Sites and apps similar to Qhanu include FloorArea AR. It's paid, but Air Measure is free. Between them they cover iPhone and iPad.
Watch the Instructional videos at the 'Developer Website' link below. That is the app using AR (Augmented Reality) to create accurately scaled areas and dimensions of the rooms you are in, You can then see and print out the floor plan to scale.
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Air Measure™ turns your phone into a ruler you can use anytime, anywhere. Just point your phone and click a point to measure almost anything. The app uses the power of Augmented Reality to measure the distance between points in 3D space.
