How do I calculate gas mileage (MPG)?

Divide the distance you drove by the fuel you used. In US units, MPG = miles driven / US gallons used, so 300 miles on 10 gallons is 30 MPG. In metric, consumption is liters used / km driven x 100; 40 liters over 500 km is 8 L/100km. Higher MPG and lower L/100km both mean better economy.

What is the difference between MPG and L/100km?

MPG measures efficiency (distance per fuel), so higher is better, while L/100km measures consumption (fuel per distance), so lower is better. They describe the same economy from opposite directions and convert with L/100km = 235.214583 / MPG (US). For example, 30 MPG equals about 7.84 L/100km.

Does this use the US gallon or the UK (Imperial) gallon?

This calculator uses the US gallon, which is 3.785 liters, matching EPA and US window-sticker figures. The UK or Imperial gallon is larger at 4.546 liters, about 20% bigger, so the same car scores a higher number in UK MPG. A car rated 30 US MPG is roughly 36 UK MPG, so always check which gallon a quoted MPG refers to before comparing.

How do I measure my car's real fuel economy?

Fill the tank fully and reset the trip meter or note the odometer. Drive normally until you refuel, then fill fully again. The pump shows exactly how much fuel you used for that distance; enter both numbers into the calculator. Averaging several full tanks gives a more reliable figure than a single short trip.

How can I improve my gas mileage?

Keep tires properly inflated to cut rolling resistance, avoid hard acceleration and braking, remove roof boxes and unnecessary weight, and hold steady, moderate speeds since economy drops sharply at high motorway speeds. Regular servicing and using the recommended oil and fuel grade also help.

How to Calculate Gas Mileage Cost for a Trip

To calculate gas mileage cost for a trip, divide the trip distance by your car's fuel economy in miles per gallon, then multiply the gallons needed by the price per gallon — or use a free calculator to get your MPG, L/100km and km/L instantly. Trip fuel cost comes down to three numbers: how far you'll drive, how much fuel your car burns per mile, and what fuel costs where you fill up. The middle number — your fuel economy — is the one most people get wrong, because it depends on real driving rather than the optimistic figure on the window sticker. A road trip planner who wants a reliable dollar estimate needs an honest MPG figure first, then a single multiplication for the trip. Using a Gas Mileage Calculator that takes the distance you actually drove and the fuel you actually used is the fastest path to a number you can budget against. This article walks through the three-step method, shows a worked cost example, and flags the US-versus-UK gallon trap that throws off every cross-border comparison.

What you need before you start

Estimating trip gas cost is one of the few driving budgets that can be calculated to within a few dollars before you leave the driveway, because the inputs are simple. You need exactly three pieces of information:

Trip distance. A Google Maps or in-car navi estimate is fine; round up slightly if your route includes detours or construction.
Your car's real fuel economy. This is the figure most drivers under- or over-estimate. A single recent full-tank measurement, or several averaged, is far more useful than the EPA sticker number, which almost no one matches in real driving.
Fuel price per gallon (or liter). Use the local price you'll actually pay at the pump; gas-price apps help if you're crossing state or country lines.

If you don't yet know your real MPG, the next section walks through the three inputs the calculator needs and where to get them. The order matters: spend five minutes measuring your actual economy once, and every future trip-cost estimate becomes a quick calculation instead of a guess.

Calculate your fuel economy with the calculator

The Gas Mileage Calculator turns two numbers you already have — distance driven and fuel used — into your fuel economy in MPG, L/100km and km/L at the same time. Because it computes all three units side by side, you see the figure your country uses alongside the one most of the rest of the world uses, with no second lookup required.

Choose your unit system. Pick imperial for miles and US gallons, or metric for kilometers and liters. The choice only changes the labels on the boxes; the underlying math is the same.
Enter the distance you drove and the fuel you used. The cleanest source is a full tank to a second full tank: fill up, reset the trip meter, drive normally, then fill again and read the pump. The gallons (or liters) on the second receipt, divided into the miles (or km) on the trip meter, give you a number that reflects how you actually drive.
Read your fuel economy instantly below. MPG, L/100km and km/L appear together. There's no submit button, and nothing is uploaded, so the result updates as you type.

Estimate trip gas cost from your fuel economy

Once the calculator gives you MPG, the trip cost formula is a one-line multiplication:

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

For the metric equivalent, the relationship is the same with liters and kilometers. A worked example with real numbers makes the chain easier to follow:

You drove 300 miles and burned 10 US gallons to do it.
The calculator returns 300 ÷ 10 = 30 MPG.
Your planned road trip is 600 miles.
Gallons needed = 600 ÷ 30 = 20 gallons.
Fuel price on the route is $3.50 per US gallon.
Trip fuel cost = 20 × $3.50 = $70.

You can do the same in metric. Suppose the calculator shows your car at 8 L/100km and the trip is 1,000 km. You need 1,000 ÷ 100 × 8 = 80 L. At $1.50 per liter, that's $120 for the trip. The unit labels change, the arithmetic does not.

US gallons vs UK gallons: don't get tripped up

Comparing a US MPG figure with a UK MPG figure is one of the most common ways trip-planning spreadsheets go wrong. The US gallon and the UK (Imperial) gallon are not the same volume, and the calculator uses the US gallon to match EPA and US window-sticker figures.

Unit	Volume in liters	Relative size	Used by
US gallon	3.785411784 L	Baseline	United States, EPA ratings
UK (Imperial) gallon	4.54609 L	About 20% larger	United Kingdom, some Commonwealth references

Because the UK gallon is bigger, the same car using the same fuel records a higher MPG number in the UK than in the US. A car rated 30 US MPG is roughly 36 UK MPG — nothing about the engine changed, only the unit. Before you compare two MPG quotes or use one to budget a trip in the other country, confirm which gallon the figure was measured in.

The link between the two common efficiency units is fixed too. MPG (US) and L/100km are reciprocals related by L/100km = 235.214583 ÷ MPG (US). So 30 MPG is about 7.84 L/100km, and 40 MPG is about 5.88 L/100km. Anyone planning a cross-border trip can convert with that single constant.

Adjust your estimate for real-world variance

A 30 MPG figure from a flat highway run doesn't survive a trip that climbs two mountain passes and crawls through a city. Real-world fuel economy can swing noticeably depending on terrain, traffic, weather, and how much of the drive is city versus highway. A few adjustments make the trip budget more honest:

Build in a 10-15% buffer. Treat the calculator's MPG as a best-case figure and divide by a buffer of 1.10 to 1.15. The 600-mile trip that cost $70 on paper becomes roughly $77 to $80 with a realistic buffer ($70 × 1.10 = $77; $70 × 1.15 = $80.50).
Add altitude and terrain. Long climbs and steep descents change fuel use substantially. Mountain driving can drop effective MPG compared with flat highway driving on the same car.
Account for idling. Traffic delays, border waits, and detours burn extra fuel that adds up over hours of idling, even on a trip that's mostly highway.
Carry a small contingency. Even careful drivers end up taking a longer route once or twice. A modest cash buffer covers the difference without wrecking the budget.

Tips to reduce gas cost on a long drive

Fuel economy is the one variable in the trip-cost formula you can actually move while you drive. A handful of habits make a measurable difference over hundreds of miles:

Tire pressure. Under-inflated tires add rolling resistance. Check the pressures on the door-jamb sticker, not the number molded on the sidewall.
Smooth throttle and brakes. Hard acceleration and late braking waste the most fuel. Anticipating traffic flow keeps the MPG up.
Steady motorway speeds. Fuel economy typically falls sharply at high motorway speeds. Holding a steady, legal cruise speed is one of the cheapest gains available.
Remove roof boxes and weight. A roof box hurts aerodynamics even when empty; cargo you don't need just adds mass.
Plan refuels. Gas prices vary noticeably across a single region. A quick price check before each fill can save real money across a long trip.

For drivers weighing gas against electricity on their next vehicle, the math is different but the planning habit is the same — see the EV charging cost calculator guide for the equivalent at-home estimate.