I have used a simple program created by Frank Crenshaw called Scale Calc.
I have used it for about 10 years and it converts everything from Miles,Yards, Feet, Inches, Kilometers, Meters, Centimeters and Millimeters. And does fractions or decimals. It has been an incredible tool and works flawlessly and instantly with both inputs displayed silmultaniously. It is an older program and is an .exe, some virus scanners flag it because of that. I downloaded it from IPMS USA years ago as a freebee. When I do a google search for it's file name (scalecal.exe) all I see is people thinking they have a virus. It is harmless, I can attest to that. You can download it here. You will probably see a warning of some sort depending on what browser you use. However it works like a charm and is easiest of all to use, it works offline so you don't need a page for it. I have it on my desktop and toolbar, it will run from anywhere.
RailSimStuff.com
Another freebee called ScaleMaster, but a little more confusing
is on Starship Modeler and can be found here. It is also an .exe file.
Starship Modeler - ScaleMaster Scale Calculator
Hope this is helpful, if anyone has a problem with running scalecalc I can answer questions.
Regards,
Jeff
Starship Modeler - ScaleMaster Scale Calculator