Spirit level
Lay your phone flat on a surface, or stand it on its long edge against a shelf or a frame.
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Waiting for the motion sensor.
No motion sensor answered. A phone or tablet has one; a laptop usually does not. The sum below gives the same angle from two measurements.
Angle from rise and run
any unit, the same for both
The reading is worked out on this device from its own sensor; nothing is sent anywhere. A thick or bent case tilts the phone: put it on a surface you trust and press Zero here first.
Made from the lab's tilt experiments, which steered particles with this same sensor reading. More from the lab