What this calculator does
Duty cycle is the fraction of one full cycle that a repeating signal spends switched on, expressed as a percentage. A duty cycle calculator like this one takes the on-time and the total period of the cycle and works out what share of that period is active.
The thing people get wrong is mixing up the total period with the off-time. The period is the full length of one repeat, on-time plus off-time together, not just the time the signal is off. Get that denominator wrong and the whole percentage is off, sometimes by a lot.
The formula
Divide the on-time by the total period and multiply by 100. If you only know the off-time, add it to the on-time first to get the total period, since on-time and off-time together make up one complete cycle.
| Term | Meaning |
|---|---|
| On-time | How long the signal, motor or device is active within one cycle. |
| Total period | The full length of one repeat: on-time plus off-time. |
| Duty cycle | On-time as a percentage of the total period. |
The inputs explained
| Field | What to enter |
|---|---|
| On-time (ms) | The active portion of one cycle, in whatever consistent time unit you are working in (milliseconds, seconds, and so on). |
| Total period (ms) | The full length of one repeat of the cycle, in the same time unit as the on-time. |
When to use it
Sizing a PWM-controlled load
Pulse-width modulation drives motors, LEDs and heaters by switching them on and off rapidly. The duty cycle sets the average power delivered, so working it out from the on-time and period tells you roughly what fraction of full power the load is receiving.
Checking equipment ratings
Welders, compressors and some power tools list a maximum duty cycle, such as running 3 minutes in every 10. Comparing your actual on-time and period against that rating shows whether you are within the safe operating limit.
Describing a repeating test or process signal
Any process that switches between two states on a fixed schedule, such as a heater cycling or a valve pulsing, can be summarised by its duty cycle for reporting or comparison purposes.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How duty cycle changes as on-time increases at a fixed 10 ms period
The same 10 ms cycle, with the on-time stretched from short to long.
| On-time | Duty cycle | Off-time |
|---|---|---|
| 1 ms | 10.00% | 9.00 ms |
| 2 ms | 20.00% | 8.00 ms |
| 3 ms | 30.00% | 7.00 ms |
| 5 ms | 50.00% | 5.00 ms |
| 7 ms | 70.00% | 3.00 ms |
| 9 ms | 90.00% | 1.00 ms |
How duty cycle changes as the period lengthens at a fixed 3 ms on-time
A fixed 3 ms active pulse, repeated over a range of total period lengths.
| Total period | Duty cycle | Off-time |
|---|---|---|
| 4 ms | 75.00% | 1.00 ms |
| 5 ms | 60.00% | 2.00 ms |
| 6 ms | 50.00% | 3.00 ms |
| 10 ms | 30.00% | 7.00 ms |
| 15 ms | 20.00% | 12.00 ms |
| 20 ms | 15.00% | 17.00 ms |
Questions
What does a 100% duty cycle mean?
It means the signal or device is on for the entire period with no off-time at all, which in the on-time/period formula happens when on-time equals the total period.
Can duty cycle be worked out from off-time instead of on-time?
Yes. Add the off-time to the on-time to get the total period, or if you only have off-time and the period, subtract off-time from the period to recover the on-time, then apply the usual formula.
Does the time unit matter?
No, as long as the on-time and total period are both entered in the same unit, whether that is milliseconds, seconds or minutes, since duty cycle is a ratio and the units cancel out.
Why do some devices have a maximum rated duty cycle?
Running a device on for a larger share of each cycle generates more average heat. Manufacturers cap the duty cycle so the device has enough off-time each cycle to cool down and avoid overheating.
For working with the elapsed time itself once you have a duration, see the Add Time Calculator.