A stud finder app and a store-bought stud finder are not two versions of the same tool. They sense different things. A phone senses steel. Most electronic stud finders sense a change in wall density. Which one you need depends on your wall and on what is behind it.
| Tool | What it senses | Finds | Does not find |
|---|---|---|---|
| Phone app (magnetometer) | Iron and steel; the field of flowing mains current | Screws and nails in studs, steel studs and pipes, a cable under load | Wood itself, copper, plastic, depth, idle cables |
| Magnetic stud finder | Iron and steel | Screws and nails in studs | Everything else; no reading, it just sticks or swings |
| Electronic stud finder | Changes in density behind the surface | Edges or center of a wooden or metal stud; many models add a live-wire warning | Reliable results on plaster and lath or very thick walls |
| Wall scanner | Several sensors, on some models radar | Wood, metal, live wiring and often plastic pipe, with an approximate depth on some models | Certainty; it still needs careful reading and costs the most |
Prices vary by model. As a rough guide, a magnetic finder is the cheapest, a basic electronic finder costs a few times more, and a multi-sensor wall scanner often costs over a hundred dollars. Ferrum costs $4.99 once.
All phone apps read the same sensor, so no app can detect more than the hardware allows. The differences are in how honestly and how usefully the reading is presented:
Ferrum measures the second you open it, shows the change in µT, costs $4.99 once with no subscription and no ads, and lists what it cannot detect on its own Learn screen. Our test numbers are in do stud finder apps really work?
For an occasional job, start with the free methods in how to find a stud without a stud finder, confirm the fastener line with a magnet or a phone app, and test with a thin pilot hole. If you are opening walls, running cable or drilling near plumbing, a wall scanner earns its price.
They sense different things. A phone app senses steel, so it finds the screws and nails in a stud. An electronic stud finder senses density, so it finds the stud itself and its edges. For ordinary drywall on wooden studs an app is usually enough; for plaster walls, pipes or professional work a dedicated tool is better.
Every phone app reads the same magnetometer, so a free app can detect the same things as a paid one. The differences are calibration, clear units, and whether ads, timers or subscription offers come before the measurement.
Only steel or iron pipes. A phone's magnetometer cannot sense copper or plastic pipe.
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.