Marine Conversion
MPG to GPH Converter
Convert miles per gallon (MPG) to fuel burn rate in gallons per hour (GPH) at a specific speed.
The MPG to GPH Converter bridges distance-based and time-based fuel economy. It is widely used in boating, sailing, and aviation, where travel speeds are measured in knots or miles per hour, and fuel limits are tracked by hours of engine runtime rather than mileage odometer readings.
How to Use This Calculator
- Enter your cruising speed. Use mph if your MPG figure is in statute miles, or knots if it is in nautical miles — the two must match.
- Enter your MPG at that speed, not a best-case or top-speed figure.
- Press Calculate GPH.
- Read the burn rate in gallons per hour, the metric equivalent in litres per hour, and how long 50 gallons would last at that rate.
The units cancel out cleanly: miles per hour ÷ miles per gallon leaves gallons per hour. That is why the speed unit and the distance unit inside MPG have to be the same. Feed the calculator knots against a statute MPG figure and the answer will be 15% out.
Why GPH Matters More Than MPG on the Water
Cars measure fuel by distance because a car's fuel use tracks distance closely. Boats and aircraft measure by time, because burn rate is set by engine load, and engine load is set by throttle position rather than by how far you have travelled. Sit at anchor with the generator running, punch into a headwind, or fight a three-knot current and you burn fuel while covering little or no ground — a distance-based figure hides all of that.
Gallons per hour is also the figure your engine's fuel-flow sensor actually reports. Converting to GPH lets you sanity-check a manufacturer's published MPG claim against what the gauge shows on your own boat, in your own conditions.
Typical Burn Rates by Engine
| Engine | Typical cruise GPH | Notes |
|---|---|---|
| 9.9 hp outboard | 0.5–1 | Tender, small skiff |
| 90 hp outboard | 4–6 | Small centre console |
| 150 hp outboard | 7–10 | Bay boat, pontoon |
| 300 hp outboard | 14–18 | Offshore centre console |
| Twin 300 hp outboards | 28–36 | Common offshore pairing |
| 370 hp diesel inboard | 12–16 | Trawler, express cruiser |
A useful rule of thumb for petrol engines is roughly 0.10 GPH per horsepower at cruise, and about 0.06 GPH per horsepower for diesels, which are meaningfully more efficient per unit of work. Use those only as a first estimate; measured flow always beats a rule of thumb.
Turning GPH Into Endurance
Once you have GPH, endurance is simply usable fuel ÷ GPH. Two cautions apply. First, usable fuel is not tank capacity — pickups sit above the tank floor, and in a seaway you cannot safely draw the last few gallons. Second, apply a reserve. The one-third rule afloat (a third out, a third back, a third in reserve) exists because conditions on the return leg are rarely the same as on the way out.
To go the other way — from a known burn rate back to distance economy — use the nautical MPG calculator for knots and nautical miles, or the boat MPG calculator for statute miles.
Got Questions?
MPG to GPH Calculator FAQ
Divide your speed (in miles per hour) by your vehicle's miles per gallon (MPG). The formula is: `GPH = Speed (mph) ÷ MPG`. For example, cruising at 30 mph in a boat that gets 3 MPG will burn 10 gallons per hour (30 ÷ 3 = 10).
Boats face varying resistance from water current, waves, wind, and hull drag, making distance-based fuel efficiency (MPG) highly inconsistent. Time-based fuel consumption (Gallons Per Hour) is more reliable for marine voyage planning and safety calculations.
A small 150HP outboard motorboat may burn 5–8 GPH at cruising speeds. Larger yachts or high-performance powerboats can easily consume 20–50+ GPH under throttle. Marine diesel engines are generally more efficient than gasoline outboards.
Yes. While idling, your speed is 0 mph (so MPG is 0), but the engine still consumes fuel. Most passenger car engines burn 0.2 to 0.5 gallons per hour at idle, while large marine engines burn much more.