The equation, and why each term is where it is
Book stock is opening stock, plus everything delivered into the tank, minus everything dispensed out of it. The dip is an independent physical measurement of the same quantity. The difference between them is the variance.
The comparison only works because the two sides are measured by different means. Book stock comes from meter readings and delivery notes; the dip comes from a stick and a calibration chart. The moment one is adjusted to agree with the other, the check stops being a check.
Take the percentage against throughput
A 40-litre gap on a tank that moved 20,000 litres is ordinary. The same 40 litres on a tank that moved 400 is not. Dividing by closing stock instead of by litres dispensed ranks those two the same way round only by accident, which is why this calculator uses throughput.
Four causes, one number
A variance does not tell you what happened. Evaporation, calibration drift, a metering error and a leak all present as the same figure, and three of the four are ordinary business while the fourth is a fire risk and an environmental liability.
What separates them is the shape of the variance over time, which is why the daily figure matters far more than any single day's result. Evaporation is small, steady and worse in summer. Drift is consistent on one nozzle and absent on its neighbours. A metering error is a step change starting on a date. A leak grows.
A month collapses thirty days of that signal into one number that any of the four could have produced. Reconciling daily costs the same total effort and produces a series instead.
Getting the dip right
Most surprising variances are measurement problems rather than fuel problems. A few habits remove almost all of them.
- Use the calibration chart supplied with that specific tank. A generic chart for a nominally identical tank manufactures a variance that is not there.
- Dip at the same time each day, ideally before the first delivery and when nothing is dispensing.
- Let the tank settle after a delivery before dipping. Fuel that is still moving reads high.
- Dip the same point on the tank each time. Tanks are rarely perfectly level.
- Record water separately. Water in the bottom of the tank reads as product on a plain dip and hides a real fuel loss.
Frequently asked questions
What is an acceptable stock variance for a petrol pump?
There is no single honest figure — it depends on throughput, ambient temperature, tank age and how the dip is taken. What matters is that your outlet knows its own normal band and can see the week it leaves that band. A pump that only ever sees a monthly number has no band to compare against.
Why is my variance positive?
A gain usually means a delivery was logged late or short-recorded, a dip was taken before the tank settled, or dispensed litres are understated because a nozzle reading was missed. Fuel does not appear from nowhere, so a persistent gain is a recording problem, not a windfall.
Does temperature affect the variance?
Yes. Fuel expands as it warms, so a tank dipped cold in the morning and warm in the afternoon holds the same mass at different volumes. It is one reason a variance measured at a consistent time of day is far more comparable than one taken whenever somebody remembers.
How much variance justifies a tank tightness test?
Not a threshold — a trend. A variance that is negative and growing week on week warrants a test regardless of whether any single day looked alarming. A one-off large figure is much more often a delivery or dip error, which is why you check the tanker invoice first.