Boat Fuel Range Calculator

Fuel range equals usable fuel (gallons) divided by fuel burn rate (gallons per hour), multiplied by boat speed (nautical miles per hour). Range (nm) = (Fuel capacity * usable fraction) / burn rate * speed. Most skippers use only two-thirds of total fuel capacity as the usable amount, keeping one-third in reserve.

The one-third rule (also called the one-third reserve rule) states that you should use one-third of your fuel to reach your destination, one-third to return, and keep one-third in reserve for emergencies. This rule is widely recommended by the US Coast Guard and seamanship guides.

Fuel burn varies widely by engine type, hull, and speed. As a rough guide: small outboard (50 hp) at cruise burns about 4 to 6 gph; mid-size inboard (200 hp) at cruise burns about 10 to 20 gph; large twin-engine cruiser can burn 30 to 60+ gph at cruise. Always check your engine manual for specific burn rate curves.

At displacement speeds, fuel efficiency (nautical miles per gallon) is relatively constant. However, many powerboats have a speed sweet spot where they are most efficient. Pushing beyond cruise speed dramatically increases fuel burn. Slowing down by 1 to 2 knots can often increase range by 20 to 30 percent on planing hulls.

Always use usable capacity, not total capacity. Most fuel tanks have unusable fuel at the bottom (typically 5 to 10 percent of total). Fuel senders also become inaccurate below about 10 percent. This calculator lets you set the usable percentage; the default 67% reflects the one-third-reserve rule.