What the calculation measures
Equivalent free air lost = system ft³ × (start PSIG − end PSIG) ÷ atmospheric PSIAInterval-average loss CFM = free air lost ÷ elapsed minutesThe volume is the entire connected test boundary, including any known piping and secondary receivers. The formula assumes the receiver air stays at the same temperature. The 14.7 PSIA default is a sea-level approximation; it is not added to the gauge-pressure difference.
CAGI describes timing a pressure drop with production stopped and the compressor off. It recommends a small 10-PSIG interval because the leakage rate changes with pressure. This calculator reports the average over your chosen interval; it applies no fixed correction to claim a rate at another pressure.
Worked example: make the comparison under the same conditions
Twenty US gallons is about 2.674 cubic feet of receiver volume. A 10 PSI drop at 14.7 PSIA atmosphere corresponds to about 1.819 cubic feet of equivalent free air. Over five minutes, that is about 0.364 CFM.
If a later observation takes 20 minutes to cross the same 120-to-110 PSI band with the same volume and temperature conditions, the estimate becomes 0.091 CFM, a 75% reduction. Enter 20 in the optional repeat field to see the two results together. This is a like-for-like comparison, not evidence that a named repair will save a particular dollar amount.
What to record before drawing a conclusion
- Use your equipment manual’s normal shutdown and observation procedure. Do not alter switches or safety devices for a measurement.
- Let temperature stabilize before comparing gauge readings. A warm-to-cold change can look like leakage.
- Record the tank gauge, start/end pressures, elapsed time and what volume remains connected. Do not substitute a changing regulated outlet reading for the receiver reading.
- Note any intentional uses, drains or automatic equipment that could remove air. If the conditions are unclear, select the unconfirmed option and the leak interpretation is withheld.
- Compare another observation over the same pressure band. Small changes may be below the gauge’s useful resolution.
Follow the manufacturer’s depressurization instructions before servicing connections. Receiver corrosion, damage or suspected vessel leakage needs qualified assessment; this guide provides no vessel-repair procedure.
Keep recovery time and leakage separate
A pump-up measurement asks how quickly air is retained while the pump runs. This decay test asks how much equivalent air disappears while the pump is off. The fill-time calculator handles the first question.
An overnight drop across a wide pressure range does not establish one constant leak CFM. Nor does it translate directly into a utility bill: cycling, loaded power and unloaded power matter. The electricity calculator uses those observed operating inputs instead.
Questions before you decide
Is it normal for a compressor to lose pressure overnight?
Cooling can lower pressure, and air loss can contribute as well. Repeat under stable conditions using the manual’s procedure. This page does not label an arbitrary overnight PSI loss acceptable or diagnose its cause.
Why is the calculated rate different from a calculator using 1.25?
Some methods add a correction to estimate leakage at normal operating pressure after a larger pressure decay. This tool reports the uncorrected interval-average loss from the stated storage model. Those are different outputs.
Does zero pressure drop mean there is no leak?
It means the entered readings resolve no drop. Measurement resolution and test conditions can hide small losses, so a zero result does not certify an airtight system.