At WWDC 2018, Apple kicked off the festivities with augmented reality, discussing the brand new ARKit 2.0 and an iOS 12 app called Measure, which you can use to measure real 3D objects. While similar apps have existed for some time, Apple's solution may be the best way to eschew your pocket measuring tape.
If you want to use Measure, you need to be on the iOS 12 beta, but you can use any iPhone that you have — not just the iPhone X. This new stock app uses your rear camera to measure things in real life, and it's pretty easy to use. It does need some work in the accuracy department, especially since AR points still drift in space, but it's a great first step for Apple's first big AR-specific app.
First, open up Measure either from the home screen, with Siri, or from the Search pane. Your iPhone may ask you to move your iPhone around a bit, and this is so it can make more accurate measurements. A white circular cursor will appear at the center of your screen. Point it at the item you want to measure. If it's a rectangle, a yellow box should appear over it, which you can tap to have it spit out rough dimensions.
You can also manually point the cursor at something, tap the plus (+) icon at the bottom to create a point, and then tap it again when you're done drawing the line to have your iPhone tell you how long it is. You can record as many lines as you want, too. If you want to save your measurements, tap the white circle on the right side to take a screenshot.
You can also tap on the recorded measurement to bring up a new screen that shows you it not just in feet and inches, but in centimeters as well. From here, you can easily copy the digits so you can paste them into a Note or wherever else you need to.
If you use the "Level" tool a lot in the Compass app, you'll probably notice it gone now. That's because it has moved to the Measure app, which makes sense considering it's more closely related to measurements than directions.
From inside Measure, tap "Level" in the bottom-right corner to begin using the tool. Your iPhone will be able to tell if you're using your phone horizontally or vertically, flat on a table, or held upright. When it's at zero degrees, your iPhone will turn green, indicating it's straight.
I'm going to add a small disclaimer for Measure: It's still in very early beta and incredibly buggy as a result. Both the app and my iPhone restarted multiple times while using it, and it always made my iPhone 7 Plus hot to the touch. While it may be because I'm using an older device, the 7 Plus will be getting iOS 12, so it should run smoothly. Additionally, Measure wasn't always accurate, so use it for approximations only right now.