Two constraints, pulling opposite ways
Ordering fuel is usually treated as one question — when am I going to run out? — and it is really two, which is why it goes wrong in both directions.
Order late and the tank has plenty of room, so a full load fits. It also means the tanker is your only source of fuel on a day you may already have run down to nothing, and a delay you do not control becomes a forecourt with bags over the nozzles.
Order early and you are safe on supply. But the tank is still fairly full when the tanker arrives, and a compartment that will not discharge is a compartment that goes back on the lorry — sometimes with a charge attached, always with the paperwork of a part-delivery.
Both constraints have to be checked against the same day, and that day is the day the tanker arrives rather than today.
Check the ullage on arrival, not now
This is the mistake worth avoiding. A tank with 5,000 litres of room today has 9,000 litres of room in two days if it sells 2,000 a day, and a 12,000-litre load that "does not fit" is turned away for no reason. Comparing a load against today's ullage makes an outlet order smaller loads more often than it needs to, and every extra delivery is another decantation to get right.
Why the minimum is not zero
Every tank has fuel that cannot be sold: the layer below the suction line, which the pump simply cannot reach. That is dead stock, and it is inventory you have paid for and will not sell while the tank is in service.
Planning down to zero therefore plans to run dry before the gauge says empty. Whatever your dead stock is, it belongs in the minimum — and most outlets add a buffer on top for the day the forecast is wrong, because the cost of being a few hundred litres conservative is nothing and the cost of being short is a closed pump.
Days of cover here is counted against usable stock, not against the whole tank, for exactly that reason.
The reorder level
The level to order at is not the minimum. It is the minimum plus everything you will sell while the order is in transit — otherwise the tanker arrives some days after you needed it.
reorder level = minimum + (daily sales x lead time)
Lead time is the honest one to get right. It is not what the supply agreement says; it is what actually happens at your outlet, including the days an order goes in late because nobody looked at the dip. If deliveries realistically take three days rather than two, use three.
Using it daily
The dip you already take is the only input that changes day to day. Everything else — capacity, minimum, lead time, your usual load — is set once.
The stock register template has the dip column this reads from, and the dip-to-litres calculator converts a stick reading if your tank chart is not to hand. Remember a horizontal tank is not linear: at a quarter of its diameter it holds under a fifth of its contents, which matters most near the bottom, which is exactly when you are deciding whether to order.
Frequently asked questions
What lead time should I use?
What actually happens at your outlet rather than what the agreement says — including the days an order goes in late because nobody checked the dip. If real deliveries take three days, use three. A lead time that is optimistic by one day moves the reorder level down by a full day of sales, which is the entire safety margin at most outlets.
How much should my minimum be?
At least your dead stock — the fuel below the suction line, which cannot be sold while the tank is in service — plus a buffer for a bad forecast. Your tank's documentation gives the dead stock figure. Planning to zero plans to run dry before the gauge says empty.
Why does it say the load will not fit when the tank looks half empty?
It should not, and if it does the numbers are worth checking — the fit test is against the space that will exist on arrival, which is larger than today's. If it still does not fit, either the load is bigger than the tank's remaining capacity even after the lead time, or the daily sales figure is too low.
Does this handle more than one tank?
One at a time, because the answer is per tank — each has its own capacity, its own dead stock and its own daily draw. Two tanks of the same grade still order separately unless they are manifolded, and if they are, treat the pair as one unit and accept that you have lost the ability to isolate a problem to one of them.