How to use the boat fuel range calculator
- Choose the fuel unit and the speed unit you use.
- Enter the tank size, the fuel burn per hour at your cruising speed and that speed.
- Enter the share of the tank you want to keep as a reserve.
- Optionally open the advanced options to add other speeds with their burn rates, then read the range, the one-way distance and the rule of thirds distance.
Formula
usable fuel = tank x (1 - reserve percent)
running time = usable fuel / burn per hour
range = running time x speed, safe one-way distance = range / 2
rule of thirds distance out = tank / 3 / burn per hour x speed
With the rule of thirds, one third of the fuel is used going out, one third coming back and one third is kept in reserve, so the farthest you plan to go is one third of the range of the whole tank.
60 gallons at 10 knots
A tank of 60 gallons, a burn of 6 gallons an hour at 10 knots and a 10% reserve leaves 54 gallons, which is 9 hours of running and a range of 90 nautical miles. The safe one-way distance is 45 nautical miles, and the fuel used is 0.6 gallons per nautical mile.
The rule of thirds splits the tank into three 20 gallon parts of 3.33 hours each, so the farthest to go out is 33.3 nautical miles. At 7 knots with a burn of 2.8 gallons an hour the range grows to 135 nautical miles, and at 14 knots with 11 gallons an hour it falls to 68.7.
Why range changes with speed
Fuel burn rises faster than speed, so a boat that runs slower often goes much farther on the same fuel. The comparison table lets you try other speeds with their own burn rates. Enter the burn you actually measure at each speed, from a flow meter or by logging the fuel used over a known distance.
To cost a trip, use the Fuel Cost Calculator. The Hull Speed Calculator and the Prop Slip Calculator look at the speed the boat can reach, and the Road Trip Fuel Stops Calculator does the same job for a car. Check the engine's cooling fluid too, with the Coolant Mix Calculator.
Reserves and safety
Winds, currents, sea state, extra load and a fouled hull can all raise fuel use, sometimes by a lot. A reserve covers them, but only a margin you are comfortable with. Recheck your figures after any change to the boat, and follow the safety rules of your waters.
Assumptions and limits
- The burn rate is the same for the whole trip, which holds at a steady cruising speed.
- The tank size is the fuel you can actually draw.
- Distance is in nautical miles when the speed is in knots, and in miles when it is in miles per hour.
- Wind, current, load and sea state are not modeled.
- These are planning figures and not navigation advice.
Frequently asked questions
What is the rule of thirds?
It is a boating guideline to use a third of the fuel to go out, a third to come back and keep a third in reserve.
Is the one-way distance the same as the rule of thirds distance?
No. The one-way distance uses your reserve percent, and the rule of thirds always keeps a third. With a 10% reserve the one-way distance is larger.
How do I find my fuel burn?
Use a fuel flow gauge or log the fuel added after a known time at a steady speed.
Why are liters and gallons both offered?
The arithmetic is the same, so the tool just labels the result in the unit you choose. Enter all fuel values in that unit.