Buck Converter Duty Cycle Calculator
A buck converter steps a higher input voltage down to a lower regulated output by rapidly switching and filtering. The duty cycle, the fraction of each switching period the main switch is on, sets the output voltage. This calculator finds the required duty cycle from the input and output voltages, with an optional efficiency factor to account for losses, and reports the on-time and off-time once you add a switching frequency. It is the first design step for any step-down switching regulator. The ideal continuous-conduction relationship is D = Vout / Vin.
Buck duty cycle formula
ideal duty cycle D = Vout / Vin
with losses D = Vout / (Vin * efficiency)
period T = 1 / switching frequency
on-time = D * T, off-time = (1 - D) * T
Efficiency is a fraction between 0 and 1. The on-time and off-time use the switching frequency entered in kilohertz, converted to seconds internally and shown in microseconds.
Design notes
- A buck duty cycle is always between 0 and 1 since the output is below the input.
- Lower efficiency raises the required duty cycle slightly.
- The relationship assumes continuous conduction mode.
- On-time plus off-time equals one switching period.
- Output voltage must be less than the input voltage for a buck topology.
Buck converter: frequently asked questions
What is the buck converter duty cycle formula?
For an ideal step-down (buck) converter in continuous conduction, the duty cycle is D = Vout / Vin. With an efficiency factor to account for losses, D = Vout / (Vin * efficiency). The duty cycle is the fraction of each switching period that the switch is on.
Why is a buck converter duty cycle always less than 1?
A buck converter steps voltage down, so the output is lower than the input and the ratio Vout / Vin is between 0 and 1. A duty cycle of 0.5 means the switch is on for half of each period, producing roughly half the input voltage in the ideal case.
How does efficiency change the duty cycle?
Real converters lose some power as heat, so the switch must be on slightly longer to deliver the target output. Dividing by the efficiency (a value between 0 and 1) raises the required duty cycle. At 90 percent efficiency the duty cycle is about 11 percent higher than the ideal value.
What is the on-time from the duty cycle?
If the switching frequency is f, the period is 1 / f and the switch on-time is D / f. For example, a 0.4 duty cycle at 100 kHz gives an on-time of 4 microseconds out of a 10 microsecond period.
Does this assume continuous conduction mode?
Yes. The simple D = Vout / Vin relationship holds in continuous conduction mode, where the inductor current never falls to zero. In discontinuous conduction the duty cycle also depends on load and inductance, which this calculator does not model.
Official sources
- NIST: SI units (volt, hertz, second).
- NASA Glenn Research Center: Switching power conversion fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.