Use your phone as a Ruler. This Ruler app supports three types of scales: metric ruler, inch ruler, fractional ruler in inches. You can use a coin or a credit card to calibrate the scale, even if you don't have a real ruler handy!
It's free of charge to download the installer of Ruler, also, you can have a look at users' reviews on Droid Informer. The app runs on Android 3.0 and above. This application is totally secure to download. Dim&Pav is the developer of Ruler that is included in the Tools category.