Fuel & units

Round-trip fuel cost formula

By RoadCost Editorial3 min readUpdated 27 October 2026

A round trip is not a different formula. It is the same one-way calculation with a larger distance. The mistake is doubling the fuel economy or the price instead of the distance.

Quick answer

Double the one-way distance, then calculate fuel use and cost as usual. If the return route is different, add both one-way distances instead of multiplying by two.

In this guide

1. The formula
2. When doubling is not enough
3. A worked example
Questions

Key takeaways

Double the distance, not the MPG or the price.
Use separate legs if the return route is different.
Add a small buffer for detours and traffic.

The formula

For a simple there-and-back trip, total distance is one-way distance × 2. Then use the normal fuel-cost method for that total distance.

round-trip distance = one-way distance × 2
fuel used = round-trip distance ÷ MPG
cost = fuel used × price per unit

When doubling is not enough

If you are taking a different route home, add the two route distances. If you plan a detour, add that distance too. A round-trip estimate based only on the outbound route can be too low.

Traffic and cold starts can also change real consumption. A small buffer is sensible for budgeting, but keep it separate from the mathematical estimate.

A worked example

A 120-mile one-way trip at 30 MPG US uses 4 gallons one way. The round trip is 240 miles and uses 8 gallons. At $3.50 per gallon, the fuel cost is $28.00.

The MPG stayed at 30. Only the distance changed.

Questions

Do I double miles or gallons for a round trip?

Double the distance. The fuel used and cost then scale with that distance. Do not double the MPG.

How do I include a detour?

Add the detour distance to the total trip distance before calculating fuel use.

Does a round trip change fuel economy?

Not by itself. The route, speed, traffic and conditions can change real economy, but the return leg is not automatically less efficient.

Sources

NIST — unit conversion — US customary and metric relationships used by the converters.
UK Weights and Measures Act 1985 — Statutory context for UK units, including the imperial gallon.
fueleconomy.gov — US fuel-economy ratings — EPA label values, test-cycle context and consumer guidance.
GOV.UK — weekly road fuel prices — UK price statistics for context, not a live station price.
US EIA — gasoline and diesel prices — US regional and national averages.

Try the calculator

Fuel cost calculator

Tick the round-trip option to double the distance automatically.

Related guides

How much fuel does 100 miles use? A simple guide

Work out fuel use and fuel cost for 100 miles in UK or US units, with example tables and the formula behind the answer.

How to budget fuel for a road trip

Estimate road-trip fuel cost, add a sensible buffer and handle unit changes across countries without guessing.

Cost per mile formula: a plain-English guide

Calculate fuel cost per mile correctly for UK and US units, with examples and the mistake that makes estimates too low.

A note on prices and rules

This guide explains arithmetic and units. It does not publish live fuel prices, electricity tariffs or changing tax rules. Use your own price and check official sources for anything that changes.

Page last reviewed: 27 October 2026. Formulas are covered by automated tests. Prices are inputs, not live data.