How to use the gas cost calculator
- Choose US units (miles, MPG, dollars per gallon) or metric (kilometers, liters per 100 km, price per liter).
- Enter the trip distance — a map app will tell you the driving distance.
- Enter your car's fuel economy and the current fuel price.
- Tick round trip to double the distance, and enter the number of people to split the cost.
How to calculate gas cost for a trip
Example: a 300-mile trip in a car that gets 25 MPG uses 300 ÷ 25 = 12 gallons. At $3.50 a gallon, that's 12 × 3.50 = $42 one way, or $84 for the round trip — $21 each if two people split it.
Finding your car's fuel economy
- Check the window sticker or the EPA rating at fueleconomy.gov. Use the highway figure for road trips and the combined figure for mixed driving.
- For a real-world number, fill the tank, reset the trip meter, drive, refill and divide the miles driven by the gallons added.
- Roof boxes, heavy loads, air conditioning, high speeds and hills all lower fuel economy — budget 10%–15% extra for a loaded car at highway speeds.
MPG to L/100 km
The two measures are inverted: MPG is distance per fuel, L/100 km is fuel per distance. To convert, divide 235.21 by the value: 25 MPG = 235.21 ÷ 25 = 9.4 L/100 km, and 7 L/100 km = 235.21 ÷ 7 = 33.6 MPG (US gallons).
Tips to save on gas
- Drive at a steady speed; fuel economy usually drops quickly above 50–60 mph.
- Keep tires inflated to the recommended pressure.
- Compare prices along your route — fuel near highway exits and in some states costs noticeably more.
Frequently asked questions
How do I calculate gas cost for a trip?
Divide the distance by your car's miles per gallon to get the gallons needed, then multiply by the price per gallon. 300 miles at 25 MPG and $3.50 a gallon costs $42.
How much gas money should I give for a ride?
Work out the total fuel cost of the trip and divide it by the number of people in the car, including the driver. Some people add a little extra for wear and tear.
How do I convert MPG to L/100 km?
Divide 235.21 by the miles per gallon (US). 30 MPG is about 7.8 L/100 km. The same formula works the other way.
How much does it cost to drive 100 miles?
At 25 MPG and $3.50 a gallon, 100 miles uses 4 gallons and costs $14. A 35 MPG car would spend $10 for the same distance.
How much CO₂ does a gallon of gas produce?
Burning one gallon of gasoline produces about 8.9 kg (19.6 lb) of carbon dioxide, according to the US EPA — roughly 2.3 kg per liter.
Results are provided for convenience and rounded for display. Sales tax rules, tipping customs and date conventions differ by location, so check local rules when it matters.
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