Inclinometer.

Grade (or gradient) is the ratio of vertical rise to horizontal run, expressed as a percentage. A 10% grade means for every 100 meters of horizontal distance, the elevation changes by 10 meters. It's the standard unit for roads, ski runs, and cycling climbs.

Roof pitch is expressed as rise/run in inches per foot (imperial standard). A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run — a 26.6° angle. This tool shows pitch in the X/12 format.

Hold your phone flat against the roof surface and read the angle. The pitch is displayed automatically in both degrees and X/12 format. 'Hold Reading' locks the value so you can step back safely.

A spirit level tells you if a surface is perfectly horizontal (0°). An inclinometer measures any angle of incline — from 0° (flat) up to 90° (vertical). The spirit level is a special case of the inclinometer at 0°.

Consumer phone accelerometers are accurate to ±0.5–1°. For precise engineering work, a calibrated hardware inclinometer is recommended. For everyday use — hanging shelves, grading driveways, measuring ski slopes — phone accuracy is more than sufficient.