U-Converter is a spreadsheet-based units conversion program. It handles over 900 different units in 70 categories. It converts values, and can display a conversion factor. U-Converter has conversion factors or conversion formulae for virtually every type of simple and complex unit. Select the unit from the worksheet, select the desired unit from U-converter, and it makes the change. All conversion factors are traceable to values from NIST Special Publication 811.
U-Converter is $10, or (if requested) comes along free with any other purchased Thermart program.