There's an endless supply of Web-based tools that can convert just about any measurement from one unit to another. If you need to convert really obscure units, though, or need a conversion tool on a regular basis, try Unit Conversion Professional from Inventive Design. It's a free Flash-based tool that converts more than 800 pairs of units in 32 categories. It also includes a calculator and a library of scientific formulas.
Unit Conversion Professional's dialog-style interface opens with its menu page, which offers two scrolling lists, View Formulas and Convert Units. The conversion units are categorized from Acceleration to Volume. Clicking on a category called up applicable conversion tools; clicking Menu took us back to the menu page. We could also select Browse A-Z, which brought up a list ranging from Abampere-Current to Zip 250-Data Storage; an impressive range. Conversions are ultrasimple: just enter the values and units and the tool does the rest. In this way we learned that 13 acres equals 5.33 darcy (approximately). The View Formulas list worked in similar fashion; for instance, clicking Mechanics called up a list of variables and equations for velocity, acceleration, angle, and more. We could print any view or expand or contract the view from the interface by clicking plus and minus icons. The Calculator tool has an interesting angled 3D perspective and a realistic rendition. It's a basic calculator, not a scientific tool. This program has no Help file other than an About file for Adobe Flash Player, but it needs none.
The only drawback to Unit Conversion Professional, and it's a stretch to call it that, is a bar of links to the developer's other software. If you're looking for an easy-to-use conversion tool that offers just about every unit you can think of and then some, plus handy formulas, look no further.