The free-to-try stud finder app for iPhone & Android.

Every iPhone since the 3GS — and every Android phone with a magnetic field sensor — ships with a magnetometer, the same chip the compass uses, exposed to developers through Apple's Core Motion framework and the Android Sensor framework. A stud finder app reads that chip directly. The detection happens entirely on your device, in real time, using sensor data the phone already gathers.

There's nothing recurring to run: no cloud inference, no external API call, no 9V battery to consume. The marginal cost of one more scan is zero, which is why nearly every stud finder app is free to download. What a subscription actually funds is the ongoing work — updates for new phones and OS versions, support, and keeping the app ad-free. The honest version of that is a clear free trial up front and no ads, ever; the dishonest version is a fake "free" listing that springs a surprise paywall after a fake scan.

Search the App Store or Google Play for "free stud finder" and most results fall into one of three traps:

• Hidden paywall. The app installs free and fakes a scan, then locks the real result behind a surprise subscription — no trial, no disclosure.
• Ad-heavy. Free to use, but with a full-screen ad between every scan and banner ads covering the UI.
• Fake scanning. A polished UI that shows a "scanning" animation and returns a result, but never actually reads the magnetometer. Check reviews — the one-star reviews will always tell you.

Stud Finder is free to download and try, ad-free, because the sensor is already in your phone — and it's funded by a subscription rather than ads, so nothing competes with your scan and nothing about you gets sold. That's the whole story.

Not every "free" listing is equal. Here's how to read them:

The distinction that matters isn't free-versus-paid — it's honest-versus-not. A clearly disclosed free trial is fine. A fake scan, an ad buried in the detection view, or a paywall that ambushes you after pretending to be free is not.

"Free" in the App Store and Google Play covers a wide range. The version worth installing has these, regardless of price:

• Real magnetometer detection. It reads the live sensor, not a scripted "scanning" animation. A live reading in microteslas is the tell.
• A live signal, not a yes/no. You can see the spike rise and fall as you cross a fastener, so you can verify the reading yourself.
• A baseline before it scans. It learns the background field — automatically, in about a second — before it tries to detect spikes against it.
• No ads in the scan view. A banner on a settings screen is tolerable; an ad while you are marking a stud is dangerous.
• No signup to start. A stud finder does not need your email to read a sensor.
• Honest pricing. If it is a paid app, it says so plainly with a real free trial — not a fake-free listing, and not detection hidden behind an undisclosed paywall.
• Works offline, no tracking SDKs. Reading a sensor requires no network and no third-party telemetry.

Stud Finder meets all of these on both iPhone and Android: free to try, ad-free, offline, no signup to start.

No. The signal is the same on a free trial or a paid plan. "Quality" for a stud finder app is mostly the visualization — does the live graph make spikes obvious, does it settle on a clean baseline quickly, does the UI stay out of your way. Those are design decisions, not pricing decisions. A well-designed app beats a poorly-designed one at any price.

The criteria for a good stud finder app — live signal graph, a baseline before it scans, no fake AR overlays — are independent of cost. We compare the apps on those criteria, including paid competitors, in the best stud finder app.

A good stud finder app, used correctly, finds studs in:

• 1/2-inch or 5/8-inch drywall over wood framing — accurate to within about an inch, per the 16-inch on-center spacing required by IRC §R602.3.1 for most US/Canada residential construction
• Multi-layer drywall in fire-rated walls — slightly attenuated but still readable
• Engineered framing like LVL beams or I-joists — fasteners are still ferrous
• Steel studs — the stud itself is steel, so it reads as a broad band rather than a spike; mark both edges and use the middle

It does not work on masonry or thick plaster — and no app, paid or free, would have helped on those. Those are sensor limitations, not software limitations. For the full breakdown of where the app works and doesn't, see do stud finder apps work.

The app you pick will only be as accurate as your technique. A 60-second routine that gets the most out of any magnetometer app:

1. Take the phone case off if it has a MagSafe ring or magnetic wallet — they saturate the sensor.
2. Hold still for a second as the scan starts, away from magnets and big metal. That is when the app takes its baseline against Earth's 25–65 μT ambient field.
3. Sweep slowly at chest height. About 2 inches per second. Mark each sharp spike with a pencil.
4. Sweep again 30 cm higher or lower. A real stud spikes at the same horizontal position both times.
5. Tap test, then drill a 1/16-inch pilot. Wood within the first quarter inch confirms.

The two-sweep step is what turns "promising" readings into reliable ones. No app skips it for you. For the full step-by-step, see how to use a stud finder app.