Boat Fuel Consumption & Range Calculator
Estimate a boat’s fuel burn from engine horsepower, engine type and throttle (or RPM) — litres and gallons per hour, nautical miles per litre, safe range and endurance with a reserve rule you choose, fuel and cost for a planned trip, and a burn table for every throttle setting. Already know your burn rate? Enter it directly.
How it works
gal/h = hp used × BSFC (lb/hp/h: petrol 4-stroke 0.45, 2-stroke DI 0.50, 2-stroke carb 0.55, diesel 0.38) ÷ fuel density (6.1 lb/gal petrol, 7.1 diesel) × (1 + extra %); hp used = hp × % power, or hp × (RPM ÷ max RPM)³; L/h = gal/h × 3.785; economy = speed ÷ burn; range = tank × (1 − reserve) ÷ burn × speed; trip fuel = distance ÷ speed × burn
Marine engines burn fuel roughly in proportion to the power they are producing, so a specific-fuel-consumption figure predicts consumption without a flow meter: a petrol four-stroke uses about 0.45 pounds of fuel per horsepower per hour, which works out to the old boater’s rule of a tenth of the horsepower in gallons per hour at full throttle, and diesels burn about 40% less for the same power. Pick a throttle setting, type a percentage, or enter your cruise RPM and the engine’s top RPM — power follows the propeller curve, roughly the cube of RPM, so 4,000 of 6,000 RPM is only about 30% power. If you already know your burn from a fuel-flow gauge or your fill-up log, switch to “I know my burn rate” and skip the estimate. Dividing speed by burn gives nautical miles per litre or gallon, and the tank size turns that into endurance and range. The rule of thirds — a third out, a third back, a third in reserve — is the standard safety margin; you can choose a smaller reserve, and the planned-trip line tells you how much fuel to carry for a given run. Because planing hulls need most of their power to climb onto the plane and then cruise efficiently at 60–75% throttle, the best economy is usually at a fast cruise, not at idle.
Worked example
Twin 125 hp four-strokes (250 hp) at 75% throttle and 24 knots: about 52 L/h (13.8 gal/h), 0.46 nm per litre; a 300 L tank gives a safe round trip of about 92 nm with a third in reserve, so a 20 nm run out and back (40 nm) needs about 87 L, or 131 L with the reserve. At 1.80 per litre, each hour costs about 94. Entering 4,000 RPM against a 6,000 RPM maximum instead gives (4,000 ÷ 6,000)³ = 30% power, or about 21 L/h.
Frequently asked questions
Why does my boat burn more than this?
Load, a dirty bottom, the wrong propeller pitch, rough water and running just below planing speed all raise burn. Add an allowance in More options, or switch to “I know my burn rate” and enter what your fuel-flow gauge or fill-up log shows.
What is the most economical speed?
For planing boats, the slowest speed that keeps the hull cleanly on the plane (often 3,500–4,000 rpm); for displacement boats, well below hull speed — fuel use rises with the cube of speed.
How do I estimate burn from RPM?
Choose “From engine RPM” and enter your cruise RPM and the wide-open-throttle RPM. A correctly propped engine follows the propeller curve, where power rises with the cube of RPM: 80% of top RPM is about 51% power, 67% of top RPM about 30%.
How much fuel should I carry for a trip?
Enter the one-way distance and whether you are coming back. The calculator works out the fuel for the run at your speed and burn, then adds your reserve — with the rule of thirds, the trip fuel is divided by two-thirds — and tells you whether it fits the tank.
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Planning estimate for education. Always follow the manufacturer’s manual, official charts and your training; conditions on the water and in the air change quickly.