Yes, they really are. I'm talking about the conversion utilities that apply to common units of measurement for gas concentration.
For some years, the good folks at Interscan have made available on their website downloadable Excel spreadsheets, to perform such conversions as parts-per-million to milligrams per cubic meter. Since these calculations take into account ALL necessary parameters (unlike most other web-based "converters") as well as maintain at least two decimal places, they truly are the most accurate on the Web.
Recently, Interscan updated the converters. They are now easy-to-use script based utilities, requiring only a few keystrokes. The conversions are made on the fly.
By way of background, Interscan also offers an informative Knowledge Base article on the stupidity of some of the alternative units of measurement.