Boat Fuel Range Calculator
Calculate boat range on a full tank from engine type, cruising speed, burn rate, and capacity.
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
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.