Measuring With Your Phone

How to level a shelf, read a roof pitch, measure without a ruler, pick the right screw, judge a noise level, check a turntable's speed and trust your compass — using the sensors you already carry.

• Using your phone as a spirit levelWhat the accelerometer actually measures, how to zero the reading against a known-flat surface, and the situations where a bubble level still wins.
• Measuring roof pitch and slopeThree ways of writing the same slope, how to convert between them, and how to take a pitch reading safely from the ground or the attic.
• Measuring without a rulerCalibrate your screen with a bank card, use standard objects as references, and know when a phone measurement is good enough.
• Metric screw sizes, spanners and drill sizesWhat the M number means, the spanner size, tapping drill and clearance drill for each common metric thread, and how to identify an unknown screw.
• Measuring noise with a phone microphoneWhat the microphone can and cannot tell you, why the numbers differ between apps, and a reference table of everyday noise levels.
• Measuring rotation speed with a strobeThe stroboscopic effect explained, how to check a turntable at 33⅓, 45 and 78, and the harmonic trap that makes people read half the real speed.
• Getting an accurate heading from your phoneMagnetic versus true north, the figure-eight calibration, and the everyday objects that quietly pull your heading off by tens of degrees.
• The measuring instruments already in your pocketAccelerometer, gyroscope, magnetometer, microphone and camera: which measuring tool each one powers, and what happens when a sensor is missing.