If you even find yourself needing to convert years to hours, miles to kilometers, liters to pints, or grams to pounds, the free Units is a fine addition to your app repertoire (or appertoir, if you prefer).

Units lets you convert numbers for area, temperature, time, weight, speed, length, pressure, power, volume, and data storage from one unit of measurement to many others. It can tell you, for example, that there are 31,557,600 seconds in a year or 1,024 drams in a gallon. More usefully, I've used it to convert lengths in centimeters to inches, and temperatures from Celsius to Fahrenheit.

To use Units, you tap one of the conversion options (Weight, for instance), then tap the box underneath the measurement you want to convert from (for example, Kilogram). Doing so brings up the standard iPhone keyboard, on which you input the number you want to enter. The app will make the calculation immediately, although you can't see most of the other measurement boxes until you tap the Done button (the on-screen keyboard takes up half the screen until you dismiss the keyboard).

Units also has the ability to convert among 35 different currencies (updated wirelessly for accurate results, with a "Last Updated" stamp on the bottom of the screen to let you know how current the rates are). There's even an on-screen ruler that lets you measure small items up to two-and-a-half inches or seven centimeters.

Units is definitely not going to win any awards for design--the app's layout and implementation are simple, without any bells and whistles--but it just works. For simple, on-the-go conversions, Units does the trick.