What the fuel cost calculator works out
A fuel cost calculator turns four numbers — trip distance, your car’s fuel efficiency, the price of fuel, and whether you’re driving one way or there-and-back — into exactly how much a trip will cost. The formula behind it is short:
fuel used (L) = distance (km) × fuel efficiency (L/100 km) ÷ 100 cost = fuel used (L) × price per liter
Enter a 400 km trip in a car that uses 6.5 L per 100 km, at $6.50 a liter, and this fuel cost calculator tells you it’ll burn 26.00 L and cost $169.00 — before you even start the engine.
Beyond that headline number, the calculator also shows cost per km or per mile, cost per person when you split the trip with passengers, what a round trip does to the total, and a price-sensitivity chart showing what the same trip costs if fuel gets 10% or 20% cheaper or more expensive. You can enter fuel efficiency in whichever format your dashboard or manual actually shows — L/100 km, km/L, MPG (US) or MPG (UK) — and enter the price per liter, per US gallon or per UK gallon; the calculator converts everything internally, so you never have to do the unit math yourself. (All amounts are in whatever currency you’re pricing fuel in — the calculator doesn’t convert currencies, it just does the arithmetic; we use “$” in the examples below purely as a placeholder.)
This calculator is deliberately about the cost of one specific trip, not your car’s general fuel economy. If your only question is “how efficient is my car, and what does that look like in the other unit,” that’s a narrower question you can answer straight from your dashboard or a fill-to-fill log — here, the fuel efficiency figure is always just an ingredient on the way to one number: what this trip will cost.
How to use the fuel cost calculator
- Enter the trip distance and pick km or mi next to it.
- Pick your fuel-efficiency format — L/100 km, km/L, MPG (US) or MPG (UK) — whichever your car’s manual or trip computer shows, and enter the number.
- Enter the fuel price and pick per liter, per US gallon or per UK gallon. Use today’s price at the pump — the calculator doesn’t fetch prices automatically, and it can’t convert between currencies.
- Turn on “Round trip” if you’re driving there and back — it doubles the distance before calculating anything else.
- Optional — Number of people sharing the cost, in advanced options. Leave it at 1 if you’re paying alone; set it higher to see the cost split evenly, with a chart showing your share versus the rest of the group.
The main result — fuel cost — updates as you type, together with fuel used, cost per km or mile, and cost per person.
How the fuel cost calculator works out the price of a trip
Every trip cost comes from the same two-step formula, run on values converted to a common base (liters and kilometers) first:
- Fuel used (L) = distance (km) × fuel efficiency (L/100 km) ÷ 100
- Cost = fuel used (L) × price per liter
If you enter fuel efficiency as km/L, the calculator first converts it to L/100 km (100 ÷ km per liter). If you enter MPG (US) or MPG (UK), it converts using the exact definitions of a mile and a gallon: 1 mile = 1.609344 km (NIST SP 811), 1 US gallon = 3.785411784 L (NIST Handbook 44), 1 UK (imperial) gallon = 4.54609 L (Weights and Measures Act 1985). If your price is per gallon rather than per liter, the same gallon-to-liter constants convert it to price per liter before the cost formula runs.
Step-by-step example
Trip: 400 km, car uses 6.5 L/100 km, fuel costs $6.50 per liter.
- Fuel used = 400 × 6.5 ÷ 100 = 26.00 L.
- Cost = 26.00 × $6.50 = $169.00.
- Cost per km = $169.00 ÷ 400 km ≈ $0.42/km.
Round trips and splitting the cost between passengers
Turning on “Round trip” doubles the distance before anything else is calculated — it’s the very first step, so every other number (fuel used, cost, cost per km) reflects the full there-and-back distance.
Example: a 180 km trip one way, round trip on, car uses 7.2 L/100 km, fuel at $6.79/L.
- Effective distance = 180 × 2 = 360 km.
- Fuel used = 360 × 7.2 ÷ 100 = 25.92 L.
- Cost = 25.92 × $6.79 ≈ $176.00 for the whole round trip.
If you’re sharing the trip, set “Number of people sharing the cost” above 1 and the calculator splits the total evenly, showing your individual share and a donut chart of your share against the rest of the group (the chart only appears once you set 2 or more people — at 1 person, your share is simply the full cost).
Example: a 300 km trip, 6.0 L/100 km, fuel at $6.20/L, split between 4 people.
- Cost = (300 × 6.0 ÷ 100) × $6.20 = 18.00 L × $6.20 = $111.60 total.
- Cost per person = $111.60 ÷ 4 = $27.90.
- Rest of the group = $111.60 − $27.90 = $83.70.
Cost per km, and what a regular commute really adds up to
Once the calculator has your cost, it also shows cost per km (or per mile) — useful for comparing routes, or for quickly pricing a different distance in the same car without re-entering everything.
Example: at $0.4225 per km (the exact, unrounded cost-per-km from the 400 km / $169.00 trip above), a flat 100 km costs 100 × $0.4225 = $42.25 — no need to rebuild the whole calculation for a shorter version of the same drive.
The same logic scales up to a commute. Say your daily round-trip driving totals 120 km, your car does 15 km/L, and fuel costs $1.45/L: fuel used is 120 ÷ 15 = 8.00 L, so the trip costs 8.00 × $1.45 = $11.60 per day. Multiply that per-trip figure by however many days you actually drive it — over a 20-workday month, that’s 20 × $11.60 = $232.00; over 12 such months, $2,784.00 a year. The calculator itself only prices one trip at a time; the monthly and yearly totals are just that one number multiplied by your own trip count.
How rising or falling fuel prices change your cost
Because cost scales directly with price, the calculator also shows a price-sensitivity chart and table: the same trip’s cost at fuel prices 20% and 10% below and above what you entered, plus the price you entered itself.
Example, using the 400 km / 6.5 L/100 km / $6.50 trip above (cost $169.00 at the entered price):
| Price change | Cost |
|---|---|
| −20% ($5.20/L) | $135.20 |
| −10% ($5.85/L) | $152.10 |
| 0% ($6.50/L) | $169.00 |
| +10% ($7.15/L) | $185.90 |
| +20% ($7.80/L) | $202.80 |
That’s a $67.60 swing across a 40-percentage-point price range on this trip — a quick way to see how exposed a given trip (or commute) is to fuel-price swings before they happen.
Comparing L/100 km, km/L, MPG (US) and MPG (UK)
The calculator accepts fuel efficiency in four formats, because different cars, countries and dashboards show different ones. Behind the scenes, everything is normalized to L/100 km before the cost formula runs:
- km/L → L/100 km: 100 ÷ (km per liter)
- MPG (US) → L/100 km: 235.214583 ÷ MPG (US)
- MPG (UK) → L/100 km: 282.480936 ÷ MPG (UK)
Those constants come straight from the mile and gallon definitions above — they’re not separate assumptions, just the same conversion factors rearranged.
MPG (US) example: 250 miles, 28 MPG (US), fuel at $3.499 per US gallon → fuel used ≈ 8.93 US gal, cost ≈ $31.24.
MPG (UK) example: 150 miles, 45 MPG (UK), fuel at $1.459 per liter → fuel used ≈ 15.15 L, cost ≈ $22.11.
Because a UK (imperial) gallon (4.54609 L) is about 20% bigger than a US gallon (3.785411784 L), the same car’s MPG (UK) figure reads roughly 1.2× higher than its MPG (US) figure — a common mix-up if you’re comparing a US car’s spec sheet with a UK one, or vice versa.
Whichever format you enter, the calculator also converts your fuel efficiency to L/100 km for the “compare with other drivers” chart, so a MPG (US) entry and an L/100 km entry land on the same scale. For example, 30 MPG (US) converts to 235.214583 ÷ 30 ≈ 7.84 L/100 km — that’s the number your car’s efficiency gets measured against, not your cost, since cost is in whatever currency you entered and can’t be compared fairly across different currencies.