Fuel Cost for a Journey
Calculate the total fuel cost for a journey from distance, MPG, and fuel price per litre.
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What this tool does
This calculator computes the total fuel cost for a journey using distance, fuel economy (UK MPG), and the price per litre. It converts UK miles per gallon to litres per mile using the UK imperial gallon constant (4.54609 litres), multiplies by distance to find litres needed, then multiplies by the fuel price. The tool supports both miles and kilometres as distance units.
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Formula
How Fuel Cost for a Journey works
This tool estimates the total cost to fuel a single journey by multiplying the distance travelled by the amount of fuel consumed per unit distance, then multiplying that fuel volume by the price per litre. The calculator accepts distance in miles or kilometres, fuel economy in UK MPG, and outputs cost in pounds sterling. It converts UK MPG into a litres-per-mile rate, calculates total litres required, and applies the fuel price to produce a monetary result.
The formula
The calculation proceeds in three steps. First, if distance is entered in kilometres, it is converted to miles (1 km = 0.621371 miles). Second, UK MPG is inverted and scaled by the UK imperial gallon volume: litres per mile = 4.54609 ÷ MPG. Third, fuel cost is computed as Cost = Distance (miles) × (4.54609 ÷ MPG) × Price per litre. The constant 4.54609 represents the volume of one UK imperial gallon in litres, which differs from the US gallon (3.78541 litres).
Where this method is most accurate
The formula assumes the stated MPG figure is correct and representative of the journey conditions—motorway, city, mixed driving, load, and weather all affect real-world consumption. It treats fuel economy as constant across the entire distance, which is a simplification; actual consumption varies with speed, terrain, and driving style. The calculator is most reliable for planning purposes when the input MPG closely matches expected driving conditions.
What this tool does not do
This calculator does not measure actual fuel consumption, adjust for traffic or weather, include toll or parking costs, or certify that any vehicle will achieve a specific MPG. It does not convert between US MPG and UK MPG (US MPG values are roughly 20% higher for the same efficiency due to the smaller gallon). The tool performs arithmetic only; it does not verify manufacturer-claimed economy figures or provide driving advice.
Disclaimer
This tool is for educational and planning purposes only. It is not financial advice, vehicle advice, or a guarantee of actual fuel costs. Real-world fuel consumption varies with driving behaviour, vehicle condition, load, weather, and road type. MotorMath publishes the formula and performs the arithmetic; users supply all inputs and remain responsible for verifying their own figures and decisions.
Questions
- Why does the calculator use 4.54609 litres per gallon?
- The constant 4.54609 is the volume of one UK imperial gallon in litres, defined by UK weights and measures standards. This differs from the US gallon (3.78541 litres), so UK MPG and US MPG are not directly comparable.
- Can I use US MPG with this calculator?
- No. The tool expects UK (imperial) MPG. US MPG figures are approximately 20% higher for the same efficiency because the US gallon is smaller. Using a US MPG value will underestimate fuel cost.
- Does this tool account for different driving conditions?
- No. The calculator treats MPG as constant across the journey. Real fuel economy varies with speed, traffic, terrain, load, and weather. The result is an estimate based on the MPG figure supplied.
- What if my car's display shows litres per 100 km instead of MPG?
- You will need to convert litres per 100 km to UK MPG before using this tool. The conversion is MPG (UK) ≈ 282.481 ÷ (L/100km). Many online converters perform this calculation.
- Does the calculator include other journey costs like tolls or parking?
- No. The tool calculates fuel cost only. It does not include tolls, congestion charges, parking fees, or vehicle wear. Those costs vary by route and jurisdiction and are not part of the formula.
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Sources & Methodology
The tool multiplies distance in miles by litres per mile (derived from UK MPG using the imperial gallon constant 4.54609 litres) and then multiplies by the fuel price per litre. If distance is entered in kilometres, it is first converted to miles (× 0.621371). The UK imperial gallon is defined by UK weights and measures legislation.
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