Nautical MPG Calculator

The Nautical MPG Calculator is a specialized utility for sailors, captains, and marine boaters to calculate range and planning profiles in nautical measurements. Knowing your NMPG helps you map out offshore cruise limits and determine your maximum range on a voyage.

1. Enter your vessel speed in knots at the throttle setting you actually cruise at, not the hull's top speed.
2. Enter your hourly fuel burn in GPH (gallons per hour) at that same speed. Most modern engine displays report this directly; otherwise divide the fuel used on a run by the hours under way.
3. Press Calculate NMPG (or hit Enter in either field).
4. Read your nautical miles per gallon, along with the statute-mile equivalent and metric km/L for comparison against road figures.

Marine navigation runs on nautical miles, not statute miles. One nautical mile is 1,852 metres — defined as one minute of latitude — which makes it 1.15078 statute miles. Charts, GPS plotters, and speed in knots all use this unit, so expressing fuel economy in nautical miles per gallon keeps your range planning in the same units as everything else on the bridge.

The practical consequence is that an NMPG figure always looks lower than the equivalent statute MPG for the same boat. A vessel returning 3.0 NMPG is doing 3.45 statute MPG. Mixing the two is a common and expensive mistake when planning an offshore leg: treating an NMPG figure as statute miles overstates your range by 15%.

What Good NMPG Looks Like

Boats are far less efficient than cars because water resistance rises steeply with speed, and a planing hull must first climb over its own bow wave. Typical cruising figures:

These are cruising figures, not wide-open throttle. Most planing hulls hit their best NMPG just after the boat settles on plane, then lose economy steadily as speed climbs from there.

How to Improve Your NMPG

• Find the efficiency sweet spot. Run a speed sweep in 200–300 RPM steps and log NMPG at each. Nearly every planing hull has a band just above planing speed where NMPG peaks; it is often 15–20% better than the next step up.
• Keep the bottom clean. Marine growth is the single largest recoverable loss. A fouled hull can cost 20–30% of your fuel economy, and a fouled prop adds more on top.
• Trim deliberately. Correct trim tab and engine trim settings reduce wetted surface and bow-up drag. Watch the NMPG figure change as you adjust.
• Watch your load. Full water and fuel tanks, ground tackle, and gear all sit in the hull. Weight matters far more on a boat than on a car because it changes how the hull sits in the water.
• Match the prop. A prop that will not let the engine reach its rated WOT RPM is over-pitched and burns fuel to no purpose.

Planning Range Safely

The long-standing rule for fuel planning afloat is the one-third rule: one third of your fuel to get there, one third to get back, one third held in reserve. Take the NMPG figure from this calculator, multiply by usable tank capacity, then plan to use no more than two thirds of the result. Sea state, current, and headwind can each cost you 10–20% against a calm-water number, and unlike a car you cannot pull over at the next exit.

If your engine reports burn rate rather than economy, use the MPG to GPH converter to move between the two. For inland and lake boating measured in statute miles, the boat MPG calculator works in the same units as your car.