Boat fuel math comes down to two numbers: how many gallons per hour your engine burns, and how many hours the trip takes. Multiply them, add a proper reserve, and you have your fuel plan. This guide gives you the formulas, typical burn rates by engine type, a worked example, and the reserve rule that keeps the calculation honest.
- Trip fuel = GPH x hours, and hours = distance in nautical miles / speed in knots.
- Rules of thumb at full throttle: gasoline engines burn about 0.10 gallons per horsepower per hour, diesels about 0.055.
- Cruising at 70-80 percent throttle cuts burn dramatically compared with wide open.
- Apply the one-third rule: a third out, a third back, a third in reserve.
- A route planner with your vessel profile does this per leg automatically.
#The core formula
Hours underway = distance / speed. A 60 nautical mile run at 20 knots is 3 hours. Fuel = GPH x hours: if your engine burns 18 gallons per hour at that cruise, the trip needs 54 gallons before reserve. That is the whole calculation; everything else is getting a truthful GPH number.
#Estimating GPH from horsepower
If you do not have measured numbers, the marine rules of thumb are: a gasoline engine at wide-open throttle burns roughly 0.10 gallons per horsepower per hour, and a diesel roughly 0.055. So a 250 hp gasoline outboard can drink about 25 GPH flat out, while a 250 hp diesel runs near 14. Nobody cruises at wide open: at a typical fast cruise around three-quarters throttle, expect roughly 60-70 percent of those figures, and displacement-speed motoring in a sailboat or trawler uses a small fraction of it.
- 150 hp gasoline outboard: about 15 GPH wide open, 8-10 GPH at fast cruise.
- 300 hp gasoline outboard: about 30 GPH wide open, 17-20 GPH at fast cruise.
- 40 hp sailboat diesel: about 2 GPH at full power, nearer 0.8-1.2 GPH at cruising revs.
- Trawler with a 120 hp diesel at displacement speed: commonly 2-4 GPH.
Treat these as planning figures and calibrate against reality: fill the tank, run a normal trip, fill again, and divide gallons by engine hours. Two or three fills give you a personal GPH that beats any table.
#The one-third rule
The classic reserve rule: one third of your fuel to get there, one third to get back, one third held in reserve. For a one-way passage, the practical version is to plan the trip on no more than two-thirds of usable tankage. Wind, current, and a rougher-than-forecast day all raise burn, and the reserve is what turns those surprises into a non-event.
Fuel burn rises steeply with speed on planing hulls, and weather effectively slows you for the same burn. If the numbers look tight, the answer is rarely more jerry cans: it is slowing down 3-4 knots or picking a better weather window.
#A worked example
Miami to Nassau is about 192 nautical miles by the practical sea route (see our sea distance pages for more pairs). A 32-foot outboard boat cruising at 22 knots takes about 8.7 hours. At 20 GPH, that is roughly 175 gallons; applying the two-thirds rule means you want around 260 gallons of usable fuel or a planned fuel stop. A 7-knot sailboat motoring the same route at 1.2 GPH needs about 33 gallons, and the same reserve logic still applies.
#Per-leg fuel with a route planner
Doing this by hand for one leg is easy; for a route with eight waypoints and a lunch stop it gets tedious. A route planner with a vessel profile does it continuously: the Marine OS route planner stores your cruising speed and GPH, and every leg you draw shows distance, time, and fuel, with totals updating as you drag waypoints. Change the speed and the whole fuel plan recalculates.
Marine OS is marina management software, and our route planner for boaters calculates per-leg time and fuel from your saved vessel profile, alongside auto sea routes, weather, and a departure scanner.
Get fuel estimates on every leg you draw
Set your speed and burn rate once in a vessel profile, and the Marine OS route planner shows distance, time, and fuel for every leg and the whole trip.
#Frequently asked questions
Frequently asked questions
Get a truthful GPH, respect the one-third rule, and let the planner do the per-leg arithmetic. Related reading: how to plan a boat route and sea distances between ports.
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