Every compressed air cost calculator does the same three things: it turns a leak into a flow rate, turns that flow rate into a compressor power draw, and turns that power draw into an annual dollar figure using an electricity rate and a run-time assumption. The dollar-and-rate part is mechanical once you have a flow rate. Where calculators actually diverge, sometimes by a factor of two or more on paper-identical inputs, is the first step: how they arrive at the flow rate in the first place.
Two different starting points
Broadly, compressed air cost calculators fall into one of two families. The first asks you to estimate or guess an orifice size (a hole diameter, picked by eye or by comparing the hiss to a reference description) and looks up a standard CFM figure for that size from a chart. The second starts from a measured or estimated flow rate that's already been derived from an actual reading at the leak itself, and just does the cost arithmetic on top of it. Both will hand you a dollar figure. Only one of them is telling you something about your actual leak rather than an assumption about its shape.
Inside an orifice-size calculator
The standard orifice-to-CFM tables (the ones behind most quick online calculators, including the leak-size slider on this site's own ROI calculator) are built around choked, sonic flow through a sharp-edged hole at a stated reference pressure, commonly 100 PSI. That reference pressure matters more than it looks like it should. Choked flow through a fixed orifice scales with absolute upstream pressure, not gauge pressure, so running the same size hole at a different system pressure changes the real flow by roughly the same proportion as the change in absolute pressure. A system running at 90 PSI rather than the 100 PSI a chart assumes is putting out on the order of a tenth less air through an identical hole, not the figure printed in the table.
The bigger source of error usually isn't the pressure mismatch, though. It's the hole itself. A chart assumes a clean, round, sharp-edged orifice with a known discharge coefficient. A real leak is a worn thread, a cracked O-ring, or a corroded seam, none of which behave like a drilled hole, and the actual discharge coefficient for an irregular opening can differ enough from the chart's assumption to move the estimate well outside what the stated hole size would suggest. Estimating "about a 3mm hole" by ear or by eye is a genuine skill, and experienced techs get reasonably close, but it's still an estimate of a shape, feeding a formula built for a different shape.
A hole-size calculator is only ever as accurate as the guess about the hole. The arithmetic after that is exact.
Inside a measured-flow calculator
The other family skips the hole-size guess entirely. An acoustic imager like the Fluke ii900 measures the sound level a leak actually produces and the distance to the source, then converts that reading to an estimated flow rate using a regression model built from measuring a large number of real leaks at known flow rates. There's no fixed formula that turns sound directly into flow (unlike the orifice chart, which is a fixed lookup), and the result is still an engineering estimate rather than a lab-grade measurement. But it's an estimate of the sound the leak is actually making, not an estimate of what shape the hole might be, which removes the biggest source of error in the other approach. The full mechanics of how that reading gets made are covered in how the technology works.
The electricity side matters just as much
Getting the flow rate right only solves half the problem. The same measured or estimated CFM figure can still produce wildly different annual costs depending on what electricity rate and running-hours assumption a calculator applies, and this is where a lot of otherwise careful tools quietly go wrong: using a US electricity price, using a bare usage rate with no demand-charge allocation, or applying production hours instead of the hours the air system actually stays pressurised. Those variables get their own full treatment in what leaks actually cost in Australia and how to work out your real cost per CFM, so they're not repeated here. The short version: a calculator that gets the flow rate exactly right can still hand you a number that's out by a factor of three if the rate and hours behind it aren't yours.
Five questions worth asking before you trust the number
- Is the flow rate measured or guessed? A figure derived from an orifice size someone estimated by ear carries more uncertainty than one derived from a sound-level reading at the leak.
- What pressure did it assume? If the calculator doesn't ask what your system pressure is, it's assuming one, usually 100 PSI, and pricing every leak as if your site runs at that pressure.
- Does the electricity rate include demand charges? A bare usage rate understates a typical Australian commercial site's effective cost per kilowatt-hour.
- Whose hours is it using? Production hours and the hours your compressed air system stays pressurised are often not the same number.
- Is it pricing one leak or your actual leak population? A single-leak figure is a building block. Most sites are carrying dozens at once, and the total is usually what gets a repair budget approved.
Where this site's own calculator sits
Worth being straightforward about which family the calculator on this site's home page belongs to: it's the orifice-size kind. Until an acoustic survey has actually measured your leaks, there's no measured flow rate to work from, so the calculator asks for a leak size, a leak count, an electricity rate and an operating schedule, and prices them using Australian rate defaults and the same 4.5 CFM/kW mid-point efficiency figure used across this site's articles. It's directional, useful for seeing how the numbers move, and says so.
The flow figures in an actual LeakHawk audit report are the other kind. Every leak logged during a site walk-through is priced from its measured sound level and distance to source, not an assumed hole size, which is the difference between a planning estimate and the number a repair-vs-leave decision should actually be made on.
Run the assumptions against your own site in the ROI calculator, or see what a report built on measured flow rates instead of guessed ones actually looks like in a sample report.