ToolnozaDuty-cycle formula
For a compressor that alternates between a defined on period and off period, duty cycle (%) = on time ÷ (on time + off time) × 100. If a unit runs for 6 minutes and rests for 4 minutes, its observed cycle is 6 ÷ (6 + 4) × 100 = 60%. Use times from the same complete cycle and the actual load/unload definition used by the manufacturer.
A field-observed run fraction is not automatically the machine’s rated allowable duty cycle. The nameplate, manual, cooling arrangement, pressure, ambient temperature, and service type determine what operation the manufacturer permits.
Worked examples and reading the result
If a compressor runs 12 minutes and is off for 8, the run fraction is 12 ÷ 20 = 0.60, or 60%. If the total cycle is 10 minutes and the run period is 7, it is 70%. The remaining fraction is time not in the defined run state; it may be unloaded, stopped, or in another control state depending on the machine.
Use repeated cycles under representative demand rather than one short observation. Record pressure, flow demand, ambient conditions, and control state. A result that varies widely between shifts may indicate a variable load, leaks, undersized storage, or a control issue; the percentage alone does not identify the cause.
Duty cycle is not the same as load or efficiency
Duty cycle is a time ratio. Motor load describes how much power is being drawn relative to a rating at a given moment. A compressor can remain running while unloaded, so simple motor-on time may not equal time producing useful air. Efficiency measures useful output relative to energy input and requires flow and power data.
For a plant compressor, look at the control mode and measured delivered air. A low run fraction does not prove the compressor is oversized, and a high run fraction does not by itself prove the system is unsafe. Compare with the actual equipment rating and system demand.
Match the machine to intermittent or continuous demand
Some small reciprocating compressors are intended for intermittent operation and require a cool-down period. Many rotary-screw and other industrial machines are designed for different duty patterns and control strategies. Product rules vary by model, so do not apply a generic 50% or 100% rating to every compressor.
When demand is steady, confirm the rated delivered flow at the required pressure. When demand swings, consider the storage and control strategy, sequencing of multiple compressors, and whether a variable-speed unit is appropriate. A professional system assessment is useful when the plant operates near its limits or cycling is frequent.
Collect better cycle data
Use a clock, data logger, or compressor controller logs to capture multiple representative cycles. Define exactly what counts as “on”: motor energized, loaded, or delivering above a chosen flow threshold. Use the same definition for each reading. Note operating pressure and whether other compressors are running.
For energy decisions, pair cycle data with kW and flow readings. A machine that runs less often can still consume substantial energy during loaded periods, and a machine that stays on may spend significant time unloaded. Do not infer annual kWh from duty percentage without a measured or rated power profile.
Separate repeated short cycles from longer steady runs. Frequent starts can indicate a small usable pressure band, a receiver that is too small for the event, a leaking system, a control setting issue, or a demand pattern that crosses the control threshold repeatedly. Log the start and stop pressures alongside cycle duration. Do not change the pressure switch or controller outside the manufacturer’s approved procedure.
Troubleshoot a high or unstable run fraction
A rising run fraction across comparable shifts can reflect a real increase in demand, a new leak, longer operating hours, or reduced compressor output. Compare the same production state, pressure, and temperature before concluding that the compressor is deteriorating. Check whether maintenance, a changed filter, a new tool, or a different control sequence coincides with the change.
A low run fraction can also be misleading if a second compressor supplies much of the load or the machine spends time unloaded. Review controller trends and measure total system flow where possible. Use the duty-cycle result as a clue to investigate, not as a pass/fail score for the whole compressed-air system.
Use the duty-cycle calculator
- Enter observed on time and off time from the same cycle in consistent units.
- Read the percentage and compare it with the equipment manual’s duty-cycle limit for the actual operating conditions.
- Repeat during representative periods and investigate repeated short cycling or operation above the stated rating.
The calculator performs the time-ratio arithmetic. It does not identify control states, estimate compressor life, establish a safe operating limit, or substitute for manufacturer guidance.
Frequently asked questions
Can I run a compressor above its duty-cycle rating? Follow the manufacturer’s instructions; a calculator cannot approve it. Excessive operation may cause overheating or accelerated wear depending on design.
Does duty cycle mean capacity? No. Capacity is delivered airflow at a specified condition; duty cycle is a time share.
Why does my compressor cycle often? Possible causes include demand variation, receiver volume, pressure controls, leaks, and system restrictions. Diagnose using pressure and flow data.
Is 100% always better? Not universally. The appropriate duty pattern depends on machine type, operating point, cooling, and manufacturer design.
Further reading
CAGI’s compressed-air equipment and system guidance: https://www.cagi.org/working-with-compressed-air/ and U.S. DOE resources: https://www.energy.gov/eere/amo/compressed-air-systems. Related Toolnoza calculators are linked below.