How to Calculate Gas Cost for a Road Trip (and per Mile)
How much gas a road trip uses and what it costs: the formula, how to find your car's real mpg, a worked example and what to charge each rider per mile.
Before any road trip, the same question lands in the group chat: how much are we going to spend on gas, and what does each of us owe? Three numbers answer it in a minute: the distance, your car’s fuel economy and the price of gas. This guide gives you the formula, how to find your car’s real mpg, a worked example with four friends, and the benchmarks for deciding what to charge per mile when you give someone a ride.
If you just want the number, the free gas split calculator runs the math as you type, tolls, round trip and each rider’s share included.
The formula, in two steps
1. How much gas the trip uses. Divide the miles by your car’s mpg:
Gallons = miles ÷ mpg
In metric, divide the kilometres by km/L, or use litres per 100 km:
Litres = km ÷ km/L, or km × L/100 km ÷ 100
The units convert both ways: 235.2 ÷ mpg = L/100 km, so 30 mpg is about 7.8 L/100 km.
| mpg (US) | L/100 km |
|---|---|
| 20 | 11.8 |
| 25 | 9.4 |
| 30 | 7.8 |
| 35 | 6.7 |
| 40 | 5.9 |
2. What it costs. Multiply the gallons by the price per gallon and add tolls and parking:
Cost = gallons × price per gallon + tolls + parking
In the UK, fuel economy is in mpg but petrol is sold by the litre, and a UK gallon is 4.546 litres (a US gallon is 3.785). Enter the price per litre and pick “mpg (UK)” in the calculator, and it converts for you.
How to find your car’s real mpg
From easiest to most accurate:
- The dashboard. Most cars show average mpg or L/100 km. Reset it when you leave and you’ll have the real figure for that trip when you arrive.
- The window sticker or the manual. The combined figure is measured in a lab. Highway driving usually comes close; city driving tends to miss it by 10 to 20%.
- Measure it yourself. Fill the tank, reset the trip meter and drive until the next fill-up, again to full. Miles driven ÷ gallons added = mpg. Two or three tanks give a reliable average.
As a reference, a compact car gets about 30 to 40 mpg on the highway, a midsize sedan 25 to 35, and a pickup or SUV 18 to 25. Steady highway speeds use less than stop-and-go traffic; above 70 mph consumption climbs fast. Air conditioning, a full trunk, a roof box and five people instead of two all add to it.
A worked example: 180 miles each way, four friends
Maya, Tom, Priya and Leo are driving to the coast: 180 miles each way, back on Sunday. Maya’s car gets 30 mpg and gas is $3.465 a gallon. There’s $21 in tolls across both ways.
- Total distance: 180 × 2 = 360 miles.
- Gas used: 360 ÷ 30 = 12 gallons.
- Gas cost: 12 × $3.465 = $41.58.
- With tolls: $41.58 + $21 = $62.58. That’s about 17 cents a mile.
If all four ride the whole way, each pays $62.58 ÷ 4, about $15.65. Maya paid for everything, so Tom, Priya and Leo each pay her back their share.
When someone rides only part of the way
Say Leo gets picked up halfway, 90 miles in, and dropped there on the way back. The fair deal is that he pays in proportion to the distance he rode: half of what a full-trip rider pays. In parts: 1 for Maya, 1 for Tom, 1 for Priya and 0.5 for Leo, 3.5 parts in total.
| Person | Rode | Parts | Pays |
|---|---|---|---|
| Maya | 360 miles | 1 | $17.88 |
| Tom | 360 miles | 1 | $17.88 |
| Priya | 360 miles | 1 | $17.88 |
| Leo | 180 miles | 0.5 | $8.94 |
| Total | 3.5 | $62.58 |
$62.58 ÷ 3.5 = $17.88 per part. Tolls are split with the same parts. This is the example the gas split calculator opens with: swap in your numbers and share the link with the group.
What to charge per mile
“How much should I charge for gas?” has three answers, depending on what you want to cover.
1. Fuel only. Price per gallon ÷ mpg. In the example, $3.465 ÷ 30 = 11.6 cents a mile. That’s the floor: what it costs to move the car.
2. Fuel and tolls. What was actually paid, divided by the miles: $62.58 ÷ 360 = 17.4 cents a mile. That’s the usual deal among friends, and what the calculator shows as “Per mile”. Split among the riders, everyone pays their share and nobody comes out ahead or behind.
3. With wear and tear. Tires, oil, maintenance and depreciation cost money per mile too, even if nothing shows up during the trip. The official mileage rates are a benchmark, because they’re designed to cover all of it:
- In the US, the IRS standard mileage rate is 72.5 cents a mile for the first half of 2026 and 76 cents from July.
- In the UK, HMRC’s approved rate is 45p a mile for the first 10,000 miles a year.
At 30 mpg and $3.465 a gallon, fuel alone is 11.6 cents a mile; the official rate is more than six times that, because it also prices in the car. For a typical car the rule of thumb is simpler: the full cost of driving runs at about twice the cost of gas alone, more for a car that’s worth a lot, less for an old one.
Which one to use? For a trip with friends, gas and tolls are enough: the car was going anyway. For a daily carpool, where one person puts their car on the line for months, agreeing on a per-mile rate closer to the official one up front avoids arguments later. And the driver can pay a share or ride free as a thank you: that’s the group’s call, and the calculator shows it both ways.
Diesel, hybrids and electric cars
The math is the same; only the units change:
- Diesel: the price per gallon and the car’s economy on diesel, which is usually higher than on gas.
- Hybrid: use the real average from the dashboard, which depends a lot on how much of the trip is highway.
- Electric: consumption in kWh per 100 km (it’s on the dashboard) and the price per kWh. In the calculator, enter the consumption in the L/100 km field and the price per kWh as the price per litre. Charging at home, use your tariff; at a public charger, the price the app shows.
Before you leave: three agreements that prevent arguments
- Who pays and how it’s split. Say it in the chat before anyone gets in the car: gas and tolls among everyone, driver included or not.
- Tolls get noted as they happen. A photo of the receipt or the toll transponder charge is enough; nobody remembers on the way back.
- If there are more expenses, make a group. Lodging, groceries, dinners: in a Spliteo group everyone adds what they pay and sees who owes whom. Gas goes in as one more expense, split by parts if someone rode half the way. The guide to splitting travel expenses with friends covers the whole trip, from the booking to settling up.
Summary
- Gallons = miles ÷ mpg (or litres = km × L/100 km ÷ 100). Cost = gallons × price + tolls.
- The dashboard gives the real mpg; the sticker figure runs high in the city.
- Among friends, split what was paid: gas and tolls, by the miles each person rode.
- To charge mileage that covers wear, the official rates (76¢ a mile in the US, 45p in the UK) are the benchmark; driving costs about twice the gas alone.
