Boat Fuel Calculator
Enter your trip distance in nautical miles, cruising speed in knots and fuel burn in gallons per hour, then set a safety reserve. The calculator returns the hours you will be underway, the trip fuel with no reserve, and the total fuel to carry once the reserve is added. Because a knot is one nautical mile per hour, distance over speed gives time with no conversion factor.
How to plan boat fuel
- Enter the trip Distance in nautical miles.
- Enter your Cruise speed in knots.
- Enter your Fuel burn at that speed in gallons per hour (gph).
- Set the Reserve percent you want to hold back as a safety buffer.
- Read Hours underway, Trip fuel and Total with reserve in the result cards.
- Click Copy to save the time underway and the fuel figures as one line.
Examples
100 nm cruise at 20 knots, 10% reserve
distance = 100 nm, speed = 20 kn, burn = 8 gph, reserve = 10%
5 h underway, 40 gal trip fuel, 44 gal with reserve
60 nm run at 25 knots, 20% reserve
distance = 60 nm, speed = 25 kn, burn = 18 gph, reserve = 20%
2.4 h underway, 43.2 gal trip fuel, 51.84 gal with reserve
Frequently asked questions
How does the calculator work?
It divides distance by speed to get hours underway, multiplies those hours by your gallons-per-hour burn to get trip fuel, then multiplies by one plus the reserve percent over 100. So 100 nm at 20 knots and 8 gph is 5 hours, 40 gallons, and 44 with a 10 percent reserve.
How do I find my gallons per hour?
Check the engine maker's fuel curve for your cruising rpm, or measure it by noting fuel used against engine hours over a known run. Burn climbs steeply with speed, so use the gph for the speed you will actually hold.
How much fuel reserve should I keep?
The tool defaults to 10 percent, and many cruisers plan 10 to 20 percent. The well-known third tank rule, a third out, a third back and a third held in reserve, is more conservative and works out near a 50 percent reserve.
Why nautical miles and knots?
One knot is exactly one nautical mile per hour, so nautical miles divided by knots gives hours directly with no factor to remember. Charts and marine electronics use these units already.
Can I plan a round trip?
Yes. Either enter the full round-trip distance, or compute one way and double the trip fuel before adding your reserve. For a fixed speed and burn the total scales straight with distance.
Does it account for current, wind or sea state?
No. It assumes a steady speed and a constant burn rate. Head seas, adverse current, extra load and a fouled hull all push consumption up, so raise the reserve when conditions are rough or uncertain.
What inputs does it reject?
Distance, speed and fuel burn must all be greater than zero, the reserve cannot be negative, and every field must be a number. If any of those fail, the tool shows a short message instead of a result.
Is my trip data private?
Yes. The calculation happens entirely in your browser, so your route, speed and fuel figures are never uploaded or stored on a server.
Learn more
- The one-third fuel rule and boating reserves
How to plan boat fuel with the third tank rule, why gph beats miles per gallon on the water, and when to add extra reserve.
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