How to Calculate Fuel Cost for a Trip

In imperial units (miles and MPG):

Gallons needed = distance (miles) ÷ MPG

Fuel cost = gallons × price per gallon

In metric units (km and L/100km):

Liters needed = (distance (km) × L/100km) ÷ 100

Fuel cost = liters × price per liter

Worked example (U.S. style)

You drive 300 miles in a car that averages 25 MPG. Fuel is $3.60/gal.

• Gallons = 300 ÷ 25 = 12 gallons
• Cost = 12 × 3.60 = $43.20

Worked example (metric)

You drive 480 km at 8.0 L/100km. Fuel is $1.45/L.

• Liters = (480 × 8.0) ÷ 100 = 38.4 L
• Cost = 38.4 × 1.45 = $55.68

Same logic either way: find volume of fuel first, then multiply by unit price. The free fuel cost calculator supports both unit systems so you do not have to convert by hand.

MPG (miles per gallon) answers: “How many miles do I get from one gallon?” Higher is better. L/100km (liters per 100 kilometers) answers: “How many liters do I burn to go 100 km?” Lower is better. They point in opposite directions, which is why mixing labels is a classic error.

Approximate conversion

• L/100km ≈ 235.215 ÷ MPG (U.S. gallon)
• MPG ≈ 235.215 ÷ L/100km

Example: 30 MPG ≈ 235.215 ÷ 30 ≈ 7.84 L/100km. A European sticker of 6.5 L/100km ≈ 235.215 ÷ 6.5 ≈ 36.2 MPG.

Which number should you use?

Use the efficiency unit that matches your distance unit and local fuel price. Do not take a European L/100km figure and plug it into an MPG formula without converting. For mixed labels on a trip abroad, convert efficiency first with a unit converter, then run the cost formula once in a single system.

Window stickers often list city, highway, and combined MPG. For highway-heavy trips, highway MPG is closer; for stop-and-go commuting, city MPG is safer. When in doubt, use your own tank average: fill up, reset the trip odometer, fill again, and compute miles ÷ gallons for that tank.

One-way cost uses one-way distance. Round-trip cost usually doubles distance if the return route and conditions match the outbound leg.

Round-trip gallons ≈ (2 × one-way miles) ÷ MPG

Round-trip cost ≈ round-trip gallons × price per gallon

Worked round-trip

One way: 180 miles, 28 MPG, $3.40/gal.

• One-way gallons = 180 ÷ 28 ≈ 6.43 gal → cost ≈ $21.86
• Round-trip gallons ≈ 12.86 gal → cost ≈ $43.71

When doubling is wrong

• Different return route — detours, scenic legs, or mountain passes change miles and efficiency
• Different load — full car and luggage on the way out, empty on the way back
• Price changes — you refuel in a higher-tax state on the return
• Idling and stops — long waits, ferries, or traffic add fuel without adding map miles

For multi-stop itineraries, sum segment distances instead of trusting a single map pin. Add 5–10% for local driving at destinations if you will not stay parked the whole visit.

Comparing two routes (short mountain pass vs longer flat freeway)? Run both distances with the same MPG first, then optionally lower MPG on the steeper route by a few points if you know hills hurt efficiency.

Cost per mile = fuel cost ÷ miles driven for that trip, or more directly:

Cost per mile ≈ (price per gallon) ÷ MPG

At $3.50/gal and 25 MPG: 3.50 ÷ 25 = $0.14 per mile in fuel alone. At 40 MPG: 3.50 ÷ 40 = $0.0875 per mile.

Weekly commute example

One-way commute: 22 miles, 5 days/week, round-trip 44 miles/day.

• Weekly miles = 44 × 5 = 220 miles
• At 30 MPG: gallons/week = 220 ÷ 30 ≈ 7.33 gal
• At $3.60/gal: weekly fuel ≈ $26.40; monthly (≈4.3 weeks) ≈ $113

That is fuel only — not parking, tolls, insurance, or wear. Still, fuel is the line item that moves fastest when prices spike or you change vehicles.

Compare two cars

Same 12,000 miles/year, $3.50/gal:

Vehicle	MPG	Gallons/year	Annual fuel	$/mile
A	22	545	$1,909	$0.159
B	34	353	$1,235	$0.103

Difference ≈ $674/year in fuel at those assumptions. For “B saves what percent of A’s fuel bill?” use the free percentage calculator after you have both dollar totals.

Price is the most volatile input. Check a recent station near your route, not a national average from last month. If your trip crosses regions, average two or three plausible prices or run a low/high scenario.

What moves real MPG

• Speed — sustained 75–80 mph often costs several MPG vs 60–65 mph
• A/C and heat — climate control and cold starts use extra fuel
• Cargo and roof boxes — weight and drag cut efficiency
• Tire pressure — underinflated tires waste fuel
• Hybrid vs pure highway — some hybrids shine in city cycles and look closer to gas cars at steady highway speed

Sensitivity check

Trip: 600 miles, sticker 32 MPG, $3.40/gal → gallons = 18.75 → cost = $63.75.

If real MPG is 28: gallons ≈ 21.43 → cost ≈ $72.86 (~14% higher). Always prefer a measured tank average over brochure optimism for budgets you care about.

Electric driving cost is a different formula (kWh × rate). If you are comparing a plug-in or EV leg of a trip, see the electricity cost guide and convert that leg to dollars separately, then add fuel legs.

Enter distance, efficiency, and unit fuel price. Read fuel volume, total cost, cost per mile or km, and optional round-trip totals.

1. Open the free fuel cost calculator.
2. Choose miles/MPG or km/L/100km so units stay consistent.
3. Enter one-way or total trip distance from your map.
4. Enter realistic MPG or L/100km (tank average beats sticker when you have it).
5. Enter current fuel price in the same volume unit (per gallon or per liter).
6. Toggle or double distance for a round-trip estimate if the tool supports it.
7. Note total cost and cost per mile for budgeting or vehicle comparisons.
8. Rerun with a second MPG or price to build a low/high range.

Tips for cleaner estimates

Use the same price when comparing two cars. Separate pure highway days from mixed days. Add non-fuel trip costs (hotels, food, tolls) outside this tool so fuel stays an honest line item.

Most bad estimates come from unit mix-ups or optimistic efficiency.

• Mixing MPG with L/100km formulas — pick one system end to end
• Using city MPG for a pure highway trip (or the reverse)
• Forgetting the return leg on “round trip” plans
• Ignoring local driving at the destination
• Using last year’s fuel price when rates moved 30–50¢
• Comparing cars on sticker only without your driving pattern

Fuel is one operating cost among many. For other everyday money math on Generatr, see the discount calculator guide and the tip calculator guide. For unit conversions when labels disagree, use the unit converter guide.

This guide is educational planning math, not a guarantee of pump totals. Your odometer, tank fills, and local prices control what you actually spend.