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 20degrees Celsius (°C), which exactly equals both 293.15kelvin (K) and 68degrees Fahrenheit (°F).

Due to thermal expansion, precision length measurements need to be made at (or converted to) a defined temperature. ISO1 helps in comparing measurements by defining such a reference temperature. The reference temperature of 20°C was adopted by the International Committee for Weights and Measures on 15 April 1931, and this temperature was used in ISO recommendation number 1 in 1951. It soon replaced worldwide other reference temperatures for length measurements that manufacturers of precision equipment had used, 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, an ISO Recommendation.

See also

• Metre

• International Organization for Standardization

References