Boating Calculators
Boat fuel range.
Enter your tank, the fuel burn per hour at your cruising speed, and the speed, and get usable range with a reserve held back. Runs in your browser; nothing is sent or stored.
Inputs
Result
Fill the form and press Calculate.
Range = speed x usable fuel / burn per hour. A 100 gal tank burning 10 gal an hour at 20 knots, holding back one third as reserve, gives 133 nautical miles of range, so the farthest you should go and still return is about 67 nautical miles.
Worked example
100 gal tank, 10 gal/h at 20 knots, one third kept in reserve. Usable fuel: 66.7 gal. Endurance: 66.7 / 10 = 6.67 h. Range: 6.67 x 20 = 133 nautical miles (153 statute miles, 247 km). Efficiency is 20 / 10 = 2.0 nm per gallon, and at $4 a gallon that is 2.00 dollars per nautical mile.
How it works
Endurance (h) = tank x (1 - reserve) / burn per hour; range (nm) = endurance x speed. A nautical mile is exactly 1,852 metres and a knot is one nautical mile per hour (about 1.15078 mph, 1.852 km/h). Gallon conversions use 3.785411784 L per US gallon and 4.54609 L per imperial gallon.
The burn rate must be measured at the speed you enter, because fuel use rises sharply with speed and with headwinds, current, sea state, load and a dirty hull. The one-third rule is a long-standing rule of thumb, and a reserve is not optional: plan with margin, check the weather, and carry what you need for the worst leg. This is a planning aid, not navigation advice.
Sources opened 2026-10-08: Nautical mile - Wikipedia; Knot (unit) - Wikipedia. Planning aid only, not professional, engineering or safety advice. Your vehicle, trailer, tire, battery and boat manuals and the weight labels on your equipment come first; when in doubt, weigh it or ask a qualified professional.
FAQ
What is the one-third rule for boat fuel?
Plan to use one third of your fuel going out, one third coming back, and keep one third as reserve. It is a rule of thumb, not a regulation.
How do I find my boat fuel burn per hour?
Use a fuel flow meter, or divide the fuel you add after a trip by the engine hours run at that speed. Manufacturer curves are only a guide.
How far can I go and still get back?
About half of the usable range, because you must return. The result panel shows that as the round-trip radius.
How many nautical miles is a mile?
A nautical mile is 1,852 metres, about 1.15 statute miles. A knot is one nautical mile per hour.
Why is my real range shorter?
Wind, waves, current, load, hull fouling and engine condition all raise burn. Measure at your real conditions and keep a reserve.