Two-leg passage
Sixty nautical miles at six knots plus five direct engine hours gives fifteen propulsion hours. At four units per hour with two auxiliary units, trip burn is 62 units before margins.
Boat Trip Fuel Burn & Reserve Calculator
Created by: James Porter
Last updated:
Estimate propulsion and auxiliary fuel burn by leg, then add measured-burn uncertainty, an explicit reserve, unusable fuel, and departure-fuel shortfall checks.
Boat Trip Fuel Burn & Reserve Calculator
BoatingBuild a leg-by-leg fuel scenario with measured burn, auxiliary use, uncertainty, reserve, unusable fuel, and departure-fuel checks.
nm
nm
h
kn
unit/h
h
unit/h
%
%
units
units
What is a Boat Trip Fuel Burn & Reserve Calculator?
This calculator builds a transparent trip-fuel scenario from entered nautical-distance or engine-hour legs, measured hourly burn, auxiliary use, uncertainty, reserve, unusable fuel, and fuel aboard.
It keeps baseline burn and planning margins separate. That distinction matters because an unexplained large number does not show whether the assumption came from measured consumption, uncertainty, reserve policy, or inaccessible fuel.
Fuel planning remains the skipper’s responsibility. The tool does not know tank shape, transfer capability, sea state, current, weather, fouling, load, engine condition, fuel quality, route changes, or safe refuge options.
How the Boat Trip Fuel Burn & Reserve Calculator Works
Distance legs are divided by cruise speed unless direct engine hours are entered. Each leg is multiplied by the measured propulsion burn rate.
Auxiliary hours and burn are added separately. Uncertainty increases expected trip burn, then reserve is calculated from that planning burn and unusable fuel is added.
Entered departure fuel is compared with required departure fuel, and usable remaining after planning burn is reported without counting unusable fuel.
Formulas and assumptions
Trip burn = Σ leg hours × burn + auxiliary hours × auxiliary burn
Planning burn = trip burn × (1 + uncertainty %)
Required departure fuel = planning burn + reserve + unusable fuel
Example Calculations
Visible margins
Ten percent uncertainty raises 62 units to 68.2. A twenty-percent reserve adds 13.64, and three unusable units produce 84.84 required departure units.
Common Applications
- Passage fuel briefing
- Comparing route legs
- Making auxiliary use visible
- Checking fuel-aboard shortfall
Fuel-Planning Tips
Use measured fuel flow for the vessel and operating mode.
Reconcile logged fuel used against tank replenishment and progress.
Plan safe alternatives before margins are approached.
Frequently Asked Questions
Where should the burn-rate input come from?
Use repeatable measurements from the actual vessel or documented manufacturer data for the relevant engine, RPM, load, and conditions. Tank gauges and one short outing can be misleading. Keep the source, units, sea state, load, trim, fouling, and test method with the value.
What reserve percentage should I use?
The calculator deliberately does not prescribe one universal reserve. The skipper must choose a policy appropriate to vessel documentation, route, weather, current, traffic, fuel availability, transfer arrangements, local requirements, and safe alternatives.
Is calculated range or fuel use guaranteed?
No. Wind, waves, current, loading, fouling, trim, manoeuvring, idling, fuel quality, transfer problems, leaks, generator use, and mechanical condition can materially change consumption and usable fuel.
How are distance and engine-hour legs combined?
Each leg can be entered as distance or direct engine hours. Distance is divided by the one entered cruise speed; direct hours override distance for that row. All propulsion hours use the same measured burn rate, so use separate scenarios when operating modes differ.
What is unusable fuel?
It is fuel entered as physically aboard but not treated as available because of pickup geometry, tank heel, transfer limits, contamination policy, or another documented constraint. It is added to required departure fuel rather than counted as reserve.
Does buying reserve fuel mean it will remain at arrival?
Not necessarily. The result is an arithmetic allocation. Real burn can consume the planned reserve, and tank arrangement can make fuel inaccessible. Monitor fuel used and remaining, compare with progress, and retain safe alternatives.
Sources and References
- Royal Yachting Association. Passage Planning, accessed July 16, 2026; https://www.rya.org.uk/water-safety/passage-planning-and-navigation/passage-planning/.
- U.S. Coast Guard Auxiliary. Fueling and recreational boating safety resources, accessed July 16, 2026; https://wow.uscgaux.info/content.php?unit=B-DEPT.
- National Weather Service. Safe Boating and marine forecast resources, accessed July 16, 2026; https://www.weather.gov/safety/safeboating-marine.
- Mercury Marine. Propeller slip explanation and theoretical-speed convention, accessed July 16, 2026; https://www.mercurymarine.com/us/en/lifestyle/dockline/how-to-calculate-propeller-slip.
- Unit conversions and arithmetic assumptions are documented in each calculator method.
Planning limitation
This is arithmetic from user-entered fuel data. It does not guarantee range, prescribe a reserve, certify tank capacity, detect leaks or contamination, or decide whether a voyage is safe.