Converting pounds to kilograms or Pounds Sterling to dollars isn’t exactly guesswork. It requires precision. Happily, Apple’s App Store offers a healthy selection of unit converters, including some specialized unit conversion applications for scientists and engineers. The question is, why in the world would anyone need more than one decent unit conversion app? A mile is 1.609344 kilometers no matter what.

The difference comes down to interface. Is it simple or cumbersome? Too many options or two few? Easy to read or confusing to navigate? Here are four unit conversion apps geared to the everyday iPhone or iPod touch user.

Yuchao Zhou’s Units Convertor was a very popular free application, though it now costs $1— a change that angered some users. Yuchao explains that he released the program as a freebie before Apple approved paid application contract.

Units Convertor (yes, that’s how it’s spelled; no, it isn’t spelled correctly) has a clever interface: simply enter the value you want to convert and turn the wheel for the unit you want to convert to. As a 99-cent application, however, Units Convertor lacks features that other apps make available for the same price. Although an upgrade released Friday includes new conversions for computer units and speed, other conversion apps have far more weights and measures to convert. And Units Convertor still doesn’t convert currency.

Units by Crossroad Solutions combines ease of use with a greater range of units available to convert.The calculator-like interface lets users save three shortcuts for commonly used conversions, including weight, temperature, area, pressure, speed and time. You can reverse the direction of a conversion with the touch of a button. Units also has a currency converter.

The downside is changing units can cumbersome. You can scroll through the various weights and measures by touching the “from” or “to” buttons—if you touch too quickly, you cannot click back. Or you change the units by clicking a tool button in the upper right-hand corner of the calculator and clicking through a menu or two.

Converter by UnitConvertr offers a simple but colorful interface, which is the highlight of this conversion tool. The application supports 109 units in nine categories, including bits and bytes, links, leagues, astronomical units and even light-years. Incredibly, though, there is no currency converter.

But the advantage of Converter is that it displays all possible conversions. So entering one for kilograms, produces units in grams, milligrams, pounds, ounces, short and long tons, tons, and grains on the same page. One minor drawback is that you sometimes need to scroll to find the conversion you were looking for. A nice fix would let the user change the order in which the units appear.

Converter by Architechies offers an intuitive, no-frills interface with a large, easy-to-read display. Converter handles 62 measurement units in seven categories. It also includes a currency converter, supporting 29 currencies, and a calculator, which allows for quick addition of multiple weights and measures for conversion. Engineers and other practitioners of the law of thermodynamics might be pleased to find that Architechies included temperature conversions for Rankine. But this app is primarily for the non-specialist, and it’s a very nice, no-frills tool—the best of the lot.

All applications in this review are compatible with any iPhone or iPod touch running the iPhone 2.0 software update.

[Ben Boychuk is a freelance writer and columnist in Rialto, California.]