Boat Fuel & Range Calculator
Gallons an hour from engine power, miles per gallon at any throttle setting, and how far you can go while still keeping a third in reserve.
Burn and range across the throttle range
| Throttle | Burn | Speed | Miles per gallon | Range on usable fuel |
|---|
Speeds in the table scale roughly with the cube root of power, which is how a planing hull behaves once it is up. Below the planing threshold that relationship breaks down badly — those rows are the least trustworthy in the table.
Where the burn figure comes from
A marine engine burns roughly half a pound of gasoline, or four tenths of a pound of diesel, per horsepower-hour. Divide by the weight of a gallon — 6.1 pounds for gasoline, 7.2 for diesel — and you have gallons an hour at full power. A 300-horsepower gas engine lands near 24.6 gallons an hour flat out; the same power in diesel is about 16.7.
That ratio is why diesel wins on long passages despite costing more per gallon and more to install. It is also why the throttle matters more than anything else on this page: fuel flow follows power, so backing off to seventy per cent cuts the burn by nearly a third.
The rule of thirds
A third of the tank out, a third back, a third untouched. It sounds conservative until the wind gets up on the way home, the sea turns the trip into a slog, and the burn you planned with turns out to have been the calm-water figure.
There is no coasting to a fuel dock. That single fact is why the reserve on a boat is a third rather than the warning light you would accept in a car.
What these numbers do not know
- These are planning figures from engine power, not measurements. A fuel-flow meter or a carefully logged tank beats them, and the difference is often twenty per cent.
- Every figure here assumes a clean hull, a flat sea and no wind. A fouled bottom, a head sea or a heavily loaded boat all push the burn up while pushing the speed down, which hits range twice.
- The most economical speed for a planing hull is usually just above the point where it settles onto the plane — pushing through the hump burns fuel for very little speed, and staying below it is slow but cheap.
- A boat is not a car. There is no coasting to a station, the current can turn against you, and the reserve is what covers the day being longer than planned.
How to use
- Enter engine horsepower, fuel type and tank size.
- Set the throttle you actually cruise at and the speed you make there.
- Read the range on usable fuel, and the turn-around distance in the table.
Frequently asked questions
How do I work out gallons per hour from horsepower?
A marine engine burns roughly half a pound of gasoline per horsepower-hour, or four tenths of a pound of diesel. Divide by the weight of a gallon — 6.1 pounds for gasoline, 7.2 for diesel — for gallons an hour at full power.
How much fuel does a 300 hp engine use?
About 24.6 gallons an hour flat out on gasoline, or 16.7 on diesel. At a more typical seventy per cent throttle those drop to roughly 17 and 12, because fuel flow follows power.
What is the rule of thirds?
A third of the tank to get there, a third to get back, a third untouched. It is more conservative than anything you would accept in a car, for the simple reason that a boat cannot coast to a fuel dock.
Why is diesel more efficient?
Higher compression extracts more work from each pound of fuel, and diesel packs more energy per gallon to begin with. The combination is why diesel wins on long passages despite costing more to buy and more to install.
Is this accurate enough to plan a passage on?
It is a planning figure, not a measurement, and the honest gap is often twenty per cent. A fuel-flow meter or a carefully logged tank beats it. Every number here also assumes a clean hull and a flat sea — fouling and a head sea raise the burn while cutting the speed, which hits range twice.
What speed is most economical?
For a planing hull, usually just above the speed where it settles onto the plane. Pushing through the hump burns a lot of fuel for very little speed, so the worst place to sit is right at the transition.
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