Boat Fuel Range Calculator
Calculate how far your boat can travel on a tank of fuel based on engine type, speed, and tank capacity.
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
Range (nautical miles) = (Tank Capacity × Fuel Efficiency) / Fuel Consumption Rate
Or more practically:
Range (nm) = Tank Capacity (gallons) / GPH × Speed (knots)
Where GPH = gallons per hour consumed at a given speed.
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 | 7.5–8.0 |
| Gasoline outboard (4-stroke) | 0.45–0.55 | 5.5–6.5 |
| Gasoline inboard | 0.50–0.60 | 6.0–7.5 |
| Diesel inboard | 0.35–0.40 | 3.5–4.5 |
| Diesel outboard | 0.33–0.38 | 3.3–4.0 |
Gasoline weighs approximately 6.1 lbs per gallon. Diesel weighs approximately 7.1 lbs per 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 (displacement) or get fully on plane — do not cruise in between.
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: ~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. Safe range (one-third rule): 165 / 3 = 55 nm out, 55 nm back, 55 nm reserve.
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. A 15-knot headwind can increase fuel consumption by 10–20%. Always factor in expected conditions when planning range.