The basics, explained

Protractor is an iOS app that measures angles, slopes, inclines, and level using your iPhone's built-in motion sensors and camera, replacing a physical protractor, spirit level, and inclinometer. It's free to try, ad-free, and works offline.

The iPhone has an accelerometer and gyroscope that sense gravity and orientation — the same sensors behind the level in Apple's Compass app. Protractor turns that into the exact tilt of the phone, so resting it on a surface gives that surface's angle in degrees.

For everyday DIY, woodworking, and home tasks, the sensor-based reading is accurate to within about a degree once the phone is held flat against the surface. Re-zeroing against a known flat reference before measuring improves accuracy further.

Yes. The same angle reading can be shown as degrees, percent slope, or a ratio, so the app works as an inclinometer for ramps, driveways, paths, and land grade.

Yes. Hold the phone against the roof slope or a rafter in the attic and read the pitch in degrees, or switch to the X-in-12 ratio roofers use. You can also measure pitch from a side-on photo using the camera mode.

Yes. A built-in spirit (bubble) level shows true horizontal and vertical so you can check whether shelves, frames, and appliances are straight without a separate level.

For angles you can't rest the phone on, the camera mode lets you take or open a photo and drag two on-screen arms onto the lines. The app reports the angle between them — useful for angles at a distance or in a diagram.

Yes. Rest the blade against the phone or use the camera protractor to check the bevel, so you can hold a consistent sharpening angle such as 15° or 20°.

The camera-based modes can estimate joint angles (similar to a goniometer) for range-of-motion checks. For clinical or diagnostic use, rely on a certified medical device.

Degrees, percent slope, and ratio (such as X-in-12 for roof pitch). You can switch between them without re-measuring.

iPhone 8 and newer running recent iOS versions. Every modern iPhone has the motion sensors the app needs, and the camera protractor works on any model with a rear camera.

No calibration is required for everyday use. For the most precise reading of the angle between two surfaces, tap to zero the app against one surface first.

Yes. All measuring runs on-device — no internet connection or account is needed.

It's free to download and includes a free trial. After the trial, a subscription unlocks the full feature set — there are no ads, and no signup to get started.

Yes with most cases. A thick or kickstand case that stops the phone sitting flush against a surface can throw the angle off slightly — remove it for the most accurate reading.

Yes. Core measuring works fully offline and does not require an account. Camera measuring happens on-device.