You can use the Augmented Reality capabilities of iOS to measure real-world objects through the device camera.
Open the Measure app and move your device around slowly for a moment so the app can start scanning the environment. It’s useful to point the camera toward flat surfaces like tables and floors to help it calibrate. This whole step shouldn’t take more than a few seconds.
Point the white aiming reticule in the center of the screen at any surface and press the big + button to create a virtual dot on that point. You can then move the device to track a virtual line towards another point and press the + again to complete the line. The real-world length of the line will be shown on screen.
Next, you can create more distinct lines or aim at a previous dot to lock onto an existing line. This way you can build up complex shapes and see all the measurements on screen at once. You can tap the Clear button at any time to remove all lines and start again.
The Measure app is pretty accurate, although bear in mind it can sometimes be off by a small margin, so don’t use it for intricate DIY projects!
Tapping the Level button at the bottom swaps to a secondary tool you can use to check how level a surface is. Useful for hanging pictures and putting up shelves!