TrackWeight

TrackWeight is a macOS application that turns a MacBook's trackpad into a digital weighing scale. It works by reading the pressure sensors built into Force Touch trackpads, which have been standard on MacBook models from around 2015 onward. The app interprets those pressure readings as weight measurements.

To use it: open the app, rest a finger on the trackpad, and while keeping that finger in contact, place the object you want to weigh on the trackpad. The key constraint is that the trackpad only produces pressure readings when it detects something conductive (like a finger) touching the surface. You apply as little pressure as possible with your finger while the object rests on top, and the displayed reading reflects the object's weight.

The app accesses low-level trackpad data through a library called Open Multi-Touch Support, which reads pressure events that macOS does not normally expose to standard applications. The data from the trackpad's underlying system turns out to already be in grams, which made calibration straightforward: the developer verified readings by placing a conventional digital scale underneath the MacBook and comparing the two.

A few practical limitations: you must keep a finger on the trackpad during every measurement, metal objects may register as a finger and interfere with the reading (placing a piece of paper underneath them helps), and the app is suitable for casual or experimental use but not for measurements where precision is critical.

The app requires macOS 13 (Ventura) or later and a MacBook with a Force Touch trackpad. It can be installed from a disk image download, via Homebrew, or built from source code. The source is licensed under the MIT license.