How to Calculate Fuel Cost for a Trip (Simple Formula)

Fuel is the one road-trip cost you can predict almost exactly before leaving the driveway. Three inputs — distance, your vehicle's consumption, and the price at the pump — and one line of arithmetic. Here's the formula in both measurement systems, plus the consumption conversion that trips everyone up.

The formula (US units)

Fuel cost = (Distance ÷ MPG) × price per gallon

*Example:* a 600-mile trip in a car that averages 30 MPG, with gas at $3.60/gallon:

600 ÷ 30 = 20 gallons needed 20 × 3.60 = $72

The formula (metric)

Metric consumption is stated as liters per 100 km, so:

Fuel cost = (Distance ÷ 100) × L/100km × price per liter

*Example:* 1,000 km at 7.8 L/100km with fuel at $0.95/liter:

(1,000 ÷ 100) × 7.8 = 78 liters 78 × 0.95 = $74.10

MPG ↔ L/100km: an inverted relationship

These two units run in opposite directions — higher MPG is better, lower L/100km is better — and the conversion is a division, not a multiplication:

L/100km = 235.21 ÷ MPG (and vice versa)

Because the relationship is inverted, equal MPG gains are *not* equal fuel savings: going from 20→30 MPG saves far more fuel per distance than 40→50. The MPG / L/100km Converter flips between the systems instantly.

Use your real-world consumption, not the sticker

Manufacturer figures come from standardized lab cycles; real driving typically runs 10–20% worse — more with highway speeds above 70 mph, roof boxes, under-inflated tires, or a fully loaded car. The most accurate method: fill the tank, reset the trip odometer, drive normally, refill, then divide miles driven by gallons added. Two or three tanks of that gives you a personal MPG figure worth trusting.

For planning, pad the estimate: multiply the result by 1.1 and you'll rarely be surprised at the pump.

Cost per mile: the budgeting number

Divide pump price by MPG to get your cost per mile:

$3.60 ÷ 30 = $0.12 per mile

This tiny number scales cleanly: a 40-mile daily commute costs $4.80/day, roughly $105/month. It also makes comparisons honest — a vehicle at 22 MPG costs $0.164/mile, about 36% more per mile than the 30 MPG car at the same pump price. Over 15,000 miles a year, that's a $660 gap.

Round trips, detours, and time

Remember to double one-way distances and add 5–10% for city driving at each end and detours en route. The Fuel Cost Calculator handles the arithmetic with any unit combination, and the Trip Time Calculator answers the companion question of how long you'll be driving.

FAQ

How much does gas cost per mile? Pump price ÷ MPG. At $3.60/gal: 25 MPG → $0.144/mi, 35 MPG → $0.103/mi. Recompute when prices move — it's the price term that changes weekly, not your MPG.

Does speed really affect fuel consumption? Substantially. Aerodynamic drag grows with the square of speed; most cars burn noticeably more fuel per mile at 75–80 mph than at 60–65. On long trips, modest speed cuts often save more than hunting for a cheaper station.

How do I split fuel costs between passengers? Compute the total with the formula, divide by people. The 600-mile example at $72 across three people is $24 each — driver's wear-and-tear is a separate negotiation.

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*Plan the full trip in seconds with the free Fuel Cost Calculator — any distance unit, any currency.*

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