iOS and iPadOS come with a Measure app that allows you to draw (virtual) straight lines to measure objects and surfaces both vertically and horizontally using your iPhone or iPad camera.
If your iPhone or iPad detects that the object you’re measuring is a square or rectangle, it automatically places a measurement box around the object. Tap the Add button and measurements appear for the object’s width and length. Move your device slightly, and the object’s calculated area appears.
While the measurement is displayed, you can tap the calculated area number to see the length of the diagonal as well as the area in square inches or square meters. automatically detect dimensions of rectangular objects.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)
