Overview

"Standard temperature and pressure" (often abbreviated STP) denotes agreed reference values of temperature and pressure used to report experimental results, compare measurements and calculate properties of gases. Because there is no single global definition, several different conventions coexist in chemistry, physics and engineering. These conventions are practical shortcuts for reproducibility rather than fundamental physical constants.

Common definitions

Different organizations and disciplines adopt different numerical choices. Commonly used combinations include:

  • 0 °C (273.15 K) and 1 atmosphere (101,325 Pa) — a traditional laboratory convention used in many older texts and calculations. At these conditions the molar volume of an ideal gas is about 22.414 L/mol.
  • 0 °C and 100 kPa (1 bar) — a slight variation that simplifies SI-based calculations.
  • 25 °C (298.15 K) and 100 kPa — often used as a standard ambient reference (sometimes called SATP) for chemical and materials data.
  • 20 °C and 1 atm — used in some engineering contexts as a 'normal' reference.

Uses and importance

Standard conditions are widely used when applying the ideal gas law to derive volumes, when quoting gas densities, and in tabulated thermodynamic data. Reporting results at a specified reference eliminates ambiguity: a measured gas volume depends strongly on temperature and pressure, so conversions between conditions require a clear standard. For discussion of reproducibility in lab practice see experimental reproducibility.

Distinctions and conventions

Be aware of the difference between STP and the thermodynamic "standard state." In thermochemistry the standard state is typically defined at a pressure of 1 bar (100 kPa) with a stated temperature (commonly 25 °C), and is used when tabulating standard enthalpies, entropies and free energies. Organizations and standards bodies may recommend one convention over another, so authors and practitioners should always state the exact values they adopt. For information on how standards are set and used across fields, consult general references on scientific standards.

Notable fact: because different communities use different definitions, it is good practice to give numerical temperature and pressure alongside any reference to "standard" conditions.