Fuel Calculator Free Online: MPG to L/100km, US vs UK MPG and Trip Fuel Cost

Published 1 November 2026 · By Sardar Zain, Founder of Orixiva · 12-minute read

An American review says 35 MPG, a British one says 42 MPG, a European brochure says 6.7 L/100km and a dealer in Asia says 15 km/L. They might all describe the same car. If you have searched for how to use a fuel calculator free online without login, the challenge is not the arithmetic: it is knowing that fuel economy units work in opposite directions and that a US gallon is smaller than a UK gallon.

This guide explains each fuel economy unit, shows how to convert MPG to L/100km by hand, and works through trip cost and comparison examples. You can check every answer with the free Orixiva fuel economy converter, which runs in your browser without signup, sign-in or login and shows the formula for each step.

The four fuel economy units

Why fuel economy is not a simple multiplier

Length or weight conversions multiply by a fixed factor. Fuel economy does not, because L/100km is fuel per distance while MPG and km/L are distance per fuel. One is the inverse of the other, so the conversion divides:

In practice this means you should never look for a single “multiply by” factor between MPG and L/100km. The constants 235.215 and 282.481 are divided by the figure you have, and the same constant works in both directions.

That is why doubling MPG does not halve the number in the same way you might expect, and why the Orixiva converter shows its working in two steps: first to km/L, then to the target unit.

How to convert MPG to L/100km

1. Check whether the MPG figure is US or UK.
2. Divide 235.215 (US) or 282.481 (UK) by the MPG.
3. Round to one or two decimals. 30 US MPG = 235.215 ÷ 30 = 7.84 L/100km.

• 25 US MPG = 9.41 L/100km.
• 40 US MPG = 5.88 L/100km.
• 40 UK MPG = 7.06 L/100km.
• 50 UK MPG = 5.65 L/100km.

How to convert L/100km to MPG

• 5 L/100km = 47.0 US MPG = 56.5 UK MPG.
• 7 L/100km = 33.6 US MPG = 40.4 UK MPG.
• 10 L/100km = 23.5 US MPG = 28.2 UK MPG.

km/L to L/100km

Divide 100 by km/L. A car rated 15 km/L uses 100 ÷ 15 = 6.67 L/100km, which is 35.3 US MPG.

US MPG vs UK MPG

A UK gallon is 4.546 litres and a US gallon is 3.785 litres, so the same car travels further on a UK gallon. UK MPG is about 1.2 times US MPG:

Flowchart: which unit is my fuel figure in?

1. Read the fuel economy figureBrochure, review, dashboard or trip computer
2. Is it “per 100 km”?
   YesL/100km: lower is betterNoDistance per fuel: higher is better
3. Is it miles per gallon?
   From the USUS MPG (3.785 L)From the UKUK MPG (4.546 L)
4. Is it km per litre?L/100km = 100 ÷ km/L
5. Convert all cars to one unitL/100km makes fuel use and cost easy to compare

The MPG illusion: why L/100km is easier to compare

Because MPG is distance per fuel, equal MPG gains do not save equal fuel. The US government’s fueleconomy.gov label guide gives the example: over 1,000 miles, going from 10 to 15 MPG saves about 33 gallons, but going from 30 to 35 MPG saves only about 5.

L/100km avoids this trap: it grows in a straight line with fuel used, so a car at 6 L/100km really uses half the fuel of one at 12 L/100km over the same distance.

How to calculate trip fuel and cost

1. Convert your car’s figure to L/100km (or MPG if you work in miles and gallons).
2. Multiply by the trip distance in the matching unit.
3. Multiply by the fuel price per litre or gallon.
4. Add 10–15% for traffic, air conditioning and hills.

Comparing petrol with electric running costs? See EV vs petrol running cost per kilometre. For flights and boats, the trip fuel guide uses burn rates instead of MPG, and the volume calculator guide converts litres and gallons.

What changes your real fuel use

Official figures come from standard tests. On the road, these make the biggest difference:

• Speed: air resistance rises steeply with speed, so motorway driving at high speed uses noticeably more fuel.
• Driving style: hard acceleration and braking waste fuel; smooth driving saves it.
• Tyre pressure: under-inflated tyres increase rolling resistance. Our pressure calculator guide converts psi, bar and kPa.
• Load and roof racks: extra weight and drag add consumption.
• Air conditioning and short trips: cold engines and A/C both raise fuel use.

Measure your own figure with the fill-up method

1. Fill the tank completely and reset the trip meter.
2. Drive normally until the tank is at least half empty.
3. Fill up again and note the litres added and the distance driven.
4. L/100km = litres ÷ km × 100. For example, 42 L over 600 km = 7.0 L/100km.

How far can you go on a full tank?

Plan to refuel well before the tank is empty; the last part of a tank is a safety reserve, not usable range. Many cars also let you switch the trip computer between L/100km, km/L and MPG in the settings menu, which is quicker than converting by hand while driving.

Converting fuel prices between litres and gallons

To compare prices across countries, convert the price, not only the economy. $3.50 per US gallon is $3.50 ÷ 3.785 = $0.92 per litre. A UK price of £1.45 per litre is £1.45 × 4.546 = £6.59 per imperial gallon. The volume calculator guide explains the gallon sizes.

Example: yearly fuel cost of two cars

Converting both cars to L/100km makes the comparison simple:

Car B saves 225 litres, or 405 a year at that price. Over five years, that is 2,025 before any change in fuel prices.

Hybrids, EVs and MPGe

Electric cars do not burn fuel, so their efficiency is shown differently: kWh per 100 km (or per 100 miles) in most countries, and MPGe (miles per gallon equivalent) on US labels. MPGe treats 33.7 kWh of electricity as equal to the energy in one US gallon of gasoline. The fuel economy converter handles liquid-fuel units; for kWh, use the energy converter, and for running cost per kilometre see EV vs petrol.

Fuel economy conversion chart

How to use the free fuel economy converter (step by step)

1. Open the fuel converter. Go to the Orixiva fuel economy converter. It opens on 8 L/100km to US MPG. No signup or login is needed.
2. Type your figure. Enter a value such as 35 or 6.7. Values must be greater than zero; the converter shows a clear message otherwise.
3. Choose From and To units. Litres per 100 km, kilometres per litre, miles per US gallon or miles per imperial gallon.
4. Read the result. It updates as you type and shows your figure in all four units in the table.
5. Check the formula. “How it was calculated” shows both steps, for example km/L = US mpg × 0.425144, then L/100 km = 100 ÷ (km/L).
6. Save or share. Swap units, copy the result, download TXT or print. Press Convert to keep it in Recent conversions and export the list as CSV.

So, how to use a fuel calculator free online without signup or login? Open the converter on any PC or phone browser and type: there is no account, nothing to install, and nothing is uploaded.

How to convert fuel economy in Excel or Google Sheets

Spreadsheet unit functions do not include fuel economy, so use the formulas directly. They work in any spreadsheet:

The same constant converts both ways because the relationship is an inverse: if 235.215 ÷ 30 = 7.84, then 235.215 ÷ 7.84 = 30.

Mistakes to avoid with fuel economy

Multiplying instead of dividing

L/100km and MPG are inverses. Multiplying 30 MPG by any factor cannot give the right L/100km.

Mixing US and UK MPG

The 20% gap can make a car look much better or worse than it is.

Comparing official figures with real driving

Lab test cycles differ between regions, and real-world use is often higher. Compare cars using figures from the same test standard.

Averaging MPG across trips

For an overall figure, divide total distance by total fuel. Averaging the MPG of each trip gives the wrong answer.

Forgetting tank size

Range = tank litres ÷ L/100km × 100. A 50 L tank at 7 L/100km gives about 714 km; keep a reserve.

Related guides: speed calculator online, power calculator free online, km to miles and length conversion and the automotive and transport calculators.