How do you calculate deseleleration?

Answer 1

Deceleration (not deseleration) is the negative rate of change of velocity over time.

Acceleration is [Final velocity - Initial Velocity]/Time. If the final velocity is less than the initial velocity, then the above quantity is negative and is termed deceleration.

The instantaneous deceleration is dV/dt, the derivative of the velocity with respect to time.

Sometime acceleration and deceleration are defined in terms of speed rather than velocity. This is not correct since it is inconsistent with the laws of motion.