One website I used almost every day as a design engineer (and continue to use often) is OnlineConversion.com. If you can think of an engineering or scientific application with unique units, this website will probably convert them. It’s free to use and doesn’t require any registration. The site claims “5,000 units and 50,000 conversions” and receives over half a million unique visitors a month based on Compete.com.
OnlineConversions.com has all the standard conversions you will need like Energy with 100 different units, Mass Flow Rate with 38, Force with 37, Acceleration, Density, Power, Pressure, Viscosity, etc.
Online Unit Conversions for The Common, The Complex, and The Unexpected