You've got the distance from your map app and a rough idea of your car's mileage — but turning that into an actual dollar figure, splitting it fairly between passengers, and figuring out if diesel or premium is worth it takes a few extra steps. Here's the math, worked through with real examples.
Quick answer Fuel cost is found by dividing the distance by your vehicle's MPG to get the gallons needed, then multiplying by the price per gallon: Cost = (Distance ÷ MPG) × Price per gallon. A car getting 32 MPG driving 300 miles at $3.49/gal uses about 9.4 gallons and costs about $32.72 — roughly $0.11 per mile.
The two numbers that make or break this estimate are your vehicle's real-world MPG and today's price per gallon — and both are worth double-checking. A window-sticker MPG rating is a lab average; short trips, cold starts, and city stop-and-go driving typically burn more fuel than the combined rating suggests. Pulling your MPG from your car's trip computer over a few tanks gives a more honest number than the sticker.
Price per gallon should never be treated as a fixed constant, either — it can move noticeably week to week depending on where you are and what's happening with crude oil. ToolPico's Fuel Cost Calculator is built around this: you always type in today's price rather than relying on a stored number, so the result stays accurate no matter when you check it.
Gallons per 100 miles (GPM): A more linear way to compare vehicles than MPG alone, since MPG differences aren't equally meaningful at every level. It's simply 100 ÷ MPG — a 32 MPG car uses about 3.1 gallons per 100 miles, a 40 MPG car about 2.5 gallons, and a 20 MPG truck about 5 gallons.
Round trips and splitting the cost
Quick answer For a round trip, double the one-way distance before applying the formula: Cost = (2 × Distance ÷ MPG) × Price per gallon. To split the total fairly, divide it by the number of riders: Per-person cost = Total cost ÷ Number of riders.
Say four friends are driving 300 miles one-way to a weekend trip in a car that gets 32 MPG, with gas at $3.49/gal. Round trip, that's 600 miles, using about 18.75 gallons for roughly $65.44 in total gas. Split four ways, each person's share comes out to about $16.36 — a much easier number to Venmo someone than "figure it out yourselves at the pump."
In the calculator, the round-trip switch and the "Split between" field (both under Advanced settings on the Trip cost tab) handle this automatically, so you don't have to do the doubling and dividing by hand every time the group or the destination changes.
Example: 300-mile one-way trip, 32 MPG, $3.49/gal (illustrative numbers)
| Scenario | Distance | Gallons | Total cost |
|---|
| One-way | 300 mi | ~9.4 gal | ~$32.72 |
| Round trip | 600 mi | ~18.75 gal | ~$65.44 |
| Round trip, split 4 ways | 600 mi | ~18.75 gal | ~$16.36 / person |
Regular vs premium vs diesel: which is actually cheaper?
Quick answer It depends on both the price per gallon and the MPG difference, not price alone. Diesel often costs more per gallon than regular but frequently gets better MPG, which can make it cheaper per mile. Premium usually costs 15–20% more per gallon and rarely improves MPG unless your engine specifically requires it.
It's tempting to assume the fuel with the lowest price per gallon always wins, but that ignores the other half of the equation. A vehicle burning less fuel per mile can come out ahead even at a higher price per gallon — which is exactly why diesel engines, despite costlier fuel, sometimes beat regular gas on a per-mile basis. The only way to know for your specific vehicle is to compare actual numbers rather than assume.
Worth checking your owner's manual: If it says "regular fuel recommended" rather than "premium required," using premium in that engine typically raises cost per mile without a matching efficiency gain, since most non-turbo, non-high-compression engines don't extract extra performance from the higher octane.
The calculator's "Fuel type" tab lets you enter your own MPG and price for regular, premium, and diesel side by side over the same distance, so you can see which one actually wins for your car rather than guessing from the price per gallon alone.
Monthly and annual commuting cost
Quick answer Multiply daily commute distance by days driven per month to get monthly miles, then apply the fuel cost formula. As an example, 30 miles a day for 22 days is 660 miles a month; at 32 MPG and $3.49/gal that's roughly $72/month, or about $864/year.
A single trip's gas cost is easy to shrug off, but a daily commute adds up in a way that's worth actually seeing on paper — especially when weighing whether a more fuel-efficient car, a shorter commute, or occasional carpooling would meaningfully change your monthly budget. The "Monthly / Annual" tab in the calculator takes your daily distance and driving days per month and projects both figures automatically, so you don't have to keep re-multiplying by hand every time gas prices shift.
For example (illustrative only, not a guarantee of your actual costs): someone commuting 30 miles a day, 22 days a month, in a 32 MPG car at $3.49/gal lands around $864 a year in fuel just for that commute — a useful baseline for comparing against a hybrid, an EV, or simply working from home an extra day a week.
Frequently asked questions
How do you calculate fuel cost for a trip?
Trip fuel cost is found with: Cost = (Distance in miles ÷ MPG) × Price per gallon. First divide the distance by your vehicle's miles-per-gallon rating to get the gallons needed, then multiply by the current price per gallon. For example, a car getting 32 MPG driving 300 miles at $3.49 per gallon uses about 9.4 gallons and costs about $32.72.
How do you calculate round-trip fuel cost?
For a round trip, double the one-way distance, then apply the normal formula: Cost = (2 × Distance ÷ MPG) × Price per gallon. In the calculator, simply turn on the round-trip switch in advanced settings; the distance is doubled automatically and the total fuel cost updates.
How do you split a road trip's gas cost between passengers?
Per-person cost equals the total fuel cost divided by the number of people sharing the ride: Per-person cost = Total cost ÷ Number of riders. For example, if a trip costs $32.72 in gas and 3 people split it, each person owes about $10.91. Fill in the "Split between" field in advanced settings and the calculator shows the per-person amount automatically.
Is diesel or regular gas cheaper per mile?
It depends on both the price per gallon and the MPG difference, not price alone. Diesel often costs more per gallon than regular gas, but diesel engines frequently get better MPG, which can make diesel cheaper per mile overall. Use the "Fuel type" tab to enter your own MPG and price for regular, premium, and diesel and see which one wins per mile for your numbers.
How do you estimate monthly commuting fuel cost?
Multiply your daily commute distance by the number of days you drive per month to get monthly miles, then apply the fuel cost formula. For example, 30 miles a day for 22 days is 660 miles a month; at 32 MPG and $3.49 a gallon that's roughly $72 a month, or about $864 a year. The "Monthly / Annual" tab automates this calculation.
Get your trip's exact fuel cost
Enter your distance, MPG, and price per gallon for round-trip totals, per-rider splits, and a regular/premium/diesel comparison — free, no sign-up.
Try the free Fuel Cost Calculator →
Related guides
A note on the numbers: The dollar figures in this guide use illustrative example inputs (a 300-mile trip, 32 MPG, $3.49/gal, etc.) to show how the formula works, not predictions of your actual costs. Real fuel prices, vehicle efficiency, and driving conditions vary, so always plug in your own current numbers. This article is for general informational purposes and isn't financial advice.