About the Boat Fuel Consumption Calculator
This boat fuel consumption calculator estimates how many gallons per hour (GPH) your engine burns from its rated horsepower, the share of that power you use at cruise, and the engine type. From the burn rate and your cruising speed it works out nautical miles per gallon, the fuel and cost for a planned run, how long the trip takes, and how far you can go on a tank while keeping a reserve.
It is meant for powerboat owners planning a day trip or a passage, renters budgeting fuel, and buyers comparing outboard and diesel setups. Use it before you leave the dock to check that your tank covers the round trip with fuel to spare, or to compare the cost of running at wide-open throttle versus an efficient cruise setting.
The estimate uses typical brake-specific fuel consumption figures: about 0.50 lb per horsepower-hour for four-stroke gasoline engines, 0.55 for direct-injection two-strokes, 0.60 for older carbureted two-strokes and 0.40 for diesels. Real burn depends on hull, load, trim, sea state and propeller, so treat the result as a planning figure and check it against your fuel-flow gauge or the engine maker’s performance bulletin.
With the default inputs, the fuel burn is 12.3 gal/h. Change any value above to recalculate instantly.
How to use the boat fuel consumption calculator
- 1Choose your engine type and enter total horsepower.
- 2Set the throttle load you plan to cruise at — 60% is a typical planing cruise.
- 3Enter your cruising speed in knots and the total trip distance in nautical miles.
- 4Add tank size, the reserve you want to keep and the local fuel price.
- 5Check burn, trip cost and range, and use the table to compare throttle settings.
Formula and method
Engines burn fuel in proportion to the power they actually produce. Multiplying rated horsepower by the throttle load gives the working horsepower; multiplying that by the brake-specific fuel consumption (pounds of fuel per horsepower per hour) gives pounds of fuel per hour, and dividing by the fuel’s weight per gallon (about 6.1 lb for gasoline, 7.1 lb for diesel) converts it to gallons per hour.
In practice this means a four-stroke gasoline engine burns roughly 1 gallon per hour for every 12 working horsepower, and a diesel roughly 1 gallon for every 18. Trip time is distance ÷ speed, trip fuel is burn × time, and range is the usable fuel after your reserve divided by the burn rate, multiplied by speed. Load is approximate because a propeller’s power demand rises steeply with rpm.
- HP
- Total rated horsepower of all engines
- load
- Share of rated power used at cruise (0–1)
- BSFC
- Brake-specific fuel consumption, lb/hp/h (0.40–0.60)
- fuel weight
- Pounds per US gallon (6.1 gasoline, 7.1 diesel)
Worked examples
250 hp four-stroke outboard at 60% load
At 60% load the engine produces 150 hp; 150 × 0.50 lb ÷ 6.1 lb/gal ≈ 12.3 gal/h. At 25 knots that is about 2.03 nm per gallon, so a 40 nm run (1.6 hours) uses about 19.7 gallons costing $88.52. Keeping a 33% reserve of an 80-gallon tank leaves 53.6 usable gallons, enough for about 109 nm.
Twin 300 hp diesels on a cruiser
Working power is 600 × 0.70 = 420 hp; 420 × 0.40 ÷ 7.1 ≈ 23.7 gal/h. A 60 nm passage at 18 knots takes 3.33 hours and burns about 78.9 gallons ($394). With 240 usable gallons after a 20% reserve, the range is about 183 nm.
90 hp carbureted two-stroke on a skiff
At half throttle the engine makes 45 hp; 45 × 0.60 ÷ 6.1 ≈ 4.43 gal/h, or about 4.5 nm per gallon at 20 knots. A 15 nm outing takes 45 minutes and burns about 3.3 gallons, roughly $13 of fuel.
Frequently asked questions
How many gallons per hour does a boat use?+
A rough rule is 1 gallon per hour for every 10–12 horsepower actually produced by a gasoline engine and about 1 gallon per 18 hp for a diesel. A 150 hp four-stroke outboard cruising at 60% load burns roughly 7–8 gal/h, and at wide-open throttle roughly 12–15 gal/h depending on the engine.
What is the rule of thirds for boat fuel?+
The rule of thirds is a common boating safety guideline: use one third of your fuel going out, one third coming back and keep one third in reserve for weather, currents or detours. Set the reserve to 33% in this calculator to apply it.
Why does fuel economy drop so much at full throttle?+
Propeller power demand rises roughly with the cube of rpm, while speed rises far more slowly once the hull is planing. The last few knots therefore cost a lot of extra fuel. Most planing boats get their best miles per gallon just above planing speed, around 50–65% load.
How accurate is a horsepower-based fuel estimate?+
It is a planning estimate for a typical, properly propped boat, and real burn can differ noticeably from it. Heavy loads, poor trim, a fouled bottom, rough water and the wrong propeller all increase burn. A fuel-flow meter or the engine maker’s performance bulletin for your hull gives the most accurate number.
How do I convert gallons per hour to miles per gallon?+
Divide speed by burn rate. At 25 knots and 12.5 gal/h you get 2 nautical miles per gallon. For statute miles multiply knots by 1.1508 first, so the same boat gets about 2.3 mpg.