What the calculator is actually working out
The leak cost calculator turns a leak you can hear or feel, a small hiss at a fitting, a hole in a hose, a leaking quick-connect, into a figure in pounds a year. It does this by converting the leak into a flow in litres a minute, then into the electrical power a compressor spends making that air, then into a cost over however many hours the system runs. Each of those steps rests on a number you supply: how big the leak is, what pressure the system runs at, and how many hours a year the compressor is on load. Change any one of those and the answer changes with it, which is why the figure is only as good as the estimate that goes into it.
None of this is abstract for a workshop with a compressor running most of the working day. Air that leaks out through a fitting is air the compressor has already compressed and paid for in electricity, and it leaks whether or not a tool is being used at the time. A leak found on a Monday morning keeps costing money every hour after that until it is fixed, including the hours nobody is in the building if the compressor is left running or the receiver is left pressurised overnight.
Why hole size drives the cost
Flow through a small hole rises steeply with pressure and with the diameter of the hole. A hole twice the diameter of another does not leak twice the air. It leaks roughly four times as much, because the flow depends on the cross-sectional area of the hole, which grows with the square of the diameter. That is the reason a handful of pinhole leaks on a long pipe run can matter less than a single split hose or a worn quick-connect seal, and why counting leaks tells you less than sizing them.
Pressure works the same way in the other direction. A leak on a line held at 10 bar loses air faster than the same hole would at 6 bar, so a system run higher than it needs to be for the tools connected to it pays twice: once in the extra power needed to compress air to that pressure at all, and again in every leak on the line losing air faster than it would at a lower setting. Running pressure and leak cost are connected for that reason, and the running-cost guide covers the pressure side of that arithmetic in more detail (Running Costs: Electricity, Pressure Settings and Where the Money Goes).
The chain from a hole to a pound figure
The calculation runs in a fixed order, and each step depends on the one before it.
- Flow: the hole size and system pressure together give an estimated leak rate in litres a minute. This is the volume of air escaping.
- Power: replacing that lost air means the compressor has to produce it, and producing a given flow at a given pressure takes a known amount of electrical power. This is where free air delivery matters: the flow a compressor's motor is sized to replace has to be stated at atmospheric pressure, the same basis a leak rate is calculated on, or the two numbers cannot be compared honestly.
- Hours: a leak on a system that runs 2,000 hours a year costs less than the same leak on a system that runs 4,000 hours a year, even though the leak rate itself has not changed. A compressor left running outside production hours, whether to keep a receiver pressurised or by habit, adds those hours to the total.
- Price: the power figure is converted to a cost using the current non-domestic electricity price (the unit rate for business electricity, published quarterly by the Department for Energy Security and Net Zero). The calculator states the price and the quarter it applies to, because a rate from an earlier quarter understates or overstates the true cost depending on which way prices have moved since.
A worked example
Say a fabrication shop has three leaks along a compressed air ring main, each roughly the size of a pinhole, and the system runs at 7 bar for most of the working week. Put the hole size and pressure into the calculator and it returns an estimated flow in litres a minute for each leak. Add the hours the compressor actually runs under load, and it converts that flow into a power draw and multiplies by the current electricity price to give an annual figure. The result is illustrative only in the sense that the leak sizes here are assumed: put in the shop's actual hole sizes and running hours and the same steps give a figure specific to that system.
Where the estimate can go wrong
A hole size entered by eye is the weakest link in the whole calculation, because a leak that looks small by ear can be considerably larger once air is actually flowing through it, and the reverse is also true. An ultrasonic leak detector gives an estimate, but even a careful measurement of an irregular tear in a hose or a worn seal is an approximation.
Running hours are the second common source of error. It is easy to enter the hours the workshop is staffed and forget the hours the compressor keeps running unattended, whether because it cycles on standby overnight or because nobody turns it off at the end of a shift. Either mistake pulls the annual figure away from what the system is actually costing, in one direction or the other.
The electricity price itself moves. A figure calculated against an old quarter's rate will be wrong in a predictable direction once prices have changed, which is why the calculator states the quarter its price applies to.
The calculator can return a wrong answer if any of these inputs is wrong, and it has no way to check a hole size or a running-hours estimate against the real system. Treat the result as a working figure to act on.
Common mistakes worth naming
- Costing only the leaks that make noise. A slow leak at a push-fit connector or a cracked plastic fitting can lose more air over a year than a louder leak at a hose joint, simply by running continuously and unnoticed.
- Ignoring standby running hours. A compressor cycling to hold pressure overnight or over a weekend is still spending power on every leak in the system during that time, even with no tools connected.
- Comparing flow figures on different bases. A leak rate and a compressor's rated output only compare fairly when both are stated as free air delivery at the same reference conditions; mixing a leak estimate with a displacement figure overstates or understates the leak's share of the compressor's output.
- Treating the annual figure as fixed. A leak that is not repaired tends to get worse, as vibration and wear open the hole further, so a figure calculated today understates next year's cost if nothing is done.
Finding and pricing leaks in practice
Fittings, quick-connects, hose ends and old thread seals are where most leaks start, and a walk around the system with the compressor running and the workshop quiet is enough to hear most of them. Soapy water brushed over a suspect joint shows a leak as bubbles where sound alone is inconclusive. Once a leak is found, sizing it as closely as possible, even a rough estimate against a known reference, gives a more useful figure than assuming a standard size for every leak in the building.
Once a leak is costed, the decision of whether to repair it immediately or schedule it with other maintenance is a judgement about the shop's own downtime and spares. What the figure does is put a number against that judgement, so a leak that would cost more to leave running for a year than to fix outright is easy to justify acting on straight away.
The receiver and pipework the leak sits on matter too. A system with a receiver sized correctly for the demand on it runs its compressor in longer, steadier cycles, which makes it easier to notice a leak by its effect on cycling frequency (Sizing the Air Receiver, and the 250 Bar-Litre Line). Sizing the system correctly in the first place is covered in the demand sizing guide (How Much Air Your Tools Need: Sizing in Litres per Minute).
Where to go next
The cost of leaks page sets out the same arithmetic with the reasoning behind each step in full, including how the electricity price feeds into the final figure (The Cost of Leaks, in Pounds). For the terms used throughout this page, free air delivery, displacement, bar and the rest, the glossary gives a short working definition of each (Compressed Air Glossary). Where the pressure a system runs at is itself part of the leak cost, the running-cost guide covers what changing that setting does to the electricity bill (Running Costs: Electricity, Pressure Settings and Where the Money Goes).