unit converter app for iphone 4s

Can anybody recommend a converter app that can convert units back and forth such kilometers to miles or liters to gallons?

Convert

I'm surprised a quick search in the App Store couldn't have turned up anything obvious for you.

PowerOne calculator is the Swiss army knife of calculators. I've used it on other smartphone platforms over the years and the iOS version is the best yet. Just wish they made a Mac version. Does conversions as well as all the usual types of statistics, and allows for custom templates. Does algabraic or RPN mode, and has a feet-inches mode for woodworkers. The iPad version is very well laid out to take advantage of the larger screen, and I think (not sure) I got both versions when I bought it for my iPhone.

If you're looking for a free one to just convert units I use 'Convert Units FREE'
Works well enough