A floating ruler that you can drag it everywhere.
Measure the real objects outside the screen (CM and Inches).
Measure the virtual objects inside the screen (Pixels and Dots). It is very useful for UI/UE designer.
Switch between centimeters and inches easily.
Calibrate this ruler precisely and easily.
Change the color of ruler.
Set vertical or horizontal ruler.
And more will be added soon.
>>> Known Issue:
Android 6.0, Settings - Apps - Configure apps (Settings Icon at the Upper-Right corner) - Draw over other apps - This app (On-screen Ruler) - Permit drawing over other apps
How to enable floating app "On-screen Ruler" on XiaoMi/MI/MIUI devices:
How to enable floating app "On-screen Ruler" on Huawei devices: