Boat Fuel Calculator
Plan how much fuel to carry for a boat trip. Enter your burn rate, cruising speed and the distance in nautical miles to see fuel burned, the reserve, running time, cost, range and endurance on a full tank.
Boat & trip
Estimate fuel burn from horsepower
Compare throttle settings
Enter fuel burn and speed from your engine's performance data to find the most economical cruise for this trip.
| Setting | Burn (GPH) | Speed (kn) | NM per gal | Trip fuel | Time | Range |
|---|
Estimate only. Burn rates rise with load, sea state, wind and fouled hulls – always carry extra fuel and check local regulations.
How to use the Boat Fuel Calculator
- Choose the fuel unit and enter your boat's fuel burn per hour at cruise (or estimate it from horsepower).
- Enter the cruising speed in knots and the trip distance in nautical miles; tick round trip if you are coming back.
- Pick a reserve rule – the rule of thirds is the standard for safe boating.
- Add your tank capacity and fuel price.
- Press Calculate fuel to see fuel to carry, running time, cost, range and whether your tank is big enough.
Boat fuel formulas
- Running time (h) = distance (NM) ÷ speed (knots)
- Trip fuel = running time × fuel burn per hour
- Rule of thirds: fuel to carry = trip fuel × 1.5 (the trip uses ⅔, ⅓ stays in reserve)
- Percentage reserve: fuel to carry = trip fuel × (1 + reserve %)
- Economy = knots ÷ GPH (nautical miles per gallon)
- Range = tank × economy; endurance = tank ÷ GPH
Worked example: 30 NM to an island and back (60 NM) at 20 knots with a 12 GPH burn. Time = 60 ÷ 20 = 3 h, trip fuel = 3 × 12 = 36 gal, with the rule of thirds carry 36 × 1.5 = 54 gal. A 60 gallon tank works (90% full). At $5 per gallon the fuel burned costs $180. Economy is 1.67 NM/gal, so a full tank gives 100 NM of range or 5 hours of running.
Typical fuel burn at cruise
| Boat | Power | Cruise speed | Approx. burn | NM per gal |
|---|---|---|---|---|
| Small skiff | 40 hp gas | 18 kn | 2–3 GPH | 6–9 |
| Center console 21 ft | 150 hp gas | 25 kn | 7–8 GPH | 3–3.5 |
| Bowrider 24 ft | 300 hp gas | 28 kn | 13–15 GPH | 1.9–2.2 |
| Twin outboard 30 ft | 2 × 300 hp gas | 32 kn | 28–32 GPH | 1.0–1.1 |
| Trawler 40 ft | 2 × 220 hp diesel | 8 kn | 5–6 GPH | 1.3–1.6 |
| Sailboat auxiliary | 30 hp diesel | 6 kn | 0.6–0.8 GPH | 7.5–10 |
These are rough figures. Your engine manufacturer's performance bulletin, or a fuel flow meter, gives the real burn at each rpm.
Tips for planning fuel on the water
- Use your usable tank capacity – the last 10% of a tank is often unreachable in a seaway.
- Plan for headwinds, current and chop: burn can rise 20–50% in rough water, which the reserve is meant to cover.
- Most planing hulls are most economical just above planing speed; running at wide open throttle can halve your range.
- Fuel is about 6.1 lb per gallon for gasoline and 7.1 lb for diesel, so a full load changes trim and speed.
Frequently asked questions
How do I calculate boat fuel for a trip?
Divide the distance in nautical miles by the speed in knots to get hours, then multiply by the fuel burn per hour. A 60 NM round trip at 20 knots takes 3 hours; at 12 GPH that is 36 gallons.
What is the rule of thirds for boat fuel?
Use one third of your fuel to go out, one third to come back and keep one third in reserve. In practice you carry 1.5 times the fuel the trip needs: 36 gallons of trip fuel means carrying 54 gallons.
How do I convert GPH to nautical miles per gallon?
Divide speed in knots by gallons per hour. At 20 knots and 12 GPH the boat goes 20 ÷ 12 = 1.67 nautical miles per gallon.
How much fuel does a boat engine use per hour?
A rough rule is about 0.1 gallon per hour per horsepower for gas engines at cruise and about 0.055 for diesels. A 250 hp gas outboard at 60% throttle burns roughly 12 GPH. Check your engine's performance bulletin for real numbers.
What is a nautical mile?
One nautical mile is 1,852 meters, about 1.15 statute miles. One knot is one nautical mile per hour.
How do I find my boat's range?
Multiply the usable tank capacity by nautical miles per gallon. A 60 gallon tank at 1.67 NM per gallon gives 100 NM, but with the rule of thirds you should only plan trips of about 67 NM total, or 33 NM out and back.