Duty cycle is calculated as:

• A. On time divided by (on time plus off time)
• B. On time plus off time
• C. On time divided by off time
• D. On time times off time

Answer: (A)

Duty cycle is the fraction of each cycle when the output is on. It is calculated by dividing the time the signal is on by the total length of the cycle, which is the sum of the on time and the off time. So the correct relation is on time divided by (on time plus off time). If you want a percentage, multiply by 100.

For example, if the signal is on for 2 ms and off for 8 ms, the duty cycle is 2 / (2 + 8) = 0.2, or 20%.

The other formulations don’t describe the fraction of time the signal is active: the sum of on and off times is the period, not the duty cycle; the ratio of on time to off time is a high/low duty ratio, not the duty cycle itself; and multiplying on and off times has no standard meaning for duty cycle.