Quick answer
A 300-mile round trip... actually a 300-mile trip each way (600 miles total) at 30 MPG and $3.50/gal costs about $70 in gas.
How it works
Divide your total miles by your car's MPG to get gallons needed, then multiply by the price per gallon. Round trips double the one-way distance.
Formula
Gas cost = (Miles ÷ MPG) × Price per gallon
Assumptions & limitations
- Constant fuel economy for the whole trip
- Round trip doubles the one-way distance
- Gas price stays the same along the route
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
How do I find my car's real MPG?
Reset your trip computer at a fill-up and check it at the next one, or divide miles driven by gallons pumped.
Should I add extra for city driving or detours?
Adding 10% to the distance is a good buffer for detours, traffic, and driving around at your destination.