Fuel Cost Calculator

The Fuel Cost Formula (With Worked Examples)

The core math is simple. First you find how much fuel the trip needs, then you multiply by price.

If your efficiency is in MPG (miles per gallon):

Fuel needed (gallons) = Distance (miles) ÷ MPG
Trip cost = Fuel needed × Price per gallon

If your efficiency is in L/100km (common outside the US):

Fuel needed (liters) = Distance (km) × L/100km ÷ 100
Trip cost = Fuel needed × Price per liter

Example 1 — US road trip

Driving 300 miles in a car that gets 30 MPG, with gas at $3.50/gallon:

Fuel needed = 300 ÷ 30 = 10 gallons
Trip cost   = 10 × $3.50 = $35.00

Example 2 — European commute

Driving 120 km in a car rated at 6.5 L/100km, with diesel at €1.75/liter:

Fuel needed = 120 × 6.5 ÷ 100 = 7.8 liters
Trip cost   = 7.8 × €1.75 = €13.65

Example 3 — Comparing two vehicles

Same 300-mile trip, but a truck at 18 MPG versus a hybrid at 50 MPG, gas at $3.50:

Truck:  300 ÷ 18 = 16.67 gal × $3.50 = $58.33
Hybrid: 300 ÷ 50 = 6.00  gal × $3.50 = $21.00
Savings: $37.33 on a single trip

That gap is exactly why a fuel cost calculator is worth running before you decide which car to take.

MPG vs. L/100km: Why the Conversion Trips People Up

The two most common ways to express gas mileage measure opposite things, and confusing them leads to real errors.

• •MPG (miles per gallon) — higher is better. It measures distance covered per unit of fuel.
• •L/100km (liters per 100 kilometers) — lower is better. It measures fuel consumed per unit of distance.

Because they're inversely related, you can't just swap the number — you have to convert. The magic constant is 235.215:

L/100km = 235.215 ÷ MPG(US)
MPG(US) = 235.215 ÷ L/100km

Watch out: a US gallon (3.785 L) is smaller than an Imperial (UK) gallon (4.546 L). UK MPG figures look about 20% better than US ones for the identical car. Use MPG(UK) = 282.481 ÷ L/100km if you're working in Imperial gallons.

Quick sanity check: 30 MPG(US) → 235.215 ÷ 30 ≈ 7.84 L/100km. A frugal 50 MPG car → about 4.7 L/100km. Knowing this keeps you from accidentally comparing a US window sticker against a European spec sheet.

Fuel Efficiency and CO2 Benchmarks

Use this reference table to sanity-check your inputs and see where your vehicle falls. CO2 figures use standard tailpipe factors: about 8.89 kg CO2 per US gallon of gasoline and 10.16 kg per gallon of diesel.

Fuel price ranges (2026 ballpark, varies widely):

These are guideposts, not quotes — always enter the actual price you'll pay for the most accurate result.

How to Estimate Your Real-World Fuel Consumption

The MPG on your window sticker is a lab number. Your real fuel consumption depends on how and where you drive. Here's how to get an honest figure:

1. •Fill the tank completely and note the odometer (or reset a trip meter).
2. •Drive normally until you're about half a tank down — the more miles, the better the average.
3. •Refill completely and record the gallons/liters added and the miles driven.
4. •Divide: miles ÷ gallons = your real MPG. Or liters ÷ km × 100 = your real L/100km.

Example

You drove 287 miles and it took 9.6 gallons to refill:

Real MPG = 287 ÷ 9.6 = 29.9 MPG

Factors that push consumption up (worse economy):

• •Highway speeds above ~60 mph — aerodynamic drag rises sharply.
• •Cold weather, short trips, and heavy AC use.
• •Aggressive acceleration and braking.
• •Roof racks, trailers, and heavy cargo.
• •Under-inflated tires (can cost 3% or more).

Using your measured MPG rather than the sticker number can change a trip estimate by 15–25%, so it's worth the five-minute test.

How to Use This Fuel Cost Calculator

The calculator is built around three inputs and three outputs.

Inputs

• •Distance — the length of your trip. Enter one-way for a single journey, or double it for a round trip. Use miles or kilometers to match your efficiency unit.
• •Fuel efficiency — your gas mileage as MPG or fuel consumption as L/100km. Use your real-world figure if you have it; otherwise use the EPA/manufacturer rating.
• •Fuel price — the current price per gallon or per liter at your local station.

Outputs

• •Total trip cost — the headline number, in your currency.
• •Fuel needed — gallons or liters required, so you know if you'll need to refuel mid-trip.
• •CO2 emissions — estimated tailpipe carbon for the journey, in kilograms.

Tip: To split costs among passengers, take the total cost and divide by the number of people. A $58 trip shared four ways is under $15 each — often cheaper than a bus ticket.

Change any single input and the results update instantly, which makes it easy to run "what-if" scenarios: what if gas rises to $4? What if I take the more efficient car?

Strategies to Cut Your Fuel Costs

Once you can see the cost, you can shrink it. The highest-leverage moves:

• •Drive the efficient vehicle for long trips. As Example 3 showed, choosing a 50 MPG car over an 18 MPG truck saved $37 on 300 miles. Over a year of commuting that's thousands.
• •Ease off the highway speed. Dropping from 75 to 65 mph can improve economy by 10–15% because drag scales with the square of speed.
• •Keep tires properly inflated and remove roof boxes when not in use.
• •Combine errands into one warmed-up trip — cold engines are least efficient in the first few miles.
• •Time your fill-ups. Apps that map local prices can save $0.10–$0.30 per gallon; on a 20-gallon fill that's up to $6.
• •Consider the total cost of the route. A shorter but stop-and-go city route can burn more fuel than a slightly longer highway route at steady speed.

Small habits compound. Improving real-world economy by just 10% on a car that costs $2,000/year in fuel puts $200 back in your pocket annually.

Common Mistakes to Avoid

Even a simple gas trip calculator gives wrong answers if the inputs are wrong. Watch for these:

• •Mixing units. Entering kilometers of distance with an MPG efficiency (a per-mile figure) produces nonsense. Keep distance and efficiency in the same measurement system.
• •US vs. UK gallons. A UK car's "40 MPG" is not the same as a US "40 MPG." The UK figure is about 20% higher for the same real consumption. Convert before comparing.
• •Using the sticker MPG for everything. Real-world economy is usually 10–20% below the lab rating, especially in winter or heavy traffic. Your budget should use your number.
• •Forgetting the return trip. Always confirm whether your distance is one-way or round-trip. This is the single most common doubling error.
• •Ignoring load and conditions. A car full of passengers and luggage climbing mountain passes burns far more than the flat-highway rating suggests.
• •Confusing MPG and L/100km direction. Remember: high MPG is good, but low L/100km is good. Don't assume a bigger number is always better.

Correct these and your estimate will typically land within a few percent of what you actually pay.