The basics, explained

Protractor is an iOS and Android app that measures angles, slopes, inclines, and level using your phone'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.

Yes — Protractor is on Google Play as well as the App Store, with the same modes on both. It needs Android 8.0 or newer and reads the accelerometer and gyroscope your phone already has. Sensor quality varies more across Android models than on iPhone, so let the reading settle before taking it.

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, and Android 8.0 (Oreo) and newer. Every modern phone 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.

Recalibrate from Settings on a known flat surface, hold the phone firmly against the surface without wobbling, and move away from strong magnetic sources such as speakers or electronic equipment.

Remove your phone case, since it can stop the phone sitting flush against the surface, make sure both the phone and the surface are clean and flat, and reset the calibration from the Settings menu.

Place the phone on the first surface and lock the reading, then place it on the second surface. The app shows both the current angle and the difference between the two measurements.

On iPhone, check the silent mode switch. On Android, check that the ringer and vibration are not muted and that Do Not Disturb is off. Sound and haptic feedback can also be toggled in the app's own Settings screen.

On iOS, yes — the protractor dial adjusts automatically in portrait and landscape. The Android version currently runs in portrait only; this does not affect accuracy, since the angle is read from the motion sensors rather than the screen orientation.

Subscriptions are billed by the store, so you cancel there. On iPhone: Settings → your name → Subscriptions → Protractor → Cancel Subscription. On Android: Google Play → profile icon → Payments & subscriptions → Subscriptions. Deleting the app does not cancel a subscription, and you keep access until the end of the current period.

Refunds are handled by the store that processed the payment. For App Store purchases, request one at reportaproblem.apple.com; for Google Play purchases, use your Google Play order history. Developers cannot issue refunds directly.

No. Subscriptions are tied to the store account that purchased them, so an App Store subscription cannot be used on Android and a Google Play subscription cannot be used on iOS. Manage or cancel each subscription in the store where you bought it.

Email support@protractorapp.com and we'll help.