Boat Fuel Burn Calculator
Enter engine horsepower, fuel type, and load to estimate burn rate. Add hours underway, pump price, and tank size to see trip fuel, cost, and how long the tank lasts — including a one-third reserve.
Calculate
Calculate
Measurements convert when you switch units
Result
The formula
Marine fuel burn is estimated from brake-specific fuel consumption (BSFC) and fuel weight. Gallons per hour at a given load equal (SFC × horsepower × engines × load%) ÷ fuel specific weight. Litres per hour follow from US gallons × 3.785.
Worked example
- 300 hp gasoline, 1 engine, 100% load (WOT)
- GPH = (0.50 × 300 × 1 × 1.00) ÷ 6.1 ≈ 24.59 gal/h
- 4 hours underway → fuel ≈ 98.4 gal · at $4.00/gal → ≈ $393
Result: ≈ 24.6 gal/h · 98.4 gal · $393
How boat fuel burn is estimated
Unlike cars (miles per gallon), boats are usually planned in gallons or litres per hour because speed and sea state change how far you get for each litre burned.
BSFC rule of thumb
The industry estimate multiplies specific fuel consumption by horsepower and divides by fuel weight per gallon. Typical planning constants are about 0.50 lb/hp/h and 6.1 lb/gal for gasoline, and 0.40 lb/hp/h and 7.2 lb/gal for diesel — close to wide-open-throttle averages for many recreational engines.
Load percentage
100% models rated power at wide-open throttle. Cruising often uses far less prop load than WOT, so lower the load % toward your typical cruise setting. Propeller load does not scale linearly with RPM, so manufacturer fuel-flow charts beat a single percentage when you have them.
Multi-engine boats
Enter horsepower per engine and the engine count. Twin 150 hp engines at the same load burn about twice a single 150 hp setup under this estimate.
What we do not model
No hull type, sea state, wind, fouling, generator load, or ethanol blend density. Figures are planning estimates — verify with a fuel-flow meter or fill-up logs before a long crossing.
Interesting facts
GPH beats MPG on the water
Wave height and current change speed more than road hills change a car trip, so skippers track burn over time (GPH or L/h) and convert to nmpg only after measuring speed.
Gasoline vs diesel constants
At the same horsepower, gasoline WOT burn is often near HP ÷ 12, while diesel is nearer HP ÷ 18 — reflecting different BSFC and fuel density assumptions.
The rule of thirds
Many safe-boating guides suggest planning one-third of the tank outbound, one-third return, and one-third reserve for weather, diversion, or waiting for a bridge.
Generators add burn
Hotel loads and gensets can add litres per hour even when the main engines are idling or shut down at anchor — keep a separate allowance for overnight stays.
Frequently asked questions
Multiply specific fuel consumption by horsepower (and engines and load fraction), then divide by fuel pounds per gallon. This calculator uses common gasoline and diesel planning constants and converts to litres when you choose metric units.
Start near 50–70% if you cruise well below WOT, then calibrate with your own logs. If you have a manufacturer RPM/fuel chart, prefer those numbers over a single load guess.
No. Boats usually quote gallons or litres per hour. You can derive nmpg as speed ÷ GPH once you know both burn and boat speed.
Yes — enter horsepower for one engine and set Engines to 2 (or more). Burn scales with engine count at the same load.
A common planning practice is to keep about one-third of usable fuel as reserve. This tool shows full-tank runtime and a one-third reserve so you can see both.
Prop pitch, fouling, altitude, humidity, injector condition, and sea state all move BSFC. Treat the result as a planning band and update it after a few fill-ups.
References
- Calculating Boat Fuel Consumption Classic recreational BSFC / GPH estimating approach for gasoline and diesel.
- Fuel Planning and the Rule of Thirds Safe-boating guidance emphasizing reserve fuel for weather and diversion.
- Marine SI Engine Certification Context on marine spark-ignition engine emissions and certification.