Fuel Range at Sea Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026Fuel Range at Sea Calculator
How far can you actually go and safely make it back? This calculator applies a fuel reserve before calculating usable cruising range, following the well-known boating "1/3 rule."
Formula
Usable Fuel = Tank Capacity x (1 - Reserve%)
Endurance Hours = Usable Fuel / Fuel Consumption Rate
Range = Endurance Hours x Cruising Speed
Example
100-gallon tank, 8 GPH consumption, 20-knot cruising speed, 33% reserve:
Usable Fuel = 67 gallons → Endurance = 8.38 hours → Range = 167.5 nm
The "1/3 Rule"
A well-known boating safety rule of thumb recommends using 1/3 of fuel to reach a destination, 1/3 to return, and keeping 1/3 in reserve for unexpected conditions - headwinds, currents, weather delays, or emergencies that increase fuel consumption or require extra running time beyond the planned route.
Why Building in a Reserve Matters
Running out of fuel at sea is a genuinely dangerous situation, not merely an inconvenience - this calculator explicitly subtracts your chosen reserve percentage before calculating usable range, rather than assuming full tank capacity is available for planned cruising distance, helping build safety margin directly into trip planning rather than as an afterthought.