Measure With Your Phone Without an Account
Search for a ruler app or a sound meter and you'll find plenty that want an email address, a "pro" subscription to unlock inches, or a permission list longer than the feature list. None of that is needed. Every measurement below comes from a sensor already in your phone, is computed on the phone, and works in airplane mode. Here is what each one can honestly do, and what to calibrate.
Ruler
The screen is a grid of pixels of known size, so drawing a ruler is easy — but phones report their pixel density with a few percent of error, which is why a 10 cm line on screen often measures 9.6 cm with a tape. Calibrate once against something standard: a bank card is 85.60 mm long everywhere in the world. Lay it on the screen, stretch the scale until the guide lines match, and the ruler is true. Also check that the ruler uses the whole screen; apps that put a banner at the bottom lose you the last centimetre and a half.
Level and angle
The accelerometer gives tilt to a fraction of a degree once zeroed on a flat surface (see the level guide). A protractor uses the same sensor with a twist: rest the phone's long edge on one surface, tap "set reference", then rest it on the second surface and read the angle between them. Good for mitre cuts, roof pitch, ramp grades and checking that a shelf bracket is square.
Light
The ambient light sensor next to the front camera reports lux — real, calibrated-at-the-factory lux, because the phone uses it to set screen brightness. It is accurate enough to check a desk against the 300–500 lux recommended for reading, compare grow lamps, or find the brightest spot for a plant. Don't cover it with your thumb; readings range from 1 lux in a dark room to 100,000 in direct sun, so a log scale is the only sensible display.
Sound
Here honesty matters. The microphone can measure relative loudness well — quiet room vs. traffic vs. concert — but it is not a calibrated sound level meter: every phone model has a different microphone, and the OS applies automatic gain. Treat readings as a guide ("this is around 70 dB"), not as evidence for a noise complaint. An app that shows two decimal places on a phone dB reading is decorating a guess.
Compass and metal
The magnetometer gives heading with no location permission (see the compass guide) and doubles as a metal detector: set a baseline in a clean spot, then a change of 15 µT or more means something ferrous is near — a stud, a nail, a pipe, a hidden screw. Its range is a few centimetres, which is plenty for finding a wall stud through plaster.
Why one toolbox instead of twelve apps
Because you use each of these a few times a month, and twelve single-purpose apps means twelve icons, twelve permission prompts, twelve ad networks and twelve privacy policies. An honest all-in-one keeps only the tools whose sensors can actually deliver, puts calibration on screen instead of in a menu, and asks for a permission only when you open the tool that needs it — camera for the magnifier and QR scanner, microphone for the sound meter, nothing for everything else.
Truekit
Truekit is built exactly that way: level, ruler, protractor, unit converter, compass, metal detector, sound meter, light meter, flashlight, magnifier, QR scanner and timer. No account, no internet, no permission on first launch, calibration on screen, ads kept off every measuring surface, and a home screen widget for the four tools you actually use. Free, in 16 languages.