How to use this calculator
Select U.S., metric or UK units. Enter your best estimate of the fuel currently in the tank and the amount you want to leave unused. Enter measured efficiency from similar driving conditions. The result is the estimated distance using only the fuel above your selected reserve. Use our MPG calculator to derive efficiency from recorded fill-ups.
This is a constant-efficiency planning estimate, not a promise that a fuel stop is reachable. Gauge readings, usable tank volume, terrain, weather, speed and idling can change the outcome. The reserve is your entered assumption, not a manufacturer-approved minimum. Do not use this result to justify driving until empty.
The formula
A worked example
With 12 U.S. gallons available, a 2-gallon reserve and 30 U.S. mpg, the estimate uses 10 gallons and gives 300 miles. At 25 mpg the same fuel gives 250 miles. In metric units, 40 liters available minus a 5-liter reserve at 7 L/100 km gives 500 km.
Range sensitivity to fuel economy
Ten U.S. gallons available for driving after reserve. Illustrative values, not vehicle specifications.
| U.S. mpg | Estimated miles |
|---|---|
| 20 | 200 |
| 25 | 250 |
| 30 | 300 |
| 35 | 350 |
Common questions
Can I use the fuel gauge fraction times tank capacity?
That can be a rough starting estimate, but the gauge and usable volume may not be exact or linear. Enter a conservative estimate and plan refueling early. The calculator does not read your vehicle or know its low-fuel warning threshold.
What happens if all available fuel is reserved?
Estimated planning range is zero. If the reserve exceeds available fuel, the tool shows an input error rather than a negative driving range.
Why is the answer different from dashboard distance-to-empty?
The dashboard may use recent driving and its own estimate of fuel remaining. This tool uses a fixed efficiency and your chosen reserve, so the assumptions can differ substantially.
Should I enter highway or city MPG?
Use an efficiency that represents the intended route, ideally measured in comparable conditions. Recalculate with worse efficiency to see how sensitive the range is. Do not assume the example MPG is your vehicle rating.
Are U.S. and UK MPG interchangeable?
No. Use U.S. gallons with U.S. MPG, or Imperial gallons with UK MPG. The metric mode pairs liters with L/100 km. Use the fuel-economy converter if your inputs use different systems.
Sources & calculation method
The formula follows the definition of fuel economy: distance per fuel volume. For L/100 km, invert the consumption rate to obtain distance per liter. DOE/EPA: Many Factors Affect MPG explains why real driving can differ from an efficiency estimate. The subtraction of a user-chosen reserve is this tool’s planning assumption, not a reserve recommendation from that source.
References accessed September 11, 2026. Sources provide context and have not reviewed or endorsed these calculators.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.