Short answer: yes, for metal. A stud finder app can find the steel screws or nails that hold drywall to a stud, and that is enough to locate the stud. It cannot see the wood itself, it cannot measure depth, and it cannot see copper or plastic. Below is how it works, what we measured with our own app, and when you should buy a real stud finder instead.
Every iPhone has a magnetometer. It is the sensor behind the compass, and it measures the magnetic field around the phone in microtesla (µT). Iron and steel bend that field. When the phone passes over a steel screw, the reading changes, and the app shows you the change.
That is the whole trick, and it explains both what works and what does not. There is no radar in a phone and no density sensor. An app that claims to "see" wood or pipes of any material through a wall is describing hardware the phone does not have.
| Target | Phone app | Why |
|---|---|---|
| Drywall screws and nails in a wooden stud | Yes | Steel fasteners disturb the magnetic field |
| Steel studs, steel pipes, steel brackets | Yes | Large iron mass, strong reading |
| The wooden stud itself | No | Wood is not magnetic; you find it through its fasteners |
| Copper pipe, aluminum, brass, plastic pipe | No | Not ferromagnetic |
| A cable with current flowing | Sometimes | Flowing mains current makes a 50/60 Hz field; an idle cable makes none |
| Depth of the object | No | A small screw close by and a large mass further in can read the same |
We build Ferrum: Stud Finder & Metal, so we tested it the way we would test any instrument: first at rest, then on a real wall.
These are our numbers from our tests. Yours will differ with the phone model, the wall, how deep the screw sits and what else is nearby. The point is the gap: a screw near the surface is not a subtle signal.
A phone app is a good fit for hanging a shelf or a TV mount on an ordinary drywall-on-wood wall. Buy or borrow a dedicated tool when you need to find the edges of the stud rather than its fasteners, when the wall is plaster and lath, when you are looking for copper or plastic pipe, or when you are drilling where a mistake is dangerous. We compare the options in stud finder app or a real stud finder.
Yes, for metal. An iPhone's magnetometer detects the steel screws or nails that hold drywall to a stud, which is how an app locates a wooden stud. It cannot detect the wood itself, copper, aluminum or plastic, and it cannot measure depth.
Indirectly. The phone cannot sense wood, but it can find the line of steel screws or nails driven into the stud. Several readings in a vertical line mean you have found the stud.
The most common causes are a magnetic phone case, skipping calibration, moving too fast, or passing the wrong part of the phone over the wall. The magnetometer sits near the top of the iPhone, around the camera area.
About Ferrum. Ferrum: Stud Finder & Metal is an iPhone app (iOS 17 or later) by 55DIGITS LLC that uses the phone's magnetometer to find the screws in a stud, steel behind walls and wiring that is carrying current. $4.99 once, no subscription, no ads, no data collected. It does not detect wood, copper, aluminum or plastic, and it does not measure depth.
Ferrum is not a substitute for a professional scanner where safety matters, for example when drilling near mains wiring or gas lines. Always verify by another means before drilling.