An honest, calibrated instrument built on your iPhone's magnetic sensor: finds the screws that hold a wooden stud, steel framing and pipes, live wiring that is carrying current, and lost ferrous objects. It measures the second you open it. One price, no timers, no subscription, no ads. Works offline.
In our own tests, the calibrated reading at rest stayed within 0.09 µT with zero false alarms across 5,213 samples, and a screw in a wall peaked at 62.9 µT and was detected on five of five passes. Your numbers will differ with phone model, wall and surroundings.
Questions, bug reports or feature requests are welcome at ferrum@tommano.com. We usually reply within a few days.
Partly, and it helps to know which part. An iPhone has a magnetometer, so it can detect iron and steel: the screws or nails that fasten drywall to a stud, steel framing, steel pipes. It cannot see wood, so Ferrum finds a wooden stud by finding the line of fasteners in it. It cannot measure depth and it cannot see copper, aluminum or plastic.
You find the screws that hold it. Put the phone flat on the wall in Stud mode and slide it sideways. A peak is a screw. Mark it, then check 30–40 cm above and below: several peaks in a vertical line mean you found the stud.
Take off any magnetic case — MagSafe magnets are far stronger than anything you are looking for. Hold the phone away from metal and cables and tap Calibrate; after about two seconds the reading settles near zero. Then move slowly: the number shows how much the field has changed since you calibrated, and the spot where it peaks is where the metal is.
Wire mode looks for the magnetic signature of current that is actually flowing, not for the presence of a wire. A cable feeding an LED lamp, a phone charger or a switched-off appliance draws far too little to detect. Switch on something power-hungry on that circuit — a hair dryer, heater or kettle — then rest the phone still against the spot for about three seconds. It works on both 50 Hz and 60 Hz mains, with nothing to set.
Near the top, around the camera area, though the exact spot varies between models. To find yours: switch to Metal, calibrate, then slide a screw slowly across the back of the phone. Wherever the number jumps most is your sensor — lead with that edge when you sweep.
No, and no phone app can. A magnetometer senses ferromagnetic metal — iron and steel — and the changing field of live wiring. Gold, silver, copper, aluminum, brass and plastic are invisible to it. Depth cannot be measured either: a strong reading may be a small screw close by or a large mass further in. For dead cables and non-magnetic targets, a capacitive stud finder or a wall-scanning radar is the right instrument.
A drift of a fraction of a microtesla is normal — the sensor is that sensitive. If it sits many microtesla above zero, something magnetic is close by: a case, a laptop, a speaker, a fridge door. Move away and calibrate again.
No. Ferrum is a one-time $4.99 purchase on the App Store, with no subscription, no in-app purchases and no ads.
Ferrum collects no data. The app has no account, no sign-in, no analytics, no advertising and no third-party code. It never connects to the internet: every measurement happens on your device, and the only stored values (your mode, sensitivity, units and sound settings) stay in your device's local storage. Nothing is ever transmitted to us or to anyone else.
Ferrum is a one-time purchase with no subscriptions and no in-app purchases. The transaction is handled entirely by Apple; we never see your payment details, name or email address.
Because no data is collected, there is nothing for us to share, sell or delete — and nothing is collected from children either. If you have any questions about this policy, contact ferrum@tommano.com.
Effective date: August 16, 2026.
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.