Planning a 400-mile road trip and wondering if it’s a $40 drive or a $90 one? Punch in three numbers and you’ll know before you turn the key. Trip distance, how many miles your car squeezes out of a gallon (or how many liters it drinks per 100 km), and what fuel costs right now. The calculator does the rest.
It speaks both unit systems. Americans get miles, MPG, and price per gallon. Everyone else gets kilometers, L/100km, and price per liter. Flip the toggle and your efficiency number converts on the fly, so you don’t have to do the 235.215 division in your head.
How it works
There are really only two formulas hiding behind those tidy result cards.
For US units, fuel needed is just distance divided by MPG. Drive 280 miles in a car that does 35 MPG and you’ll burn 8 gallons. Multiply by the pump price and there’s your cost.
Metric flips the logic because L/100km already bakes in the distance. Fuel needed equals distance ÷ 100 × the L/100km figure. A car rated at 7 L/100km on a 450 km drive uses 31.5 liters. Same idea, different arithmetic.
The round-trip card simply doubles everything. People forget the drive home counts too, and that’s where a “cheap” trip quietly turns expensive.
A quick example
Say you’re driving from Denver to Santa Fe, about 390 miles one way, in an SUV that manages 24 MPG. Gas is sitting at $3.60 a gallon.
- Fuel needed: 390 ÷ 24 = 16.25 gallons
- One-way cost: 16.25 × $3.60 = about $58.50
- Round trip: roughly $117 in fuel alone
Now you know to budget around $120 for gas, not the $60 your gut guessed. That gap is exactly why people blow their travel budgets.
Things that throw the estimate off
Your real MPG isn’t the sticker number. Highway cruising at a steady 65 beats the rated figure; crawling through city traffic, idling at lights, and short cold-start hops drag it down hard. A roof box or a trunk full of camping gear can shave several miles per gallon all by itself.
Weather plays too. Winter fuel blends, a cold engine that runs rich until it warms up, and the heater all nudge consumption up. Headwinds on a flat interstate are sneakier than you’d think. If you want a number you can actually trust, pad the result by 10 to 15 percent.
One more thing: nothing you type here gets sent anywhere. The math runs in your browser the instant you change a field. No account, no upload, no tracking.
Common questions
Where do I find my car’s MPG or L/100km? The window sticker when you bought it lists the EPA or manufacturer rating. For a real-world figure, fill up, reset the trip meter, drive normally, then divide the distance by the gallons it took to refill.
Does it work for diesel or electric cars? Diesel, yep, just enter the diesel pump price. EVs don’t use gallons or liters, so this isn’t the tool for them. You’d want a kWh-per-mile calculator instead.
Why convert efficiency when I switch units? A 30 MPG car isn’t 30 L/100km, it’s about 7.8. The toggle runs the conversion (235.215 ÷ your value) so the output stays realistic instead of giving you a nonsense number.
Can I change the currency symbol? Sure. The currency box is plain text, so swap the $ for €, £, ₺, or whatever you pay in. The price label updates to match.
Is round trip just double the one-way cost? Pretty much. It assumes the same route and same efficiency both directions, which is close enough for budgeting unless one leg is all uphill.