There are some ruler apps on Google Play but usually they are not accurately. This ruler app can show exactly like a true ruler, in cm and inch. You choose your screen size one time and done. Next time when you open app, it remember your choice and you don’t need to choose again.
And it auto hide the choose button, change cm to inch button. Just show the ruler. If need to show, you click on the word show at the left corner of the app.
The ruler can also measure area of small thing if you put it on phone screen.
This is the most accurately ruler app, so vote it 5 star if you like.
Have a ruler on your pocket, anywhere.