https://www.cnet.com/news/ios-12-turns-your-iphone-into-a-virtual-tape-measure/

The new Measure app in iOS 12 lets you figure out the approximate dimensions of real world objects. Just connect the corners and let your phone figure it out.

The demo looked simple enough -- just point your phone at whatever you want to measure, then tap and drag your finger along an edge. Your phone's software will do the rest, giving you an approximate idea of the width, depth or height of whatever you're looking at.