Gas Mileage Calculator
Find MPG, L/100km, and cost per distance.
Mileage Results
What is a Gas Mileage Calculator?
A gas mileage calculator measures your vehicle's fuel efficiency — how far it travels on the fuel it burns. Enter the distance driven and fuel used, and the tool reports your economy in miles per gallon (MPG) and liters per 100 kilometers (L/100km), the two standard measures of fuel consumption. Add a fuel price and it also works out what each mile or kilometer costs.
MPG is the convention in the United States — higher is better, since you cover more ground per gallon. Most of Europe and Asia quote L/100km instead, where lower is better. Reporting both figures lets you compare any car against ratings published in either system.
Window-sticker ratings rarely match daily driving, so measuring your economy over several tanks is the reliable way to budget fuel costs and catch problems early — a sudden drop in MPG is often the first symptom.
The Gas Mileage Formulas
Everything the calculator does reduces to a few divisions:
MPG = Distance Driven ÷ Fuel Used
L/100km = (Fuel Used ÷ Distance Driven) × 100
L/100km = 235.214583 ÷ MPG
Cost per Mile = (Fuel Used × Fuel Price) ÷ Distance Driven
- Unit System — switches the input labels between Miles/Gallons and Kilometers/Liters; either way, both MPG and L/100km appear in the results.
- Distance Driven — miles or kilometers between fill-ups, best taken from your trip odometer.
- Fuel Used — gallons or liters burned, read off the pump at your next fill-up.
- Fuel Price (optional) — price per gallon or per liter; leave blank and the cost results stay hidden.
The constant comes from exact definitions: 3.785411784 liters per US gallon and 1.609344 kilometers per mile, so 3.785411784 × 100 ÷ 1.609344 = 235.214583. Cost per 100 miles (or 100 km) is the per-unit cost × 100.
Worked Example: A 300-Mile Tank
Fill the tank, reset the trip odometer, and drive 300 miles; the next fill-up takes exactly 10 gallons at $3.50 per gallon:
- MPG: 300 ÷ 10 = 30.00 MPG.
- L/100km: 235.214583 ÷ 30 = 7.840486, which rounds to 7.84 L/100km.
- Cost per mile: (10 × $3.50) ÷ 300 = $35.00 ÷ 300 = $0.116667, displayed as $0.12 per mile.
- Cost per 100 miles: $0.116667 × 100 = $11.6667, displayed as $11.67.
Metric mode runs the same math: 500 km on 40 liters at $1.50 per liter gives (40 ÷ 500) × 100 = 8.00 L/100km, and 235.214583 ÷ 8 = 29.401823, or 29.40 MPG. Cost is (40 × $1.50) ÷ 500 = $0.12 per km and $12.00 per 100 km.
MPG to L/100km Quick Reference
Each L/100km figure is 235.214583 divided by the MPG value, rounded to two decimals.
| MPG | L/100km | MPG | L/100km |
|---|---|---|---|
| 15 | 15.68 | 35 | 6.72 |
| 20 | 11.76 | 40 | 5.88 |
| 25 | 9.41 | 45 | 5.23 |
| 30 | 7.84 | 50 | 4.70 |
How to Use
- Choose your Unit System: Miles/Gallons or Kilometers/Liters; the input labels update automatically.
- Enter the Distance Driven since your last fill-up; your trip odometer reading is ideal.
- Enter the Fuel Used in gallons or liters from the pump receipt.
- Optionally enter the Fuel Price per gallon or liter to unlock the cost results.
- Click Calculate Mileage to see MPG, L/100km, and, with a price entered, the cost per mile or km and per 100 miles or 100 km.
Frequently Asked Questions
Which is better, high MPG or low L/100km?
Both signal better efficiency — they are inverse views of the same measurement. MPG counts distance per unit of fuel, so more is better; L/100km counts fuel per fixed distance, so less is better. The two convert through 235.214583: 30 MPG equals 7.84 L/100km, and 8 L/100km equals 29.40 MPG.
Why is my actual mileage different from the window sticker?
Official ratings come from standardized lab tests. Real economy shifts with speed, traffic, hills, cargo, air conditioning, tire pressure, and cold weather. Quantify the gap: if your car averages 30 MPG in mild weather but 27 MPG on winter tanks, that is a (30 − 27) ÷ 30 = 10% drop, and a 12-gallon fill-up shrinks from 360 miles of range to 324.
Can I compare two vehicles?
Yes — run the calculator for each vehicle with the same distance and fuel used, then compare MPG. Vehicle A driving 300 miles on 10 gallons returns 30 MPG; Vehicle B on 12 gallons returns 300 ÷ 12 = 25 MPG. Over 12,000 miles a year at $3.50 per gallon, A burns 400 gallons ($1,400) versus 480 gallons ($1,680) for B — $280 less per year.
How do I measure fuel used accurately?
Use the full-tank-to-full-tank method: fill until the pump clicks off, reset the trip odometer, and drive normally. At the next fill-up, stopping at the first click again, the receipt shows fuel used and the trip odometer shows distance. Single tanks vary with pump calibration and temperature, so average three or four tanks.
How do I convert MPG to L/100km by hand?
Divide 235.214583 by the MPG figure: 25 MPG uses 235.214583 ÷ 25 = 9.41 L/100km. For the reverse, divide by L/100km — 6 L/100km is 235.214583 ÷ 6 = 39.20 MPG. The constant assumes US gallons (3.785411784 liters), not the larger UK imperial gallon.
Is upgrading to a higher-MPG car always worth it?
Savings shrink as MPG climbs, because fuel burned falls with the reciprocal of MPG. At 12,000 miles and $3.50 per gallon, moving from 15 to 20 MPG cuts annual cost from $2,800 to $2,100 — $700 saved; moving from 40 to 45 MPG saves only $116.67 ($1,050 down to $933.33). Compare gallons burned per year, not the MPG gap.
What is considered good gas mileage?
Roughly, 30 MPG (7.84 L/100km) combined is decent for a gasoline sedan, 40 MPG (5.88 L/100km) is very good, and 50 MPG (4.70 L/100km) or better is hybrid territory. For trucks and large SUVs, 20-25 MPG (11.76-9.41 L/100km) is common. Judge a vehicle against its own class, not an absolute number.