Boat Fuel Cost Calculator
| 4-stroke HP | ~GPH at cruise | 4 hr trip @ $4.50 |
|---|---|---|
| 25 | 2.5 | $45 |
| 90 | 9 | $162 |
| 150 | 15 | $270 |
| 250 | 25 | $450 |
Boat fuel burn has a rule of thumb that holds up surprisingly well: a modern 4-stroke outboard burns about 10% of its rated horsepower in gallons per hour at cruise - a 150 hp engine drinks roughly 15 GPH, a 90 hp about 9. Older 2-strokes run about 40% thirstier at the same rating. Throttle is the multiplier that matters: WOT can double the burn, while easing from full to 70% throttle typically halves it, which is why the same lake day can cost $40 or $90 depending on how the throttle is treated.
The other line item is where you buy: marina fuel routinely runs $1-2 per gallon over road prices because dockside volume is small and the fuel dock is captive. Fueling from jerry cans at the road station before launch is the boater's version of the fill-line trick - and the calculator prices both so the season total is honest either way.
How to use
- Enter engine horsepower and type (modern 4-stroke or older 2-stroke), then your fuel price - marina or road.
- Enter hours on the water per trip and trips per season for the per-trip and season totals.
- Read GPH at cruise, the per-trip cost, and the season bill - then try 70% throttle against WOT to see what the throttle lever earns.
Frequently asked questions
How much fuel does a boat use per hour?
The 10% rule: a modern 4-stroke outboard burns roughly 10% of its rated horsepower in gallons per hour at cruise - a 150 hp engine about 15 GPH, a 90 hp about 9, a 25 hp tin-boat about 2.5. Older carbureted 2-strokes burn closer to 14% of rated HP. Real numbers drift with load, hull and sea state, but the rule gets you within a gallon.
How much fuel does a boat use per hour at full throttle?
A lot more per mile, only somewhat more per hour: WOT pushes a 150 hp four-stroke toward 25-30 GPH while the boat is at its top speed, so gallons-per-MILE can double or triple versus a 70% cruise. The fuel-efficient speed is the one where the hull breaks over onto plane and settles - typically 60-75% of wide-open throttle.
Why is marina fuel so much more expensive?
A marina sells a few thousand gallons a week from a dock that exists for three months of peak season, with delivery, environmental compliance and real estate baked in - so expect $1-2 per gallon over the road price. Filling portable tanks at the road station before launch is legal, common and usually saves $15-40 on a modest day.
How do I plan fuel for a long trip?
Tank capacity divided by GPH is your hours of range; multiply by cruise speed for miles of range, then apply the one-third rule: one third out, one third back, one third in reserve for weather and detours. The calculator's per-trip figure at cruise is the planning number - WOT estimates are for racing, not for going somewhere and coming back.