Open the Measure app on your iPhone
Move your iPhone to let it detect the area
Position the white dot over the starting point
Tap the plus button to set the first point
Move your iPhone to the ending point
Tap the plus button again to set the second point
Read the measured distance on the screen
To measure a person, point the camera at them and follow the on-screen prompt
To measure a rectangle or square, align the frame with the object and view the dimensions
To save a measurement, take a screenshot
To clear a measurement, tap the reset or trash option in the app