How to Work Out Miles Per Litre

Miles per litre is a fuel economy metric that expresses how far a vehicle travels on a single litre of fuel. A car achieving 12 miles per litre travels 12 miles before consuming one litre of petrol or diesel. The higher the number, the more fuel-efficient the vehicle. It is the direct metric equivalent of miles per gallon (MPG), scaled to litres rather than gallons — which makes it more useful for UK drivers since fuel has been sold exclusively in litres since 1983.

Why miles per litre vs MPG

MPG is the traditional UK standard, but it creates a practical disconnect: your fuel receipt shows litres, yet your car's economy display shows miles per gallon. Converting MPG to a fuel cost requires knowing that one Imperial gallon equals 4.546 litres — a constant most drivers do not carry in their head. Miles per litre eliminates that step entirely. If you achieve 12 miles per litre and fuel costs £1.50/litre, your cost per mile is simply £1.50 ÷ 12 = 12.5p. No gallon conversion needed.

Miles per litre vs L/100km

European vehicles and most comparison databases quote L/100km (litres per 100 kilometres). This is also a litre-based metric, but it works in reverse: lower is better. A car using 5.0 L/100km is more efficient than one using 8.0 L/100km. Miles per litre is higher-is-better, making it more intuitive for UK drivers accustomed to MPG conventions. Both are useful; the Fuel Consumption Converter on Quasar Tools converts between them and five other units simultaneously.

• Miles per litre (mi/L): Distance per litre of fuel — higher is better. Most practical for UK drivers.
• MPG (UK): Miles per Imperial gallon — higher is better. Traditional UK standard, still on most dashboards.
• MPG (US): Miles per US gallon — higher is better. US and Canada standard; about 20% lower than UK MPG.
• L/100km: Litres per 100 kilometres — lower is better. Standard in Europe, Australia, and manufacturer WLTP specs.
• km/L: Kilometres per litre — higher is better. Common in Asia and parts of Africa.

The miles per litre formula is: Miles per litre = Miles driven ÷ Litres used. The challenge is getting an accurate measurement of both inputs. Manufacturer figures are rarely representative of real-world driving — the only way to know your actual fuel economy is to measure it yourself across a complete fill-to-fill cycle.

Always measure fill-to-fill: brimming the tank at both ends gives you the only truly accurate fuel consumption figure for your real-world driving.

Converting between miles per litre and other fuel economy units requires knowing two constants: the Imperial gallon volume (4.546 litres) and the miles-to-kilometres ratio (1.609 km per mile). Every conversion follows from these two numbers.

Converting MPG back to miles per litre

To reverse the conversion — going from an MPG figure back to miles per litre — divide MPG (UK) by 4.546, or divide MPG (US) by 3.785. A manufacturer quoting 50 MPG (UK) is equivalent to 50 ÷ 4.546 = 11.0 mi/L. These conversions are exact; no rounding approximation is involved.

Converting L/100km to miles per litre

Divide 62.14 by the L/100km figure to get miles per litre. A vehicle rated at 5.5 L/100km = 62.14 ÷ 5.5 = 11.3 mi/L. The constant 62.14 comes from 100 km expressed in miles (100 ÷ 1.609 = 62.14). For all these conversions in one step, the Fuel Consumption Converter produces a complete table from any single input.

Once you know your miles per litre, calculating fuel cost for any journey is straightforward. The Fuel Cost Calculator on Quasar Tools accepts your distance, your fuel economy in any unit (including miles per litre directly), and the current fuel price — then returns total cost, litres needed, cost per kilometre or mile, and a multi-trip comparison table.

Manual fuel cost calculation

The manual formula is: Fuel cost = (Distance ÷ Miles per litre) × Fuel price per litre. For a 200-mile trip at 12 mi/L with fuel at £1.55/L: 200 ÷ 12 = 16.67 litres needed. 16.67 × £1.55 = £25.83 total fuel cost. For a weekly commute of 40 miles per day (200 miles/week), the same formula gives you weekly and monthly fuel spend with a single calculation.

Comparing two vehicles on fuel cost

If you are deciding between two cars — one achieving 11 mi/L and another at 14 mi/L — the fuel saving per mile is (1/11 − 1/14) × fuel price. At £1.55/L: (0.0909 − 0.0714) × £1.55 = £0.030 saved per mile. Over 12,000 miles per year that is £360 annual saving. The Fuel Cost Calculator runs this comparison directly — enter the same journey distance for each vehicle and compare the output side by side.

Tracking fuel economy over time

Logging miles per litre across multiple fill-ups reveals trends that a single measurement misses: seasonal variation (winter economy is typically 5 to 10 percent worse than summer), motorway vs city split, and the gradual decline that signals a maintenance issue. A sudden drop of 15 percent or more is worth investigating — common causes include under-inflated tyres, a failing oxygen sensor, or a clogged air filter.

What constitutes good miles per litre depends entirely on vehicle type. Comparing a supermini against an SUV on the same mi/L scale is misleading — the relevant question is how a vehicle performs relative to similar vehicles in its segment. The benchmarks below use real-world UK driving figures, not manufacturer WLTP lab values.

New WLTP figures vs real world

The WLTP (Worldwide Harmonised Light Vehicles Test Procedure) replaced the older NEDC test in 2017. WLTP figures are closer to real-world results — typically within 10 to 20 percent — whereas NEDC figures were often 20 to 30 percent optimistic. When a manufacturer quotes "up to 56 MPG (UK)" under WLTP, you can realistically expect 47 to 52 MPG in mixed driving, which corresponds to roughly 10.3 to 11.4 mi/L. Always treat the WLTP figure as a ceiling, not a guarantee.

Miles per litre is not a fixed property of a vehicle — it varies significantly based on how and where you drive. Understanding what degrades fuel economy lets you make meaningful improvements without changing vehicles.

Speed and aerodynamic drag

Aerodynamic drag increases with the square of speed. Driving at 70 mph instead of 60 mph does not increase drag by 17 percent — it increases it by approximately 36 percent. Most petrol cars reach peak efficiency at 45 to 55 mph. Above 60 mph, fuel economy typically falls 10 to 15 percent per additional 10 mph. For commuters using a motorway, reducing from 80 to 65 mph is the single highest-impact change for fuel economy.

Urban stop-start vs motorway driving

City driving requires frequent acceleration from rest, which is the most fuel-intensive part of driving a petrol or diesel vehicle. Hybrid vehicles recoup energy during braking (regenerative braking), which dramatically closes the gap between city and motorway efficiency — making hybrids up to 40 percent more fuel-efficient in city conditions compared to equivalent petrol-only vehicles.

• Tyre pressure: Under-inflated by 10 PSI reduces mi/L by 1 to 3 percent — check monthly.
• Air conditioning: Increases fuel consumption by 5 to 15 percent, more at lower speeds.
• Vehicle load: Every 100 kg of extra weight reduces fuel economy by approximately 1 to 2 percent.
• Cold weather: Engine warm-up in sub-5°C temperatures can cut mi/L by 10 to 20 percent on short journeys.
• Roof boxes and racks: Increase aerodynamic drag by 15 to 25 percent even when empty — remove when not in use.
• Air filter condition: A blocked air filter can reduce fuel economy by 5 to 10 percent.

Knowing your miles per litre turns fuel into a predictable line item rather than a variable expense. With an accurate mi/L figure and the current pump price, you can budget fuel for any journey — commutes, road trips, or client travel expense claims — to within a few percent.

Annual fuel cost estimation

The average UK car is driven approximately 7,400 miles per year (DfT 2023 data). At 12 mi/L and £1.55/L: 7,400 ÷ 12 = 617 litres × £1.55 = £956 per year. At 14 mi/L the same mileage costs 7,400 ÷ 14 = 529 litres × £1.55 = £820 per year — a saving of £136. The Fuel Cost Calculator on Quasar Tools produces this annual total directly from annual mileage, mi/L, and fuel price inputs.

Expense claims and HMRC mileage

For self-employed drivers and employees claiming business mileage, knowing your actual fuel cost per mile matters. HMRC's approved mileage rates are fixed at 45p/mile for the first 10,000 business miles and 25p/mile above that. At current fuel prices, a car achieving 12 mi/L at £1.55/L has a fuel-only cost of £1.55 ÷ 12 = 12.9p per mile — well within the AMAP rate. A less efficient car at 8 mi/L costs 19.4p per mile in fuel alone, leaving less of the AMAP rate to cover wear, insurance, and depreciation.