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Fuel Economy Calculator

AutomotiveFuel Economy and Running Costs
Measured in
Fuel economy (L/100 km or US mpg, matching your inputs) —
Litres per 100 km
—
Miles per US gallon
—
Miles per imperial gallon
—
Kilometres per litre
—
Consumption band
—

Fuel economy is distance divided by fuel used, or fuel per 100 km in metric countries. Driving 300 miles on 10 US gallons gives 30 mpg, which equals 7.84 L/100 km (235.215 ÷ 30) and 36.03 miles per imperial gallon. Enter your own trip and the calculator returns all four common units together.

About this tool

Drivers comparing a car's real consumption with its window sticker, fleet managers checking a logbook, and anyone moving between a US, UK or European car listing all run into the same problem: four units that do not convert linearly. This calculator takes the distance you drove and the fuel you put back in, in either kilometres and litres or miles and US gallons, and reports litres per 100 km, kilometres per litre, US mpg and imperial mpg, plus a rough consumption band. The working is shown line by line. One limit: a single tank is a noisy sample. Pump shut-off points vary by a litre or more, so average several fill-ups before drawing conclusions about a car or a driving style.

How to use it

  1. Pick your measurement system

    Choose kilometres and litres or miles and US gallons; the fields switch to match.

  2. Enter the distance

    Use the trip meter reading since your last full tank, not the total odometer.

  3. Enter the fuel used

    Type the amount the pump delivered when you filled back up to full.

  4. Read all four units

    The headline uses your chosen system; the other three units and a consumption band sit below it.

Examples

300 miles on 10 US gallons

Measured in
Miles and US gallons
Distance driven
300
Fuel used
10

Result Fuel economy (L/100 km or US mpg, matching your inputs): 30
Litres per 100 km: 7.84
Miles per US gallon: 30
Miles per imperial gallon: 36.03
Kilometres per litre: 12.75
Consumption band: Average (7–10 L/100 km)

  1. MPG (US) = 300 mi ÷ 10 gal = 30
  2. L/100 km = 235.215 ÷ 30 = 7.84
  3. MPG (UK) = MPG (US) × 1.20095 = 36.03
  4. km/L = 100 ÷ L/100 km = 12.75

A typical midsize sedan figure: 30 mpg in the US is 7.84 L/100 km and 36.03 mpg on a UK forecourt.

100 km on 7 litres

Measured in
Kilometres and litres
Distance driven
100
Fuel used
7

Result Fuel economy (L/100 km or US mpg, matching your inputs): 7
Litres per 100 km: 7
Miles per US gallon: 33.6
Miles per imperial gallon: 40.35
Kilometres per litre: 14.29
Consumption band: Average (7–10 L/100 km)

  1. L/100 km = 7 L ÷ 100 km × 100 = 7
  2. MPG (US) = 235.215 ÷ 7 = 33.6
  3. MPG (UK) = MPG (US) × 1.20095 = 40.35
  4. km/L = 100 ÷ L/100 km = 14.29

Seven litres per hundred kilometres works out to 14.29 km/L and about 33.6 US mpg.

500 km on a 40 L refill

Measured in
Kilometres and litres
Distance driven
500
Fuel used
40

Result Fuel economy (L/100 km or US mpg, matching your inputs): 8
Litres per 100 km: 8
Miles per US gallon: 29.4
Miles per imperial gallon: 35.31
Kilometres per litre: 12.5
Consumption band: Average (7–10 L/100 km)

  1. L/100 km = 40 L ÷ 500 km × 100 = 8
  2. MPG (US) = 235.215 ÷ 8 = 29.4
  3. MPG (UK) = MPG (US) × 1.20095 = 35.31
  4. km/L = 100 ÷ L/100 km = 12.5

Refilling 40 litres after 500 km gives 8 L/100 km, a middling figure for a petrol family car.

How it is calculated

L/100 km = fuel (L) ÷ distance (km) × 100; mpg (US) = miles ÷ US gallons = 235.215 ÷ L/100 km

L/100 km
litres burned per 100 kilometres; lower is better
mpg (US)
miles per US gallon of 3.785411784 L
mpg (UK)
miles per imperial gallon of 4.54609 L, equal to US mpg × 1.20095
235.215
100 × 3.785411784 ÷ 1.609344, the constant linking mpg and L/100 km

Inputs are first converted to kilometres and litres using the exact NIST definitions of the mile (1.609344 km) and the gallons. Consumption per 100 km is then fuel over distance. Because mpg is distance over fuel, the two are reciprocals: halving L/100 km doubles mpg, which is why a 10 to 20 mpg upgrade saves far more fuel than a 30 to 40 mpg one.

When not to use it

  • Do not use it for electric cars; they are rated in kWh per 100 km or MPGe.
  • Do not rely on a partial fill, since the fuel figure will not match the distance.
  • Do not compare its result directly to lab-cycle ratings without allowing for real-world losses.

Common mistakes

  • Mixing US and imperial gallons, which makes UK mpg look about 20 percent better.
  • Averaging mpg figures instead of totalling miles and gallons across tanks.
  • Using the odometer total rather than the trip distance since the last fill.

Frequently asked questions

Why is UK mpg higher than US mpg for the same car?

An imperial gallon holds 4.54609 litres while a US gallon holds 3.785411784 litres. The car travels further on the larger gallon, so its UK mpg is 1.20095 times its US figure: 30 US mpg is 36.03 UK mpg.

How do I convert mpg to L/100 km by hand?

Divide 235.215 by US mpg, or 282.481 by imperial mpg. For example, 25 US mpg is 235.215 ÷ 25 = 9.41 L/100 km. The same division works in reverse to get mpg from L/100 km.

Why does my figure differ from the official rating?

Official ratings come from standard lab cycles at set speeds and temperatures. Short trips, cold engines, air conditioning, roof racks, heavy loads and motorway speeds above about 110 km/h all raise real consumption, often by 10 to 25 percent over the label.

What is the most reliable way to measure my car's consumption?

Fill the tank to the first pump click, reset the trip meter, drive normally, then fill again to the first click. The fuel delivered on the second fill matches the distance driven. Repeat over three or more tanks and total the distance and fuel before dividing.

What do the consumption bands mean?

They are rough groupings by litres per 100 km: under 5 is very frugal (small hybrids and diesels), 5 to 7 economical, 7 to 10 average for petrol family cars, 10 to 14 thirsty, and above 14 typical of large SUVs, vans and performance cars.

Does the trip computer give the same number?

Often not exactly. Many dashboard readouts estimate fuel from injector timing and tend to read a few percent optimistic. A fill-to-fill measurement entered here is a useful check on what the dashboard shows.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 21). Fuel Economy Calculator. Tools Directory. Retrieved October 26, 2026, from https://tools.milunir.com/automotive/fuel-economy-and-running-costs/fuel-economy-calculator/

MLA

Tools Directory Editorial Team. “Fuel Economy Calculator.” Tools Directory, 21 Oct. 2026, https://tools.milunir.com/automotive/fuel-economy-and-running-costs/fuel-economy-calculator/. Accessed 26 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Fuel Economy Calculator.” Tools Directory. Last modified October 21, 2026. Accessed October 26, 2026. https://tools.milunir.com/automotive/fuel-economy-and-running-costs/fuel-economy-calculator/.

Link<a href="https://tools.milunir.com/automotive/fuel-economy-and-running-costs/fuel-economy-calculator/">Fuel Economy Calculator – Tools Directory</a>