PianoMeter is a piano tuning app developed by Willey Piano LLC. It is designed to turn your iPhone into a professional quality electronic tuning aid. Unlike regular chromatic tuning apps, PianoMeter actively measures the tonal characteristics of each note and calculates the ideal "stretch" or offset from equal temperament, creating a custom tuning for your piano.

The app offers three levels of functionality: a free version for evaluation, a paid "plus" version with basic tuning features, and a "professional" version for professional piano tuners. The free version allows you to tune notes on the piano between C3 and C5, while the full piano tuning requires an in-app purchase.

The free version includes automatic note detection, the ability to measure each note and compare its current tuning to the ideal tuning curve, and a live frequency spectrum or inharmonicity display. Upgrading to the "plus" version adds tuning functionality for the entire piano, the ability to tune to different frequency standards, historical or custom temperaments, and calibration to an external frequency source.

For professional piano tuners, upgrading to the "professional" version unlocks additional features such as saving and loading tuning files, pitch raise mode for initial tuning, custom tuning styles, and access to future enhancements. The upgrade costs approximately $25 for the "plus" version and $300 for the "professional" version.

Please note that the app requires permission to access the device's microphone and read/write files.