ISO 1 is an international standard set by the International Organization for Standardization that specifies the standard reference temperature for geometrical product specification and verification. The temperature is fixed at 20 degrees Celsius (°C), which exactly equals both 293.15 kelvin (K) and 68 degrees Fahrenheit (°F).

Due to thermal expansion, precision length measurements need to be made at (or converted to) a defined temperature. ISO 1 helps in comparing measurements by defining such a reference temperature. The reference temperature of 20 °C was adopted by the CIPM on 15 April 1931, and became ISO recommendation number 1 in 1951. It soon replaced worldwide other reference temperatures for length measurements that manufacturers of precision equipment had used before, including 0 °C, 62 °F, and 25 °C. Among the reasons for choosing 20 °C was that this was a comfortable and practical workshop temperature and that it resulted in an integer value on both the Celsius and Fahrenheit scales.

It was the first ISO standard, issued originally as ISO/R 1, a ISO Recommendation.

See also


  1. ^ "ISO 1:2016 - Geometrical product specifications (GPS) -- Standard reference temperature for the specification of geometrical and dimensional properties". 2016-08-26. Retrieved 2016-09-15.
  2. ^ Ted Doiron: 20 °C—A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements Archived 2013-02-17 at the Wayback Machine. Journal of Research of the National Institute of Standards and Technology, Vol. 112, No. 1, January–February 2007.
  3. ^ Tranchard, Sandrine (2017-02-23). "ISO celebrates 70 years". ISO. Retrieved 2023-08-10.