GPH to MPG Calculator

Your gauge tells you gallons per hour. This turns that into gallons per mile — the number that actually tells you how far a tank will take you, and the one worth tracking every time you change speed, trim, or prop.

How this is figured

MPG is simply speed divided by burn rate — the same math whether you're running a car or a boat, just with worse mileage. Statute MPG uses mph; nautical MPG (nm/gal) uses knots, which is the number that actually matches the distances on a nautical chart or GPS set to nautical miles. Range is a straight multiplication of your usable fuel by whichever efficiency number matches your fuel gauge and chartplotter units — usable fuel means what you're actually willing to burn, not the tank's rated capacity. Most captains don't plan past two-thirds of a full tank, keeping the rest in reserve for the way home, a rough sea slowing you down, or a longer run than planned.

Efficiency changes a lot with speed on a planing hull — burn rate climbs faster than speed does once you're pushing hard, so the best MPG on most boats is usually found at a specific, often surprisingly modest cruise speed just past the hump onto plane, not at wide-open throttle. Running this calculator at a couple of different cruise speeds from your own fuel-flow gauge is the fastest way to find your boat's real sweet spot.