Every litre you buy should end up either in a customer's tank or still in yours. When the numbers don't agree, the gap is your stock loss (or gain). Working it out takes three figures you already have: what was in the tank, what came in, and what went out through the meters.
Step 1: Work out your book stock
Book stock is how much fuel should be in the tank according to your records. It doesn't involve a dip stick at all, just arithmetic:
- Opening dip stock is the dip reading from the end of the previous day (or the start of this one).
- Deliveries received are the litres actually discharged into that tank, ideally checked with a dip before and after the tanker unloads.
- Litres sold come from your nozzle meter readings: closing meter minus opening meter, minus any test litres you poured back into the tank.
Step 2: Take the closing dip
At the same time every day, dip the tank and convert the depth into litres using the tank's calibration chart. This is your actual stock. If you're unsure about the dipping routine itself, see our guide on taking accurate tank dip readings.
Step 3: Compare the two
Gain / loss % = Gain or loss ÷ Book stock × 100
A negative number means fuel is missing. A positive number means you have more fuel than your records say, which is just as worth investigating: it often points to an unrecorded delivery, a wrong meter reading or a dip error that will "come back" as a loss later.
A worked example
Here's one day for a diesel tank:
| Item | Litres |
|---|---|
| Opening dip stock | 18,500 |
| + Delivery received | 10,000 |
| − Litres sold (from nozzle meters, after test litres) | 12,350 |
| = Book stock | 16,150 |
| Closing dip stock | 15,980 |
| Gain / loss | −170 (−1.05%) |
At a purchase cost of 260 per litre, those 170 litres are worth 44,200 in your currency. In a single day. If the same thing happened every day, that would be over 1.3 million a month.
Tip: value losses at what you paid for the fuel, not the selling price. That's the money that has actually left your business.
How much loss is normal?
Some variation is unavoidable. Fuel expands and contracts with temperature, a little evaporates, and no dip stick or meter is perfect to the litre. That's why a single day's number on its own can mislead you.
Many operators treat a small variance of a fraction of a percent of sales as normal, but the right tolerance depends on your product, climate, equipment and local rules. The better approach is to:
- Track it every day, for every tank, so you learn what "normal" looks like at your site.
- Watch the running total, not just single days. Small daily gains and losses should roughly cancel out over a week or a month. A total that keeps drifting in one direction is a real problem.
- Set an alert threshold. FuelBotics, for example, flags a dip when the gap is more than 1% of book stock and at least 50 litres, so tiny tanks don't trigger false alarms and big tanks don't hide big losses.
When the numbers don't add up
If a tank shows a loss beyond your tolerance, work through these in order. Most "losses" turn out to be recording mistakes, which are the easiest to fix:
- Recheck the meter readings for every nozzle on that tank. A single digit typed wrong can create hundreds of litres.
- Confirm test litres were recorded and actually poured back into the same tank.
- Check the delivery: was the full invoiced quantity discharged, and into the right tank?
- Re-dip if the reading was taken during or soon after a delivery, or while the pumps were running.
- Look at the trend. A one-off gap that reverses the next day is usually a dip or timing error. A steady daily loss points to a leak, a meter out of calibration or fuel leaving without being recorded. Our guide to the 7 common causes of fuel loss covers each one.
Doing it without a spreadsheet
The calculation is simple, but doing it by hand for every tank, every day, across several stations is where it falls apart. Readings get copied wrong, dips are skipped, and losses are only spotted at month end.
You can start with our free daily sales and dip register, which does the book stock maths for you in Excel. When you're ready to stop copying numbers between sheets, FuelBotics works out book stock from your readings and deliveries automatically, compares it with every dip, and sends an alert the moment a gap crosses your threshold.
