The SI unit of acceleration is metres per second per second because acceleration is:

1. Units follow directly from the defining equation. 2. Acceleration is velocity in metres per second divided by time in seconds. 3. So the SI units are metres per second squared, that is metres per second per second. 4. That name literally means by how many metres per second the velocity changes every second. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 2 'Kinematics', sections 2.1-2.8_