Free virtual ruler
For those who don’t bring physical rulers anywhere but seem to need them sometimes, On-screen Ruler is a great app to have. This handy, free utility app basically produces a floating ruler on your device’s screen, turning it into a measuring tool in seconds. You only need to grant permission for it to be displayed over other apps, then you’re good to go.
Developed by NeedJava, On-screen Ruler allows you to measure both tangible and intangible objects. You may use it to determine the size of the things on the screen or for objects you’re holding in your palms.