You are standing at the fuel dock looking at your engine’s horsepower rating and wondering what that actually means for your wallet on a Saturday out on the water. Boats do not work like cars. There is no odometer counting miles, no simple MPG number on a window sticker, just an engine burning fuel by the hour while the water underneath you keeps changing how hard that engine has to work. Many boaters search for a boat gas mileage calculator, but fuel consumption on the water is usually measured in gallons per hour rather than miles per gallon, since distance traveled is a poor way to track fuel use against constantly shifting current and wave conditions.
We built this marine fuel calculator so you can turn your engine’s horsepower into a real gallons-per-hour number, then into an actual dollar figure for your day on the water. This calculator works for outboards, inboards and sterndrive engines alike, since the underlying formula is the same regardless of mounting type. Enter your horsepower, pick gas or diesel, and choose your throttle setting, and the calculator handles the rest. No spreadsheet, no guessing at the pump before a trip.
Written by the FuelConsumptionCalc Research Team
Reviewed by the Marine Fuel Research Team. Formulas and figures on this page are cross-checked against published marine engine fuel consumption standards and manufacturer specific fuel consumption data.
Sources: marine engine manufacturer fuel consumption specifications (Mercury Marine, Yamaha Marine, Honda Marine), published specific fuel onsumption (SFC) benchmarks for gasoline and diesel marine engines, and boating industry fuel planning guidance (BoatUS, Boating Magazine).
See our full methodology page for how we verify very figure on this site.
Last Reviewed: July 2026
BOAT FUEL CALCULATOR
Estimate your engine’s GPH and trip fuel cost
Your Fuel Estimate
Use this Boat Fuel Calculator to estimate gallons per hour and trip fuel cost for a gas or diesel marine engine. Works for outboards, inboards and sterndrive engines, with separate Economy, Normal, and Fast cruise modes alongside wide open throttle.
Quick Answer
GPH = (Specific Fuel Consumption x Horsepower) / Fuel Specific Weight
Example: 300 HP gas engine at WOT = approximately 24.5 GPH
| Engine Type | Typical GPH at WOT per 100 HP |
| Gasoline (4-stroke) | 8 to 9 GPH per 100 HP |
| Diesel | 5.5 to 6.5 GPH per 100 PH |
| Gasoline at Cruise | 4 to 5.5 GPH per 100 HP |
| Diesel at cruise | 3 to 4 GPH per 100 HP |
How This Boat Fuel Calculator Works
This calculator takes your engine’s horsepower and turns it into gallons per hour, the standard way fuel use is measured on the water since there is no practical way to track miles against constantly shifting water resistance. Enter your horsepower, choose gasoline or diesel, and select whether you are running wide open throttle or a more relaxed cruising speed.
From there, add your planned hours on the water and your local fuel price, and the calculator gives you your total fuel needed and trip cost. If you know your tank size, it also estimates your usable range so you are not guessing how far you can really go before heading back to the dock.
Formula Used
The calculation follows the standard marine fuel consumption formula used by engine manufacturers and marine mechanics:
GPH = (Specific Fuel Consumption x Horsepower) / Fuel Specific Weight
Gasoline engines average roughly 0.50 lbs of fuel burned per horsepower per hour at peak output, and gasoline weighs approximately 6.1 lbs per gallon. Diesel engines average roughly 0.40 lbs of fuel per horsepower per hour, and diesel weighs approximately 7.2 lbs per gallon. Cruising speed typically reduces this consumption rate by 35 to 50 percent compared to wide open throttle.
Example Calculation
Engine: 300 HP gasoline outboard
Throttle: Wide open
Step 1, Apply the Formula:
GPH = (0.50 x 300) / 6.1
Step 2, Calculate:
GPH = 150 / 6.1 = 24.6 GPH
Step 3, Fuel Cost for 4 Hours at $4.00 per gallon:
24.6 x 4 = 98.4 gallons, 98.4 x $4.00 = $393.60
This is the type of calculation the tool above runs the moment you click Calculate, using your own engine numbers instead of this example.
How to Use This Boat Fuel Calculator
Enter your engine’s horsepower, select gas or diesel, and choose wide open throttle or cruising speed. Add your planned hours and fuel price for your total trip cost.
Enter Your Engine Hoursepower
If you are looking for an outboard fuel consumption calculator, this is the field where it starts, enter your total horsepower above to estimate fuel burn. Use your engine’s rated horsepower, found on the engine owling, your owner’s manual, or your boat’s capacity plate. If you run twin or triple engines, enter the combined total horsepower across all engines for an accurate fuel burn figure.
Select Gas or Diesel
Gasoline and diesel engines burn fuel at meaningfully different rates per horsepower, and the two fuels also weigh different amounts per gallon, so this choice changes your result directly rather than acting as a minor adjustment.
Choose Wide Open Throttle or Cruising Spee
Wide open throttle gives you the maximum fuel burn figure your engine manufacturer typically publishes. Cruising speed reflects more realistic day-to-day running, where most boaters actually spend most of their time on the water, and burns noticeably less fuel per hour than WOT.
Add Your Hours Run and Fuel Price
Enter how many hours you plan to run the engine and your local fuel price per gallon to get your total fuel needed and trip cost. For a land-based vehicle towing your boat to the launch, our fuel cost calculator handles that leg of the trip separately.
Boat Fuel Consumption Formula Explained
GPH equals specific fuel consumption multiplied by orsepower, divided by fuel specific weight. This single formula is the industry-standard way marine engine manufacturers and mechanics estimate fuel burn at peak output.
The GPH Formula
GPH = (Specific Fuel Consumption x Horsepower) / Fuel Specific Weight
Specific Fuel Consumption (SFC) measures pounds of fuel burned per horsepower per hour, averaging around 0.50 for gasoline engines and 0.40 for diesel engines. Fuel Specific Weight (FSW) is simply how much a gallon of that fuel weighs, roughly 6.1 lbs for gasoline and 7.2 lbs for diesel.
Worked Example for a 300 HP Outboard
A 300 HP gasoline outboard running at wide open throttle:
GPH = (0.50 x 300) / 6.1 = 24.6 GPH The same engine running at a more relaxed cruising speed, where consumption typically drops by 35 to 50 percent, burns closer to 12 to 16 GPH instead, a meaningful difference if you are planning a full day on the water rather than a short, fast run.
The Quick Estimate Shortcut
If you do not have detailed specific fuel consumption figures handy, a simpler shortcut divides total horsepower by 10 for gasoline engines to get an approximate WOT GPH figure. For diesel engines, the shortcut multiplies horsepower by 0.06 instead. As an example, a 300 HP diesel engine works out to 300 x 0.06, which is approximately 18 GPH. This shortcut is less precise than the full SFC formula but works as a fast mental estimate when you just need a ballpark number before heading out.
Common Engine Sizes and Their Fuel Burn
Seeing your own engine size next to a few common configurations often makes GPH easier to picture than the formula alone. The figures below use the gasoline SFC formula at wide open throttle.
| Engine Setup | Horsepower | Gas GPH at WOT |
| 115 Hp outboard | 115 HP | 9.4 GPH |
| 150 Hp outboard | 150 HP | 12.3 GPH |
| 250 Hp outboard | 250 HP | 20.5 GPH |
| 300 Hp outboard | 300 HP | 24.6 GPH |
| Twin 300 Hp outboard | 600 HP | 49.2 GPH |
A single 150 HP outboard on a smaller center console burns roughly half what a 300 HP engine on a larger boat does at wide open throttle, which is worth keeping in mind when comparing fuel cost between different boats rather than assuming all outboards burn similarly.
Wide Open Throttle vs Cruising Speed
Wide open throttle represents your engine’s maximum fuel burn rate, while cruising speed typically uses 35 to 50 percent less fuel per hour for the same engine. Most boaters spend the majority of their time on the water at cruising speed, not WOT, so your real fuel use over a full day usually lands well below the wide open throttle figure.
Running at WOT burns through fuel quickly and is generally reserved for short bursts, getting up on plane, or specific high-speed situations rather than sustained cruising. A boat that burns 24 GPH at wide open throttle might cruise comfortably at 12 to 16 GPH instead, which adds up to a significant fuel and cost difference across a multi-hour outing.
GPH vs Nautical Miles Per Gallon
GPH tells you how much fuel your engine burns per hour, but it does not directly tell you how far you can travel on that fuel, since boat speed varies by throttle setting, hull design, and conditions. Nautical miles per gallon (NMPG) fills that gap, calculated by dividing your boat speedin knots by your GPH, giving you a distance-based efficiency figure similar to a car’s MPG.
A boat running 20 knots at 15 GPH gets roughly 1.3 nautical miles per gallon, for example. NMPG is most useful when comparing efficiency across different speeds for the same boat, since a slightly slower cruising speed often delivers meaningfully better NMPG than running near wide open throttle, even though the GPH figure alone does not show this clearly. Most boaters track GPH for fuel budgeting and tank planning, then check NMPG specifically when trying to maximize range or compare cruising speeds against each other.
We Affects Your Boat’s Fuel Consumption
Beyond horsepower and throttle setting, four additional factors meaningfully change your real-world fuel consumption. Sea conditions, hull type and load, engine condition, and number of engines all compund on top of the base GPH figure.
Sea Conditions
Rough water and significant chop can increase fuel consumption by 20 to 30 percent compared to calm, flat water, since the engine works harder to maintain speed and the hull is fighting more resistance with every wave. Heading out in rougher conditions means budgeting extra fuel beyond your flat-water estimate.
Hull Type and load
A heavily loaded boat, full of passengers, gear, and a full fuel tank, burns more fuel than the same boat running light, since more weight means more resistance to push through the water. Hull design also matters significantly, with planing hulls generally more fuel efficient at speed than displacement hulls of similar size.
Engine Tunning and Maintenance
A poorly tuned engine, fouled spark plugs, or a dirty fuel filter can noticeably increase fuel consumption above the manufacturer’s rated figures. Regular maintenance keeps your real-world GPH closer to the numbers this calculator estimates rather than drifting upward over time.
Number of Engines
Twin or triple engine setups multiply your total fuel burn accordingly, though they often allow you to run on fewer engines at lower speeds for better overall efficiency on long, slower cruises. Enter your combined total horsepower across all engines running simultaneously for an accurate combined GPH figure.
Planning Fuel for a Day on the Water
A simple rule many experienced boaters follow is the thirds rule, plan to use one third of your fuel getting out, one third getting back, and keep one third in reserve for unexpected conditions, detours, or engine trouble. This buffer matters more on the water than on land, since you cannot simply pull over if you run low offshore or far from a fuel dock.
This calculator’s tank size field gives you a usable range estimate for the time you will actually spend on the water. For the drive to the marina or boat launch towing your trailer, our fuel cost calculator handles that separate, land-based leg of your trip.
GPH vs Nautical Miles Per Gallon
Some boaters, particularly those cruising longer distances or comparing trip efficiency, also track nautical miles per gallon (NMPG) alongside GPH. Nautical miles per gallon divides your boat’s speed in knots by its GPH at that speed, giving you a distance-based efficiency figure similar in spirit to a car’s MPG, just measured in nautical miles instead of statute miles.
For example, a boat cruising at 20 knots while burning 15 GPH achieves 20 divided by 15, or roughly 1.33 nautical miles per gallon. GPH remains the more practical day-to-day figure for fuel budgeting, since your speed through the water constantly shifts with throttle, current, and conditions, but NMPG is useful when comparing the relative efficiency of two different cruising speeds or two different boats covering the same route.
How to Improve Your Boat’s Fuel Economy
Trim your engine properly for the conditions and load, since poor trim increases drag and forces the engine to work harder than necessary to maintain speed and stay on plane efficiently.
Keep your hull clean of growth and buildup, since even moderate fouling on the hull bottom can increase fuel consumption noticeably by adding drag that the engine has to overcome on every run.
Run at cruising speed rather than wide open throttle whenever your schedule allows, since the fuel savings between the two throttle settings are substantial across a multi-hour day on the water.
Keep up with routine engine maintenance, including fresh spark plugs, clean fuel filters, and properly maintained propellers, since all of these directly affect how efficiently your engine converts fuel into forward motion.
Methodology
GPH on this page is calculated using the standard marine industry formula: GPH = (Specific Fuel Consumption x Horsepower) / Fuel Specific Weight, using SFC values of approximately 0.50 for gasoline and 0.40 for diesel, and FSW values of approximately 6.1 lbs per gallon for gasoline and 7.2 lbs per gallon for diesel. Industry SFC values are derived from marine engine manufacturer data and published fuel consumption studies. The calculator above offers Economy Cruise (approximately 45 percent of WOT), Normal Cruise (approximately 57.5 percent of WOT), and Fast Cruise (approximately 70 percent of WOT) in addition to wide open throttle, since real cruising behavior varies by boater and conditions rather than following one fixed percentage.
These figures reflect published marine engine industry averages and are not a guarantee for any specific engine, hull, or set of conditions. Real-world fuel consumption depends on your specific engine tuning, hull design, load, sea conditions, RPM, and propeller setup, and your own logged fuel use from an actual day on the water will always be the most accurate figure for your boat.
Frequently Asked Questions
Related Calculators
Calculate the exact fuel cost for the drive to and from the marina or boat launch, separate from your time on the water.
Trip Fuel CalculatorPlan fuel needed for a full road trip if you are towing your boat a long distance to a different lake or coastline.
Towing CalculatorEstimate how much towing your boat trailer reduces your tow vehicle’s MPG on the drive to the launch.
Fuel Unit ConverterConvert between gallons, litres, and other fuel units, useful when comparing GPH against a car’s MPG.
MPG CalculatorCalculate your tow vehicle’s real-world MPG from a fill-up before towing your boat to the water.
Generator CalculatorCalculate fuel consumption for an onboard or portable generator using a similar horsepower-based approach.
Monthly Fuel Cost CalculatorBudget your recurring fuel spend if you boat regularly through the season.
Truck Fuel CalculatorCalculate diesel fuel cost if you tow your boat with a diesel truck.
Disclaimer
This calculator provides estimates only. Actual boat fuel consumption varies based on engine condition, hull type, load, sea conditions, and maintenance. Results do not account for RPM variation, propeller setup, or specific water conditions on any given day. See our full disclaimer page for more detail.