Fuel Cost Calculator: how it works (with formulas + examples)

Fuel cost looks simple until you’re actually planning a trip: you might know the distance, but not whether your car will hit the “official” MPG; you might be doing a round trip; and you almost always drive extra miles for food stops, wrong turns, scenic routes, or picking people up. This Fuel Cost Calculator is designed to handle all of that in one place — while staying easy enough to use in 10 seconds.

The calculator does three core things: (1) converts your distance and fuel economy into fuel used, (2) multiplies by fuel price to get total fuel cost, and (3) optionally adjusts the distance using a round-trip multiplier and a detour buffer so your estimate matches real life rather than “perfect highway conditions”.

The first thing we compute is your effective distance. If you choose “Round trip”, the distance is doubled. Then we apply your detour buffer (0–20%). This buffer is intentionally small and practical — it’s not meant to represent an entire additional side-trip, just the “extra” that sneaks into most drives.

Effective distance = distance × (round trip multiplier) × (1 + detour %)

• If round trip = No → multiplier = 1
• If round trip = Yes → multiplier = 2
• If detour = 10% → multiply by 1.10

Fuel economy comes in two common formats:

• MPG (miles per gallon): higher is better (you go farther per gallon).
• L/100km (liters per 100 km): lower is better (you burn fewer liters per 100 km).

The math depends on which format you selected via “Unit system”.

Note: UK MPG uses the imperial gallon, which is larger than the US gallon. That’s why “60 MPG” in the UK is not the same as “60 MPG” in the US. This calculator keeps each system consistent so you don’t accidentally mix units.

Once we have total cost, we also compute:

• Cost per mile/km = total cost ÷ effective distance
• Cost per person = total cost ÷ people (if you entered a number > 1)

These two extra outputs are what make the tool practical for real planning. Cost per distance helps you compare driving vs alternatives. Cost per person is perfect for carpools — and for preventing the “I thought you were paying” conversation.

• Example 1 (US): 120 miles, 30 MPG, $3.59/gal, 10% detour, no round trip.
  Effective distance = 120 × 1 × 1.10 = 132 miles.
  Fuel used = 132 ÷ 30 = 4.4 gal.
  Cost = 4.4 × 3.59 = $15.80 (approx).
• Example 2 (Metric): 300 km, 7.5 L/100km, $1.70/L, round trip, 5% detour.
  Effective distance = 300 × 2 × 1.05 = 630 km.
  Fuel used = (630 × 7.5) ÷ 100 = 47.25 L.
  Cost = 47.25 × 1.70 = $80.33 (approx).
• Example 3 (Carpool split): If total fuel cost is $48 and you set people = 4, each person’s share is $12.

• City driving lowers MPG: if you’ll be in traffic, use your city MPG (or add a bigger detour buffer).
• Speed matters: driving 80 mph usually increases fuel use compared with 60–65 mph.
• Weather + elevation: wind, rain, snow, mountains can increase fuel consumption.
• Load matters: more passengers + luggage can increase fuel usage.

Bottom line: if you want a “budget-safe” estimate, consider using a slightly worse fuel economy than your best-case, and keep the detour buffer at 10–20% for road trips.

Frequently Asked Questions

• Do I enter the one-way distance or total distance?

  Enter the one-way distance if you want, then toggle “Round trip” to automatically double it. If you already have total distance, leave “Round trip” set to No.

• What detour buffer should I use?

  For quick drives: 5%. For normal driving: 10%. For road trips with stops, pickups, and “we missed the exit” moments: 15–20%.

• What if I only know my L/100km but I’m in the US?

  Switch the unit system to “Metric” and enter L/100km directly. The calculator will compute liters and cost using $/L (or your local price per liter).

• Is this the same as total trip cost?

  No — this is fuel cost only. Total trip cost may include tolls, food, lodging, parking, maintenance, and time. For that, use the Road Trip Cost Planner in the related tools section.

• Why does UK MPG differ from US MPG?

  A UK (imperial) gallon is larger than a US gallon, so MPG values aren’t directly interchangeable. This tool keeps each unit system internally consistent.

• Does this store my trip?

  Only if you click “Save Trip” — and it saves locally on this device via your browser storage. Nothing is uploaded.