Boat Fuel Range Calculator
Calculate boat range on a full tank from cruising speed, burn rate, tank capacity, and conditions.
Applies the one-third rule for safe offshore planning.
Knowing your boat’s fuel range is essential for safe passage planning. Running out of fuel on the water is dangerous.
Unlike a car, you cannot pull over and walk to a fuel station. The standard safety practice is the “one-third rule”: use one-third of your fuel going out, one-third coming back, and keep one-third in reserve for emergencies, currents, and weather changes.
Basic Range Formula
Two steps, and it is worth keeping them separate because mixing them up is how people end up with a range figure that is out by a factor of ten.
First, how far you go on one gallon: Nautical miles per gallon = Speed (knots) / Fuel burn (gallons per hour)
Then, how far the tank takes you: Gross range (nm) = Tank capacity (gallons) × Nautical miles per gallon
Collapsed into one line, Range (nm) = Tank capacity ÷ GPH × Speed.
GPH is gallons per hour at the speed you actually cruise, not at idle and not at wide open throttle. That is the number worth getting right, because everything downstream is built on it.
Fuel Consumption by Engine Type
Fuel consumption in marine engines is commonly expressed as gallons per hour (GPH) or liters per hour (LPH). A useful rule of thumb for gasoline inboard/outboard engines:
GPH ≈ HP × Load Factor × BSFC
Where:
- HP = engine horsepower
- Load Factor = fraction of full throttle (0.5 = half throttle, 0.75 = cruise, 1.0 = WOT)
- BSFC = Brake Specific Fuel Consumption (lbs fuel per HP per hour)
| Engine Type | BSFC (lb/HP/hr) | GPH at 100 HP cruise (75%) |
|---|---|---|
| Gasoline outboard (2-stroke) | 0.55–0.65 | 6.8–8.0 |
| Gasoline outboard (4-stroke) | 0.45–0.55 | 5.5–6.8 |
| Gasoline inboard | 0.50–0.60 | 6.2–7.4 |
| Diesel inboard | 0.35–0.40 | 3.7–4.2 |
| Diesel outboard | 0.33–0.38 | 3.5–4.0 |
Gasoline weighs approximately 6.1 lbs per gallon. Diesel weighs approximately 7.1 lbs per gallon, which is where the right-hand column comes from: multiply HP by load factor by BSFC to get pounds of fuel an hour, then divide by the weight of a gallon.
Speed vs. Efficiency
Boat fuel efficiency changes dramatically with speed due to hull resistance:
| Speed Zone | Hull Behavior | Fuel Efficiency |
|---|---|---|
| Displacement (< 7 knots) | Hull sits in water | Best MPG |
| Transition (7–15 knots) | Hull climbing its own bow wave | WORST MPG (hump zone) |
| Planing (15–30+ knots) | Hull rides on top of water | Moderate MPG |
| High speed (30+ knots) | Fully planing | Decreasing MPG |
The transition zone (getting “over the hump”) is the least efficient speed range.
Either go slow, in displacement mode, or get fully on plane. Do not cruise in between, because that is where you burn the most fuel for the least distance.
Typical Fuel Efficiency by Boat Type
| Boat Type | Length | Cruise Speed | GPH (cruise) | Nautical MPG |
|---|---|---|---|---|
| Small outboard (skiff) | 16–18 ft | 25 knots | 6–8 | 3.0–4.0 |
| Center console | 20–24 ft | 28 knots | 12–18 | 1.5–2.5 |
| Cabin cruiser | 28–35 ft | 22 knots | 20–35 | 0.6–1.1 |
| Sailing yacht (motor) | 35–45 ft | 7 knots | 2–4 | 1.8–3.5 |
| Trawler | 35–45 ft | 8 knots | 3–6 | 1.3–2.7 |
| Pontoon boat | 20–24 ft | 15 knots | 4–6 | 2.5–3.5 |
Worked Example: 22 ft Center Console, 100-Gallon Tank, Twin 150 HP Outboards
Cruise speed: 28 knots at 3500 RPM (75% load). GPH per engine: about 8.5 GPH (4-stroke outboard). Total GPH: 2 × 8.5 = 17 GPH. Nautical MPG: 28 / 17 = 1.65 nm/gallon. Gross range: 100 × 1.65 = 165 nautical miles. One-third rule: 55 nm out, 55 nm back, and the last 33 gallons stay in the tank. Usable range is therefore 110 nm, two-thirds of the gross figure, not 165.
The One-Third Rule
| Fuel Portion | Use | Percentage |
|---|---|---|
| First third | Outbound trip | 33% |
| Second third | Return trip | 33% |
| Third third | Reserve (weather, currents, emergencies) | 33% |
This means your usable range is only two-thirds of your gross range. Never plan a trip that uses more than two-thirds of your fuel capacity.
Current and Wind Effects
A 2-knot opposing current reduces your effective speed by 2 knots but your engine still burns fuel at the same rate. Wind and chop do the same thing from the other direction, making the hull work harder for the same speed.
The conditions selector applies this as a straight penalty on the burn rate: calm leaves it alone, moderate adds 12%, and rough adds 25%. Those are deliberately blunt. Real fuel burn depends on sea state, load, trim and how much you are fighting the wheel, and on a bad day a planing hull that keeps falling off plane can burn far more than 25% extra. If the forecast is marginal, plan on the rough figure and be pleasantly surprised.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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