Fuel Tank Fill Cost Calculator

How Tank Math Compares with Per-Gallon Math

Most drivers track fuel by what the pump shows rather than by cost per mile. The cost of one full tank maps onto how refuelling actually happens, a sum leaving the account every week or two, which makes it easier to hold in mind than an annual total. Both views are useful, and this is the one people already have a feel for.

Projecting Beyond a Single Tank

One tank is a number people know. The useful move is multiplying it out: the same fill repeated three times a month becomes a monthly figure, then an annual one, then a five-year one, and each step changes what the number is good for. A per-tank figure says what leaves the account this week. A five-year figure is the scale at which a more efficient vehicle or a shorter commute starts to pay for itself, which is a different question entirely.

Common Things People Overlook

Two things move the real number away from this one. Fuel prices are volatile in a way most household costs are not, so a calculation at today's price understates the range a year's spending can occupy. And fuel is the classic variable cost, rising and falling with use rather than sitting fixed like insurance or depreciation, which is what makes it responsive to changes in driving when most motoring costs are not.

A worked example

With the defaults, a 14-gallon tank at 3.50 a gallon and three fills a month: 49.00 per tank, 147.00 a month, 1,764.00 a year, and 8,820.00 over five years, across 36 fills.

What moves the number most

Only two of the three inputs touch the headline. Cost per tank is capacity times price and nothing else, so the same 49.00 comes back whether the tank is filled once a month or ten times. Fills Per Month drives everything beneath it instead: at three fills the year costs 1,764.00, at four it costs 2,352.00. Tank Capacity and Fuel Price are interchangeable in effect, since a 10 per cent rise in either moves every figure by the same 10 per cent.

The formula behind this

Capacity multiplied by price gives the cost per fill. That figure multiplied by fills per month gives the monthly total, then by 12 and by 60 for the annual and five-year projections. The five-year row assumes today's price holds for five years, which it will not, so read it as the cost of the current pattern at the current price rather than as a forecast.

What the figure is and isn't

A five-year total is a large number by construction, and largeness on its own settles nothing. Fuel bought is transport consumed, and whether that trade is a good one depends on what the driving is for, which sits outside anything the calculator can see. What the projection does is put a recurring cost on the same scale as the one-off decisions it competes with, such as the price of a different car. Transport is a large enough slice of household spending, on Eurostat's figures, to put the two on the same footing.