Boat Fuel Calculator
A third out, a third back, a third spare.
Work out Boat Fuel. A third out, a third back, a third spare. Free, with no account and nothing to install.
Fuel for the passage
200 l
5 h 0 min at 2 l/nm · within the usable 268 l
The third-tank rule — a third out, a third back, a third in reserve — is conservative by design, and it exists because the sea does not always let you come back the way you went. Wind, tide and a headwind on the return leg all cost fuel that the outbound passage did not. Fuel burn climbs roughly with the cube of speed near hull speed, so slowing 20% often halves the burn on a displacement hull. Best economy usually sits around three-quarters of hull speed, well below anything that feels fast. Litres per nautical mile is the figure to track, not litres per hour. It combines burn and speed into the number that actually determines whether you get there, and it is what makes a slow passage cheaper rather than just longer.
How the Boat Fuel Calculator works
Fuel for a passage with the reserve held back separately, plus range on a full tank and after reserves, cost per nautical mile, and where best economy sits on a displacement hull.
Also known as: boat fuel for a passage · litres per nautical mile · third tank rule fuel · most economical boat speed
Not financial advice. This calculator is for planning and illustration, not financial advice. Real products carry fees, taxes, and terms it does not model. Confirm figures with your lender or a qualified adviser before committing.
Frequently asked questions
What is the third-tank rule?
A third of the tank out, a third back, a third in reserve. It is deliberately conservative, because the sea does not always let you come back the way you went.
How do I work out fuel per nautical mile?
Burn rate divided by speed. That single figure combines both into what actually determines whether you get there — better to track than litres per hour.
Why does slowing down save so much fuel?
Because burn climbs roughly with the cube of speed near hull speed. Slowing 20% on a displacement hull often halves the burn, which is why best economy sits well below anything that feels fast.
What speed is most economical?
Around three-quarters of hull speed for a displacement boat. A planing hull is different — it is least efficient just below planing speed and improves once it is up.
How much reserve should I hold back?
A third is the traditional answer and this page takes whatever you enter. Headwinds, foul tide and a diversion all draw on it, and none of them announce themselves in advance.
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