MetAR is the simplest and the best add free ruler & tape measure on the market that uses augmented reality (AR) technology.

Tutorial:
1.) Open MetAR app
2.) Scan the surface with your phone camera
3.) Put blue dots near the objects you want to measure
3.1.) Use "line" icon and tap blue dots once again to measure the distance between them
3.2.) Use "circle" icon and tap blue dots once again to measure the radius
3.3.) Use "square" icon and tap blue dots once again to measure the surface
4.) And many more features

Have fun!

Note:
Please note that MetAR requires ARCore library produced by Google. ARCore is constantly improving, which, in turn, positively affects the quality and accuracy of MetAR app.