If you are using software to make conversions that are critical to your application, especially those in trade or commerce, you should determine if the program uses the appropriate conversion factors and that it rounds the values accurately and appropriately for your application (e.g., so that the final quantity is not overstated). Some Website display the actual factor on which the conversion is based and explain the rules of rounding that the programmer has used to compute the displayed result. The conversion factor and rounding rules used will help you verify the accuracy of the conversion. We also recommend that you make an inquiry to the provider of the software or web page to ask if the conversion factors used in the software are consistent with the official values in NIST Special Publication 811 Appendix B or IEEE/ASTM SI 10, American National Standard for Use of the International System of Units (SI): The Modern Metric System which is available for purchase at http://www.astm.org or http://www.ieee.org. Users should evaluate metric conversion software for compliance with the standards listed above.
All links to metric conversion software and other devices have been removed and NIST will not accept requests to add links to the Metric Program Website. We are not able to maintain an accurate list of all the metric converters available, nor can we maintain the accuracy of the web links or provide any assurances regarding the accuracy, rounding, or results of any program, nor perform a fair evaluation of their performance or features. To find a conversion program that meet your needs, we suggest you conduct a search on terms such as unit calculator, metric converter, or metric conversion program. Any one of these terms should return several URL's from which you can choose the converter appropriate for your application.
NBS Technical Note 872 - Computer Program Package for Metric Conversion (1975) - PDF
American National Metric Council (ANMC) - Guidelines for Metric Transition of Software (1985) - PDF
Charles A. Schulz, Association for Computing Machinery (ACM) – Writing Applications for Uniform Operation on a Mainframe or PC: A Metric Conversion Program (1990) - PDF
Gordon S. Novak Jr., University of Texas - Conversion of Units of Measurement (1995)
National Physical Laboratory (NPL) Software for Metrology Best Practice Guide No. 1 - Validation of Software in Measurement Systems (2007) - PDF
National Physical Laboratory (NPL) Software Support for Metrology Best Practice Guide No. 7 - Development and Testing of Spreadsheet Applications (2006) - PDF