = R/mRs​=R/m, where mmm is molecular mass, as confusion between these can lead to errors.
The U.S. Standard Atmosphere uses a value of the universal gas constant slightly inconsistent with modern values of the Avogadro and Boltzmann constants, reflecting historical conventions.

Significance and Applications

Standard temperature and pressure underpin consistent reporting in chemistry, physics, engineering, energy commerce and environmental regulation. Whether defining the molar volume of gases, calibrating instruments, establishing emissions standards or specifying gas flow for industrial processes, the use of clearly defined reference conditions supports transparency and comparability. Because multiple definitions remain in use worldwide, specifying the chosen standard explicitly remains essential to avoid ambiguity in technical communication.