How to use it
- Pick MPG, trip cost or convert.
- For MPG, enter miles driven and gallons added at each fill-up.
- For trip cost, enter distance, mpg and gas price.
- Read your mpg, cost per mile or trip cost.
How to measure your real MPG
Your dashboard estimate can be off. For an accurate figure, use the full-tank method:
- Fill the tank completely and reset the trip odometer.
- Drive normally.
- Fill up again and note the gallons it took and the miles on the trip meter.
- MPG = miles driven / gallons added.
Example: 312 miles on 10.4 gallons = 30 mpg. Add several fill-ups to average out differences in how full the tank gets. Enter the gas price to see your cost per mile and per fill-up. Everything is calculated in your browser.
Trip gas cost
Gas cost = (distance / mpg) × price per gallon
Example: a 450-mile drive in a car that gets 28 mpg, with gas at $3.40 a gallon:
- Gas needed: 450 / 28 ≈ 16.1 gallons
- Cost: 16.1 × 3.40 ≈ $54.64
- Round trip: about $109.29; split four ways, about $27.32 each
Your cost per mile is 3.40 / 28 ≈ 12.1 cents. Highway driving often gives better mileage than the city rating, while heavy loads, roof boxes, cold weather and high speeds lower it.
Electric vehicles
Tick Electric vehicle to work in kWh. Enter efficiency in kWh per 100 miles and the electricity price per kWh. At 30 kWh/100 mi and 16 cents per kWh, the same 450-mile trip uses 135 kWh and costs about $21.60 charging at home. Public fast charging often costs more per kWh, so use the price you will actually pay.
For fill-up tracking, enter miles driven and kWh added; the calculator shows miles per kWh and kWh per 100 miles.
MPG and L/100 km conversion
Americans measure fuel economy in miles per gallon (higher is better). Most other countries use liters per 100 km (lower is better). The conversion is L/100 km = 235.215 / mpg.
| US mpg | L/100 km |
|---|---|
| 20 | 11.76 |
| 25 | 9.41 |
| 30 | 7.84 |
| 40 | 5.88 |
| 50 | 4.70 |
UK mpg uses the larger imperial gallon, so the same car shows about 20% higher mpg in the UK. The convert tab handles US mpg, UK mpg, L/100 km and km/L.
Why mpg can mislead
Because mpg is a ratio of distance to fuel, improvements at the low end save far more gas than the same mpg gain at the high end. Over 10,000 miles:
| Change | Gallons saved |
|---|---|
| 15 → 20 mpg | about 167 |
| 30 → 40 mpg | about 83 |
Upgrading from a 15 mpg truck to a 20 mpg one saves twice as much fuel as going from 30 to 40 mpg. Fuel economy labels show gallons per 100 miles for this reason.
Tips to improve gas mileage
- Keep tires inflated to the pressure on the door-jamb sticker.
- Drive smoothly; hard acceleration and braking waste fuel.
- Slow down on the highway; drag rises quickly with speed.
- Remove roof racks and extra weight when you don't need them.
- Keep up with maintenance such as air filters and oil changes.
- Combine short errands into one trip; a cold engine uses more fuel for the first few miles.
- Avoid long idling. Turning the engine off while you wait saves gas in most modern cars.
For car payments, see the auto loan calculator.
Frequently asked questions
How do I calculate my gas mileage?
Fill the tank, reset the trip meter, drive, fill up again, and divide the miles driven by the gallons it took.
How much will gas cost for my trip?
Divide the trip distance by your mpg and multiply by the price per gallon. 450 miles at 28 mpg and $3.40 a gallon is about $54.64.
How do I convert mpg to L/100 km?
Divide 235.215 by the mpg. 30 mpg is about 7.84 L/100 km.
Why is my dashboard mpg different?
Onboard computers estimate fuel use and can be off by a few percent. The full-tank method is more reliable.
Does it work for electric cars?
Yes. Tick Electric vehicle and enter kWh instead of gallons.
Last updated: November 12, 2026