The Measure app uses augmented reality (AR) technology to turn your device into a tape measure. You can gauge the size of objects, automatically detect the dimensions of rectangular objects, and save a photo of the measurement. If the Measure app is not already installed on your iPhone, you can download it through the App Store.
How to take a single measurement
Open the Measure app, then follow any onscreen instructions that ask you to move your device around. This gives your device a frame of reference for the object you're measuring and the surface it's on. Keep moving your device until a circle with a dot in the center appears.
Move your device so that the dot is over the starting point of your measurement, then tap the Add button (plus sign).
Slowly move your device until the dot is over the ending point of your measurement, then tap the Add button again.
How to take multiple measurements
After you take the first measurement, move your device to place the dot over another location on or near the object.
Tap the Add button to start the second measurement, then move your device to place the dot somewhere along the existing measurement.
Tap the Add button again and the second measurement appears.
Repeat these steps to take as many measurements as you'd like.
Tap the Undo button to remove the most recent measurement, or tap Clear to start over.
Full information: https://support.apple.com/en-us/HT208924