Can I use my phone as a stud finder?

Every smartphone contains a magnetometer for the compass, and it can be read by an app to show the strength of the magnetic field near the handset. Held against a plasterboard wall it responds to the steel screws holding the board to the studs. That is a genuine physical effect and it genuinely works — the question is what it does not do.

Safety

Step by step

1. Understand what the sensor actually reads.It measures magnetic field strength. Steel screws and nails distort the earth's field locally, and that is what the app is showing you. Timber, plastic, copper and electricity are all invisible to it.
2. Find where the sensor is on your handset.It is not in the middle. Wave a small magnet slowly over the back of the phone with the app running and note where the reading peaks — usually near the top edge. Every phone is different and using the wrong part of the phone offsets every reading.
3. Take the case off.A thick case holds the sensor 5 to 10mm away from the wall, which meaningfully reduces sensitivity on a signal that is already weak through 12.5mm of plasterboard.
4. Sweep slowly and horizontally.Very slowly, keeping the phone flat and at a constant distance. Mark every peak. Screws run up the stud at intervals, so a vertical line of peaks confirms a stud far more convincingly than a single reading.
5. Confirm with the expected spacing.Find one stud and check for another at 400 or 600mm. Readings that do not fall on a regular spacing are probably something else — a nail plate, a bracket, a pipe clip.
6. Use it for fixing points only.This is a reasonable way to find somewhere to put a screw in a stud. It is not a way to decide that a spot is safe to drill, because the two questions are entirely different and the app answers only the first.
7. Use a real detector or isolate the circuit for the safety question.Before drilling anywhere near a socket, a switch, a bathroom, a kitchen or a consumer unit, use a multi-detector in live mode, stay out of the safe zones, and consider switching the circuit off at the consumer unit.

Tips

• The app is genuinely handy for confirming a stud you have already located another way — knocking, looking at skirting fixings, or measuring from a corner at 400mm centres.
• Speakers, magnetic phone mounts, magnetic wallet cases and wireless charging coils all interfere with the magnetometer. So does standing next to a radiator.
• If the app asks you to calibrate by waving the phone in a figure of eight, do it — that is the compass calibration and readings are unreliable without it.
• On lath and plaster, on solid masonry and on foil-backed board, forget the phone entirely. It is a plasterboard-and-screws tool.

Common mistakes

• Using an app as a pre-drilling safety check — It cannot detect live cables by any mechanism. A clear reading tells you there is no steel screw there and nothing whatever about wiring.
• Assuming the sensor is in the middle of the phone — It is usually near one end, so every mark is offset by several centimetres in a consistent direction — enough to miss the stud you found.
• Sweeping quickly — The magnetometer updates slowly and the field change over a screw is small. A fast sweep produces a flat, featureless reading and the conclusion that the app does not work.

Alternatives

• Phone magnetometer app: Free, always with you, and it does find plasterboard screws. Sensitivity varies enormously between handsets, the sensor is in a different place on every phone, and a phone case adds a fixed standoff. Detects metal only, and nothing electrical.
• A strong magnet on a string: Costs almost nothing, needs no calibration, no battery and no app. Dragged over a wall it sticks to every screw, and it is more reliable than the phone because it responds directly rather than through a sensor whose position you have to guess. It will also hang from the screw and mark the spot for you.
• Magnetic stud finder: The same idea, packaged: a pivoting or fixed magnet in a small body that clicks or sticks over a fixing. Cheap, immediate, nothing to go wrong. Still finds only fixings.
• Electronic multi-detector: The only option in this list that attempts live-cable detection, which is the part that matters for safety. Needs calibration and has real blind spots, but it is the tool the safety advice assumes you are using.

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