Four tools on the sensors your iPhone already has: a level, an AR tape measure, a decibel meter and a stud finder.
The short version of what these tools cannot do: the stud finder finds metal rather than wood, the tape measure estimates rather than measures exactly, and the sound meter is not calibrated. Each is explained below, and the app says the same thing before you use each tool.
Set zero on it. The sensor measures gravity, not your surface, so it has no way of knowing what you consider level until you tell it. Flat and vertical keep separate zeros, so set both.
That is expected. A case, or the camera standing proud of the back, lifts one edge by a fraction of a degree. Set zero again whenever the phone's resting face changes.
The camera has not found a surface under the reticle yet. Sweep the phone gently across the scene — tracking builds from motion, and a phone held still never starts. More light and a surface with some pattern both help; a blank white wall in a dim room is the hardest case there is.
Over a short span, a centimetre or two. Over a room, more. Distance costs accuracy, because the further away the surface the more a small aiming error becomes a large one — readings under three metres are the ones to trust. If something has to fit, check it with a tape.
Tracking has slipped, which happens after a lot of movement or a trip through a dark corridor. Tap Reset and start the session again.
It will. A phone microphone has no traceable sensitivity, its response is shaped by the case and the port, and iOS may apply its own gain. The app treats full scale as roughly 120 dB and measures down from there, which is a reference point rather than a calibration. For anything official, use a Class 1 or 2 meter.
Input clips near 120 dB, so the very loudest sounds flatten against the top of the scale instead of going past it.
It found metal. That could be a screw, a nail, a pipe or a cable. A stud is inferred from a line of hits at even spacing — one hit on its own tells you very little, and no hit tells you which of those four it is. Never drill on this reading alone.
The magnetometer needs calibrating, and that gesture is how iOS does it. It takes a couple of seconds.
Speakers, laptops, chargers and steel furniture all have magnets in them. Set zero on a clear part of the wall, away from all of that.
Bugs, refund questions or anything else: support@toolkit.slumbiapp.com. Telling us your iPhone model and iOS version makes a bug much easier to find.