Estimate fuel usage, range, and costs for your boat
Modern 4-stroke outboard
Common rail diesel engine
Twin gasoline V8 engines
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Download NowRange is decided at the fuel dock long before the throttle is opened, and it comes down to how many gallons per hour your engine actually burns at the speed you intend to cruise. Over-estimating economy is how boats run tanks dry; the honest way to plan is to compute burn from brake-specific fuel consumption and then build the passage around it.
Fuel burn climbs steeply with power, roughly with the cube once a hull is on plane, so a small reduction in cruise rpm can add hours of range. Engine type matters just as much: a modern turbo diesel sips where an old carburetted petrol V8 drinks, and the difference is written into the specific-consumption figure.
Enter horsepower, engine type and your typical throttle setting, and the calculator estimates gallons per hour, suggested tank size for a target range, and burn at several settings so you can see how speed trades against distance.
Power Used = HP × (Throttle % ÷ 100)²; Gallons/Hour = (Power Used × BSFC) ÷ Fuel Weight; Range = Tank ÷ Gal/hr| Engine Type | BSFC (lb/hp-hr) | Character |
|---|---|---|
| Turbo diesel | 0.36 - 0.38 | Most economical, high torque |
| Naturally aspirated diesel | 0.40 | Robust cruiser standard |
| Petrol EFI inboard | 0.45 - 0.48 | Clean, responsive, thirstier |
| Carburetted four-stroke | 0.50 - 0.55 | Older petrol engines |
| Two-stroke outboard | 0.58 - 0.60 | Simple, most fuel-hungry |
Typical brake specific fuel consumption by engine type.
Formula verified against standard naval-architecture references · Updated October 2026
It depends on horsepower, engine type and throttle. A rule of thumb is that a four-stroke petrol inboard burns close to a tenth of a gallon per hour per rated horsepower at cruise, and a diesel burns about half that. The precise route is power used times brake specific fuel consumption divided by fuel weight, which is exactly what this calculator does so you get a figure for your engine and setting rather than a generic guess.
Because the power needed to push a hull rises faster than speed. Once a planing boat is over the hump, horsepower and therefore fuel demand scale roughly with the cube of speed, so cutting from 30 to 24 knots can drop burn by around half. Displacement hulls show the same effect near their hull speed. Backing off a few rpm is the single most powerful tool for stretching range.
Diesel wins on fuel per mile almost every time, because a diesel converts more of each gallon into shaft horsepower and diesel carries slightly more energy per litre. Petrol engines cost less to buy and are simpler to service in some regions, but they burn noticeably more for the same work. Where range and running cost dominate, the lower BSFC of a diesel decides the argument.
Size it from the longest leg you expect to run, divided by cruise burn, plus a reserve of at least twenty percent for wind, current and detour. That gives the minimum usable capacity, to which you add the physical room and weight allowance. The calculator reports a suggested tank for your target range; when in doubt and where weight allows, carry more, because tankage is the cheapest safety margin on open water.
Greatly, and it is the most overlooked variable. A growth-covered hull adds skin friction that can raise fuel consumption by ten to twenty percent or more at the same speed, so a boat that once did 4 gal/hr may quietly drift to 5. The fix is cleaning and good antifouling, and it pays for itself in fuel, which is why real logged figures beat any table.
They are a sound planning starting point, not a substitute for your own log. Advertised BSFC is a bench average, and actual burn varies with load, trim, prop, fouling, sea state and altitude. Run one honest tank against engine hours and nautical miles, enter your measured cruise burn, and the model snaps to your boat, at which point it becomes dependable for passage planning.
Idling and manoeuvring burn little fuel per hour but very few miles, so they drag average economy down over a short hop. The steep cost is sustained high throttle, not idling. For fuel planning what matters is cruise burn over the passage, so keep dock-side idling to a reasonable level and let the calculator focus on the speed you will actually cover distance at.