Does the round trip checkbox double the distance or the cost?

The distance, and it does so before anything else runs. A 300 km one-way entry becomes a 600 km total, which takes fuel needed from 24 L to 48 L and the trip cost from $36.00 to $72.00 at the default 8 L/100km and $1.50 per liter. Because it is a plain multiply by two, a return figure always differs from the one-way figure by exactly that factor.

Can I accidentally pair a distance in kilometres with an MPG figure?

No, and the form is built that way on purpose. The unit system selector switches distance, consumption and price as one set, and the calculation reads only the three fields belonging to the system you chose. The other three stay on screen but are never referenced, so a stale number left in them cannot reach the result.

Why does the fuel price line show two dollar signs?

The report puts a currency marker in front of the price and then appends the unit, which already contains one, so the metric line reads $1.50 $/L and the imperial line reads $3.50 $/gallon. It is a cosmetic duplication in the label rather than a doubled value, and the price used in the arithmetic is the single figure you typed.

Which gallon sits behind the MPG option?

The US gallon, defined as exactly 3.785411784 litres. A UK gallon is 4.54609 litres, roughly a fifth larger, so a UK MPG figure entered as-is will understate how much fuel the trip needs. If you think in UK gallons, switch to Metric and enter your consumption in litres per 100 km instead.

Where does the second consumption figure in brackets come from?

The report converts whatever you entered into the other convention and prints it beside your value, so 8 L/100km is echoed as roughly 29.4 MPG and 30 MPG is echoed as roughly 7.84 L/100km. Both directions divide the same constant by your input, which is why the relationship is reciprocal and never a fixed multiplier.

What happens if I clear the distance box rather than typing a number?

An emptied numeric box reads as zero, and zero is below the field's floor, so the calculation runs on 0.1 km instead of the 300 that was sitting there. Consumption floors at 0.1 in metric and at 1 in imperial, and price floors at 0.01 in both. A total that comes out absurdly small is almost always a cleared box rather than a bug.

Fuel Cost Calculator for a Trip (L/100km or MPG)

Work out the fuel cost of a trip from distance, consumption in L/100km or MPG, and fuel price, with an optional round trip and a per-person split.

🔒 Private by design: everything is generated locally in your browser and never uploaded to any server.

One selector switches distance, consumption and price together

The bug that ruins most fuel calculators is a mixed unit: a distance in kilometres paired with a consumption figure in miles per gallon, producing a number that looks plausible and is wrong by roughly the ratio between a mile and a kilometre. This page removes that possibility structurally rather than warning about it. There is one Unit system control, it switches all three quantities at once, and the calculation only ever reads the three fields belonging to the selected system.

That is why distance, consumption and price each appear twice on the form when only one copy of each is read, and why every one of those labels ends with the system it applies to. Whatever is sitting in the imperial boxes while Metric is selected is never referenced. It cannot leak.

Choose Metric (km, L/100km, $/L) or Imperial (miles, MPG, $/gallon) under Unit system.
In metric, fill One-way distance, km (used only in Metric), Fuel consumption, L/100km (used only in Metric) and Fuel price, $ per liter (used only in Metric). In imperial, fill the three fields whose labels end with “used only in Imperial” instead.
Tick Round trip (doubles the distance) if you are coming back.
Set Split cost between this many people if more than one of you is paying.
Press the action button, which carries the page’s own name, Fuel Cost Calculator.

A 300 km drive at 8 L/100km, worked through to the cent

The metric defaults are a real worked example rather than placeholder numbers. Three hundred kilometres at 8 litres per 100 km needs 24 litres, and 24 litres at $1.50 each is a trip cost of $36.00. Leave the split at one person and the cost per person line repeats $36.00.

The imperial defaults tell the same story in the other convention. Two hundred miles at 30 MPG needs 200 divided by 30, which the report prints as 6.667 gallons because fuel needed is given to three decimals while everything else is given to two. At $3.50 a gallon that is $23.33.

Money is formatted by a helper imported straight from the Electricity Cost Calculator, which shows two decimals normally and switches to four for figures under ten cents. That is why a fuel price of one cent prints as $0.0100 rather than as an uninformative $0.01.

235.214583 is derived from two exact definitions

Litres per 100 km and miles per gallon do not convert with a multiplier, because they run in opposite directions. One counts fuel over a fixed distance, so lower is better; the other counts distance over a fixed volume, so higher is better. They are reciprocals, and the conversion is a constant divided by your value in both directions.

That constant is not typed in as a remembered 235.2. It is computed from the exact US gallon (3.785411784 litres) and the exact international mile (1.609344 km), which gives 235.2145833333333. Run it in either direction and the numbers land where a spec sheet says they should: 35 MPG becomes 6.72 L/100km, and 20 MPG becomes 11.76 L/100km. The bracketed figure the report prints next to your consumption is exactly this conversion, so 8 L/100km is echoed as roughly 29.4 MPG.

If you need plain unit conversions instead, distance or speed rather than fuel economy, the Unit Converter covers those categories separately.

Two dollar signs on the price line, and other rough edges

Worth flagging honestly, because it looks like a defect the first time you see it: the fuel price line in the report reads Fuel price: $1.50 $/L. The currency marker is added by the formatter and the unit label already contains one, so they double up. Nothing is doubled in the arithmetic. The imperial version reads $3.50 $/gallon for the same reason.

The currency itself is generic. There is no currency selector anywhere; the dollar sign is a placeholder, and if you enter a price in euros, pounds or pesos then every figure in the report is in that currency too. The rest of the guardrails are quiet clamps. Distance floors at 0.1, consumption at 0.1 in metric and 1 in imperial, price at 0.01, and the number of people is rounded to a whole number and held between 1 and 50, so there is no route to a division by zero.

Round trip doubles first, the split divides last

Order matters here and it is worth stating explicitly. The round trip toggle acts on the one-way distance before fuel is calculated, so it scales fuel needed and total cost together. The split acts on the finished total, so it changes only the per-person line.

Combine them on the metric defaults and you get 600 km, 48 litres, a total of $72.00 and, across four people, $18.00 each. Untick the round trip and the same four-way split gives $9.00 each on a $36.00 total. Splitting a shared drive this way is the same arithmetic the Tip Calculator applies to a restaurant bill, and the rest of the everyday money tools sit together on the calculators hub.

What a fuel estimate cannot know about your drive

A rated consumption figure is a laboratory number, and a real trip is not a laboratory. Cold starts, roof boxes, headwinds, a full car, mountain passes and city traffic all push actual consumption above the sticker figure. Nothing here models any of that, because there is no field on the form that could carry the information.

Treat the output as a floor rather than a forecast. The most useful habit is to enter the consumption your own trip computer reports on the kind of road you are about to drive, not the manufacturer’s combined figure, and to run the calculation again with a slightly worse number to see how sensitive the total is. Fuel prices are not modelled either: the figure you type is applied to the whole trip, so a long drive across a price border needs to be split into two runs.

Fuel Cost Calculator