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The inverse second or reciprocal second (s⁻¹), also called per second, is a unit defined as the multiplicative inverse of the second (a unit of time). It is applicable for physical quantities of dimension reciprocal time, such as frequency and strain rate.

It is dimensionally equivalent to:

• hertz (Hz), historically known as cycles per second – the SI unit for frequency and rotational frequency
• becquerel (Bq) – the SI unit for the rate of occurrence of aperiodic or stochastic radionuclide events
• baud (Bd) – the unit for symbol rate over a communication link
• bit per second (bit/s) – the unit of bit rate^[1]
• radian per second (rad⋅s⁻¹) - the SI unit for angular frequency and angular velocity

However, the special names and symbols above for s⁻¹ are recommend to be used in their domains of application, for clarity, as they are used for different kinds of quantity.^[a][b]

The inverse minute or reciprocal minute (min⁻¹), also called per minute, is 60⁻¹ s⁻¹, as 1 min = 60 s; it is used in quantities of type "counts per minute", such as:

• Actions per minute
• Beats per minute
• Counts per minute
• Revolutions per minute (rpm)
• Words per minute

Inverse square second (s⁻²) is involved in the units of linear acceleration, angular acceleration, and rotational acceleration.

• Aperiodic frequency
• Inverse metre
• Reciprocal length
• Shear rate
• Unit of time