Calculate what you observed
An observed pump-on fraction is running time divided by the total observation time. If a hypothetical pump runs for 3 minutes and rests for 7 minutes, its observed fraction over that ten-minute cycle is 30%. If it runs for 7 minutes and rests for 3, the fraction is 70%.
The calculator describes that observation. It does not prescribe a new run/rest schedule. Keep records of both the longest uninterrupted run and the total time observed; reducing a full session to one percentage can hide an unusually long continuous run.
Read the manufacturer’s definition
A published duty rating needs context: the applicable period, model, operating conditions and any separate duration restrictions. California Air Tools, for example, describes a standard-workday 70/30 rating for many models and also publishes continuous-operation guidance and exceptions. Its instructions must not be applied automatically to another brand or to every model in its own range.
Two hypothetical schedules can each total 30 minutes on and 30 minutes off. One alternates short cycles; the other runs uninterrupted for the first half hour. They have the same arithmetic fraction but different longest run times. The percentage alone cannot establish that both meet a particular manual.
Why the match tool still asks for review
Dividing average air demand by published compressor delivery provides a simple air-balance estimate of required pump time. It leaves out the actual flow curve, control behavior, leakage and installation effects. Treat that estimate as a reason to inspect the duty instructions, not a measured pump-on percentage.
For a buying decision, record the proposed work session and intended pauses, then check them against the exact compressor’s manual. Where those limits are absent from the verified data, ToolSpecLab reports the gap. A larger tank does not change the motor-pump’s published operating permission.
Keep the manufacturer’s observation period and run-duration limits with its duty percentage. An average-flow result cannot replace them.