Calculate Fuel Cost & MPG instantly.
Fuel Cost & MPG Calculator UK
Trip cost, cost-per-mile, running-cost projections — plus a real-world MPG calculator if you don't know your car's MPG.
Trip Details
Work Out Your Real MPG
Most of us only really think about fuel cost when we're stood at the pump watching the numbers climb. You know roughly what your journey should cost, but between changing petrol and diesel prices, real-world MPG being nothing like the number in the brochure, and half of us not knowing whether our car does 40mpg or 50mpg, working it out in your head rarely ends well. That's what the fuel cost calculator UK drivers can use above is for — built as both a proper fuel cost calculator UK households can rely on and an MPG calculator UK drivers can use even without knowing their exact figures upfront. This guide walks through exactly how to use it, how the maths behind it works, and how petrol, diesel and electric actually compare once you do the sums properly.
Depending on what you drive and what you're trying to work out, this same tool doubles as a petrol cost calculator UK drivers use for a quick top-up trip, a diesel cost calculator UK households rely on for the bigger commutes, a cost per mile calculator UK motorists use to budget properly, an electric car cost per mile UK breakdown if you're plugging in rather than filling up, or a straightforward journey fuel cost calculator for a one-off trip — the maths underneath is identical, it's just the inputs that change.
What This Fuel Cost & MPG Calculator Actually Does
This isn't just a single "type in a number, get a number" tool. It's built around how UK drivers actually think about running costs, which is rarely just "one trip" — it's the commute, the weekly shop, the school run, the whole picture over a year.
- Three fuel types — petrol, diesel and electric are all supported, with the price and efficiency fields switching automatically (pence per litre for petrol/diesel, pence per kWh for electric).
- Route type adjustment — City, Mixed or Motorway. Petrol and diesel engines tend to drink more fuel in stop-start city traffic, while electric cars usually do the opposite thanks to regenerative braking clawing back energy every time you slow down. The calculator nudges the figures accordingly rather than pretending every road is the same.
- One-way or return journeys, in miles or kilometres.
- MPG or litres per 100km — whichever you're used to.
- Trips per week, so a single commute figure automatically rolls up into a weekly, monthly and yearly running cost, not just a one-off number you forget about the next day.
- A fuel-type comparison that shows what the exact same journey would cost if you were driving a petrol, diesel or electric car instead — genuinely useful if you're weighing up your next car rather than just budgeting for this week's diesel.
How to Use the Fuel Cost Calculator (Step by Step)
- Pick your fuel type. Petrol, diesel or electric — the price and economy fields adjust themselves so you're never entering the wrong unit by mistake.
- Choose a route type. If your journey is mostly town driving pick City, motorway-heavy trips pick Motorway, and Mixed covers everything in between (this is the safest default if you're not sure).
- Set your journey type and distance. Toggle One-way or Return, then enter the distance in miles or km — Google Maps is the easiest way to get an accurate figure if you don't already know it.
- Enter your fuel economy. This is the MPG (or L/100km) figure for your specific car. If you genuinely have no idea what it is, switch to the "Don't know your MPG?" tab first — more on that below.
- Enter the current fuel price. Pence per litre for petrol and diesel, or pence per kWh if you're charging an EV. The forecourt board or your last receipt will have this.
- Set how often you make this trip. Use the quick presets (occasional, weekdays, daily commute) or type your own number of trips per week.
- Hit Calculate. You'll get the total cost for that single trip, your cost per mile, how much fuel or energy you'll actually use, and the weekly/monthly/yearly running cost based on how often you're doing it.
Don't Know Your MPG? Here's How to Work It Out
A huge number of drivers simply don't know their car's real MPG, and that's completely fair — it's not exactly written on the dashboard. The manufacturer's official figure is measured in lab conditions and is almost always higher than what you'll actually get on the road, sometimes by 15–20%. Rather than guessing, the second tab on the calculator works it out from what you've actually spent.
You only need three things: the price you paid per litre, the total amount you spent filling up, and roughly how far that tank took you. From there the calculator does the rest:
Litres used = amount spent ÷ (price per litre ÷ 100)
MPG (UK) = miles driven ÷ (litres used ÷ 4.54609)
That 4.54609 figure is the UK imperial gallon in litres — worth knowing, because it's different from the US gallon (3.78541 litres), which is why an American car's MPG figure always looks better than the same car sold here. Once you've got your real MPG, there's a button to send it straight into the fuel cost calculator so you don't have to type it out again.
How the Fuel Cost Is Actually Calculated
The formula behind every fuel cost calculator worth using is the same one the RAC and most fleet managers use:
Fuel cost = (distance ÷ MPG) × 4.54609 × price per litre
To put that into real numbers: a car doing 45mpg, with petrol at 156p a litre, covering a 24-mile round trip, would use just over 2.4 litres of fuel and cost around £3.78. Stretch that same car and price out over longer distances and it looks like this:
| Distance | Fuel used | Estimated cost |
|---|---|---|
| 10 miles | 1.0 litres | £1.58 |
| 24 miles | 2.4 litres | £3.78 |
| 50 miles | 5.1 litres | £7.88 |
| 100 miles | 10.1 litres | £15.76 |
| 200 miles | 20.2 litres | £31.52 |
Petrol vs Diesel vs Electric: Which Is Actually Cheaper Per Mile?
This is the question the comparison chart on the calculator is built to answer, because the honest answer is "it depends", and the gap is usually smaller than people expect. Using current UK averages — petrol at around 156p/litre, diesel at 174p/litre, and electricity at roughly 28p/kWh at home versus 60–80p/kWh on a public rapid charger — the cost per 100 miles looks something like this:
| Fuel type | Typical efficiency | Cost per 100 miles |
|---|---|---|
| Petrol | 45 mpg | £15.76 |
| Diesel | 55 mpg | £14.38 |
| Electric (home charging) | 3.5 mi/kWh | £8.00 |
| Electric (public rapid charger) | 3.5 mi/kWh | £18.57 |
The pattern that jumps out is that diesel usually edges out petrol on longer, motorway-heavy trips thanks to better MPG, even though it's pricier per litre — the fuel efficiency more than makes up for it. Working out your electric car cost per mile tells a different story: it's by far the cheapest option if you can charge at home, but that advantage shrinks fast, and can even disappear, if you're relying on public rapid chargers. It's exactly why the calculator lets you compare all three for your own journey rather than working off a single national average.
Why Route Type Changes the Numbers
A car's official MPG figure assumes a fairly even mix of driving conditions, but real journeys rarely are. Sit in traffic on the school run and a petrol or diesel engine spends a lot of its time idling or accelerating from a standstill, both of which are thirsty — real-world city MPG can come in noticeably below the combined figure. Electric cars tend to do the opposite: town driving means more braking, and regenerative braking recovers some of that energy back into the battery, so EVs are often at their most efficient in slow traffic and their least efficient cruising at motorway speeds, where wind resistance takes over. That's the logic behind the City/Mixed/Motorway toggle on the calculator.
MPG to Litres per 100km — Quick Reference
If you're comparing a UK car against a European spec sheet, or just prefer thinking in metric, an MPG to L/100km conversion is genuinely handy to have on hand. Here's how the two scales line up (remember: with L/100km, lower is better, which is the opposite way round from MPG):
| MPG (UK) | L/100km |
|---|---|
| 30 mpg | 9.4 |
| 35 mpg | 8.1 |
| 40 mpg | 7.1 |
| 45 mpg | 6.3 |
| 50 mpg | 5.7 |
| 55 mpg | 5.1 |
| 60 mpg | 4.7 |
| 65 mpg | 4.3 |
| 70 mpg | 4.0 |
Simple Ways to Improve Your MPG
None of these are groundbreaking, but they genuinely move the needle, and they cost nothing to try:
- Ease off the accelerator. Harsh acceleration and braking can drag real-world MPG down noticeably — smooth, anticipatory driving is consistently the biggest single factor.
- Check your tyre pressures. Under-inflated tyres increase rolling resistance, and most drivers don't check them nearly often enough.
- Clear out the boot. Extra weight means the engine works harder for every mile, especially around town.
- Use cruise control on the motorway where it's safe to do so — a steady speed is almost always more efficient than constantly speeding up and slowing down.
- Keep up with servicing. A poorly maintained engine, dirty air filter or low oil can quietly cost you MPG without you noticing.
Frequently Asked Questions
It's only as accurate as the MPG figure you put in. Manufacturer figures are usually optimistic, so for a realistic result it's worth using your car's real-world MPG — which is exactly what the "Don't know your MPG?" tab works out for you from a recent fill-up.
Official figures are measured under controlled lab conditions with no traffic, no hills and a specific driving cycle. Everyday driving — cold starts, traffic, weather, extra weight — almost always brings the real number down, typically by 15–20%.
It comes down to three things: your distance, your car's MPG, and the current price per litre. Plug those three into the calculator above and it works out the exact cost, along with a cost-per-mile figure so you can compare journeys or vehicles fairly.
For an average petrol car doing around 45mpg at 156p a litre, a 100 mile journey costs roughly £15.76. A more efficient diesel doing 55mpg comes out slightly cheaper at around £14.38, while an electric car charged at home can cost as little as £8. See the comparison table above for the full breakdown.
Per litre, diesel is usually more expensive than petrol in the UK. But diesel engines are typically more fuel-efficient, especially on longer motorway journeys, so the cost per mile often ends up lower than petrol despite the higher pump price. It depends heavily on the specific car and the type of driving you do.
If you can charge mainly at home, electric is usually significantly cheaper per mile than petrol or diesel. That gap narrows a lot — and can disappear entirely — if you rely mostly on public rapid chargers, which cost several times more per kWh than a home tariff.
As a rough guide, anything above 50mpg is considered strong for a petrol car, 60mpg+ is very good for diesel, and small city cars or hybrids can comfortably exceed both. Larger SUVs and performance cars will typically sit lower, often in the 30s.
Fill up your tank, note the price per litre and reset your trip counter. Once you've used a decent amount of fuel, note how much you spent and how many miles you covered, then enter those into the "Don't know your MPG?" tab above — it works backwards from what you actually spent to give you a real-world MPG figure, rather than the optimistic one on the spec sheet.
UK imperial gallons throughout (4.54609 litres), since that's the standard MPG figure used on UK vehicles. This matters because a US gallon is smaller, so American MPG figures for the same car will always look higher than the UK equivalent.
Figures used in the examples above are illustrative UK averages correct as of 2026 and will vary by location, retailer and vehicle. Always check current pump or tariff prices for an exact figure — that's exactly what the calculator above is for.
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