Viewlers Mobile App
Viewlers is a free mobile app that allows you to measure accurately using your mobile screen. It may require calibration first time. You can do this manually using a real ruler or finding the DPI value of your device from the website linked to in the app. You can also estimate the length of larger objects using the visual ruler and an object of known length.
Viewlers Video Demonstration
Free Visual and Digital Ruler App for Android
This video shows how you can accurately measure small objects using Viewlers digital ruler.
To measure larger objects or distances with some degree of accuracy you can use the camera based visual ruler.
Its a super two in one ruler!
Available for free download on Google Play Store: