Earth pulls everything down at about 9.8 m/s², but not equally everywhere: about 9.780 at the equator and 9.832 at the poles, slightly less on a mountain. Measure it two ways below. Nothing leaves your phone unless you press Add to the map.
WHERE YOU ARE (OPTIONAL)
Used only to compute the gravity you should expect. Rounded to 0.1° (~10 km) on your phone before anything is sent.
A · PHONE AT REST
Lay the phone flat on a solid table and don't touch it. It averages its accelerometer for 10 s. Honest limit: phone sensors are usually off by 0.5–2% straight from the factory, more than the whole equator-to-pole difference. The 6-face calibration below cancels the sensor's offset; the scale error that's left sets a floor of roughly 0.5%.
ADD TO THE CROWD GRAVITY MAP
Opt-in. Sends only: the g value and its uncertainty, the method, a device class, the rounded location if you gave one, and a random id kept in this browser. Readings more than 1% from the expected local gravity are flagged on the map, not deleted. Every reading is chained (sha256) so nobody can edit it later.
THE CROWD MAP verifying…
See it on the globe: Earth Lantern ▸ Gravity (crowd)
Physics: expected gravity uses the WGS84 normal-gravity (Somigliana) formula minus the free-air gradient, 3.086 µm/s² per metre of height. Local rock density, tides (up to ~2.5 µm/s²) and buildings make real gravity differ slightly; real gravimeters see those, phones can't.