How Much Does a Road Trip Cost in Gas?

Planning a road trip and want the gas number before you go? It comes down to three inputs and one division. The formula is trivial; the errors come from using the wrong numbers — the EPA’s MPG instead of yours, one-way distance instead of round trip, or today’s local price instead of the route’s.

The formula

fuel cost = (trip miles ÷ MPG) × price per gallon

That’s it. A 900-mile round trip at 32 MPG with $3.60 gas:

(900 ÷ 32) × 3.60 = 28.1 gallons × $3.60 ≈ $101

Per person in a carful of four, ~$25 each. Cheaper than most people guess — fuel is rarely the expensive part of a road trip.

Estimate your trip

Enter the round-trip distance, your real highway MPG, and the gas price you expect to pay:

Your numbersSaved on this device only

Vehicle MPG

mpg

Real-world, not the EPA sticker.Gas price

Local price per gallon.

Trip distance

One-way miles.Round trip (double the distance)Annual miles

Roughly how far you drive per year.Compare to MPG

mpg

Optional. Another car's MPG — 0 to skip.

This trip costs

$14.00

120 mi · 4 gal · $0.12/mi

A full year at 12,000 miles costs $1,400 in fuel.

Cost per mile: $0.12
Annual gallons: 400 gal
Annual fuel cost: $1,400
Per month: $117annual ÷ 12

The three mistakes that throw it off

Using the sticker MPG. The EPA combined number isn’t your highway number. Road trips are mostly highway, so use your car’s highway MPG — often higher than combined, but loaded cars, roof boxes, and high speeds cut it. Better still, use your trip computer’s real average.
Forgetting it’s round trip. “It’s 450 miles away” is 900 miles of gas. Double it before you divide.
Using your home gas price. Prices vary by state and along highways. For a long route, use a mid-route or destination-state price, which can differ by a dollar a gallon.

What changes the number on the road

Speed. Fuel economy drops sharply above ~65 mph — every 5 mph over costs noticeable MPG.
Load and drag. A full car, roof box, or bike rack can cut highway MPG by 10–25%.
Terrain and AC. Mountains and constant AC lower MPG; steady flat cruising raises it.
Headwinds. Real, and worse than most expect on long open stretches.

Gas vs electric for the same trip

An EV’s road-trip “fuel” cost depends heavily on whether you charge at home before leaving or rely on public fast charging en route — the per-mile cost can swing 3–4×. See Charging an EV at Home vs Public for that comparison.

What this doesn’t include

Tolls and parking — route-dependent, sometimes more than the gas.
Food, lodging, attractions — usually the real budget.
Wear and depreciation — the miles cost more than fuel alone (see True Cost of Ownership).

The one-line version

Road-trip gas = round-trip miles ÷ your real highway MPG × the price you’ll actually pay. Get those three right and you’ll be within a few dollars; fuel is almost always the smallest line in the trip budget.

Fuel Cost Calculator — per-trip and per-year fuel cost.
The Real Cost of Fuel — what a mile actually costs.
EV Charging at Home vs Public — the EV version of this question.
True Cost of Ownership — fuel is one line; here’s the rest.

AutoMath is an educational tool. Estimates depend on your real MPG and gas prices.