For one reason or another, we're stuck in a World where different people measure things in completely different ways.
Convert Units for Free eliminates the confusion of measuring in feet when you're used to meters, telling the temperature in Fahrenheit when you normally use Centigrade, or figuring our what a speed limit in kmph means if you normally experience signs in mph. In all, there 13 different types of measurements (e.g. angle, area, data, energy, force), and each is able to convert between at least two units of measurements (but usually more than this). Disappointingly, Convert Units for Free doesn't have a currency converter at present.
You don't need a degree in engineering to work out how to use Convert Units for Free. You select a conversion type from the list then spin the dials with your fingers to choose what units you want to convert from and to. This is a little more fiddly than it should be and the text is a bit too small. It's certainly not as easy to use as the scrollwheel-style interface that you'll find in Convertbot.
Convert Units for Free includes a 'Swap' button for quickly switching the conversion, and there's even a minus button for dealing with conversions in negative.
If you want a quick way to perform conversions, Convert Units for Free does its job fairly efficiently.
