The first known mechanical alarm clock that historians usually point to was about 19 inches tall (around 48 centimeters).

What that “first alarm clock” was

  • The device most often cited is a 15th‑century German iron alarm clock from Nuremberg with an open metal framework and a bell on top.
  • Descriptions of this clock state that it measured roughly 19 inches in height and was designed to hang on a wall so the driving weights had room to drop.

A bit of context

  • Earlier timekeeping devices (water clocks, candle clocks, tower clocks) could signal times, but the Nuremberg iron clock is one of the earliest surviving domestic alarm clocks with a built‑in alarm mechanism.
  • Later inventions, like Levi Hutchins’s 1787 alarm clock in New Hampshire and Antoine Redier’s 1847 adjustable alarm clock, came centuries after that tall iron design.

TL;DR: The earliest commonly cited mechanical alarm clock was about 19 inches (48 cm) tall.

Information gathered from public forums or data available on the internet and portrayed here.