Before any road trip, the same question comes up: how much is this going to cost in fuel? Whether you drive in miles and gallons or kilometres and litres, the math is the same three steps — and this guide walks through all of them, including the unit conversions that trip people up and how to split the cost fairly with passengers. When you want the answer instantly, use our free Fuel Cost Calculator.

The Fuel Cost Formula

Calculating trip fuel cost takes three steps once your units match:

1. Fuel needed = Distance ÷ Fuel efficiency
2. Total cost = Fuel needed × Price per unit
3. Cost per mile/km = Total cost ÷ Distance

Worked example in US units: a 300-mile trip in a car doing 30 MPG, with gas at $3.85 a gallon.

• Fuel needed: 300 ÷ 30 = 10 gallons
• Total cost: 10 × $3.85 = $38.50
• Cost per mile: $38.50 ÷ 300 ≈ $0.13/mile

The same trip in metric: 300 km at 7 L/100km with fuel at $1.60/litre uses 21 litres and costs $33.60. Our Fuel Cost Calculator handles either system automatically.

Understanding Fuel Efficiency: MPG, L/100km and km/L

Three measurement systems are used around the world, and mixing them up leads to big errors:

• MPG (miles per gallon) — US and UK. Higher is better.
• L/100km (litres per 100 km) — Europe, Australia, Canada. Lower is better, because it measures fuel used.
• km/L (kilometres per litre) — India, Japan, much of Asia. Higher is better.

As a rough guide, 30 MPG (US) is about 7.8 L/100km, and 15 km/L is about 6.7 L/100km. Rather than convert by hand, enter whatever your dashboard shows and let the calculator normalise it.

The US vs UK Gallon Trap

This catches out a lot of travelers. A US gallon is about 3.785 litres, but a UK (imperial) gallon is about 4.546 litres — roughly 1.2 times bigger. That means the same car appears to do more miles per gallon in the UK than in the US, purely because the gallon is larger. When you enter MPG, always pick the right gallon type so your fuel estimate is accurate.

How to Split Fuel Cost Between Passengers

Carpooling or road-tripping with friends? Splitting fuel is the easiest part: divide the total fuel cost by the number of people. A $60 fuel bill between 4 people is $15 each. Set the “split between” field in our Fuel Cost Calculator and it shows the per-person figure instantly. For dividing other shared costs like meals and tips on the trip, our Tip Calculator splits any bill between any number of people.

Round Trips and Multi-Leg Journeys

Don’t forget the return leg — a common reason real costs come in higher than expected. For a simple there-and-back journey, toggle “round trip” to double the distance automatically. For multi-stop trips, add up the legs to get the total one-way distance first.

9 Ways to Cut Your Fuel Costs

• Keep tyres inflated — under-inflation noticeably raises consumption.
• Ease off the speed — fuel use climbs sharply above ~80 km/h (50 mph).
• Lighten the load — remove unneeded weight and roof racks.
• Drive smoothly — anticipate traffic; avoid hard acceleration and braking.
• Combine errands — one warm-engine trip beats several cold starts.
• Use cruise control on flat highways for steady efficiency.
• Service regularly — clean filters and good plugs help economy.
• Shop around for fuel — prices vary station to station.
• Plan efficient routes — avoiding stop-start traffic saves more than you’d think.