Boat Fuel Consumption Calculator estimates marine fuel burn, cruising range, runtime, and efficiency from horsepower, speed, engine type, and fuel capacity. Formula: cruise burn = HP × fuel factor × 0.65.
Why Fuel Planning Starts at the Engine, Not the Tank
The most common pre-departure mistake is dividing tank capacity by a rough miles-per-gallon guess without ever accounting for how hard the engine is actually working. A 200-horsepower outboard running at cruising throttle draws fuel based on its rated power output, not the speed shown on the GPS. This calculator anchors every figure — range, reserve, and burn rate — to engine horsepower and the recognized industry multipliers for each fuel system type, so the numbers you plan around are the same ones engineers use to size marine fuel systems.
The outputs cover both what your engine will do at wide-open throttle and what it actually burns during a relaxed cruise, which are two very different numbers. Knowing both prevents the scenario where a quick full-throttle sprint to beat weather burns a third of your reserve before you realize it.
Calculator Used Formulas
Engine Multipliers (mBase) by Fuel System
- Gasoline 4-Stroke: mBase = 0.10 gal/hr per HP
- Gasoline 2-Stroke: mBase = 0.11 gal/hr per HP
- Marine Diesel: mBase = 0.055 gal/hr per HP
WOT (Wide Open Throttle) Fuel Rate
- WOT Base Rate (Gal/hr) = HP × mBase
- WOT Rate — US / Knots mode: WOT Rate (Gal/hr) = WOT Base Rate
- WOT Rate — Metric mode: WOT Rate (L/hr) = WOT Base Rate × 3.78541
Cruising Fuel Rate (65% of WOT)
- Cruise Base Rate (Gal/hr) = WOT Base Rate × 0.65
- Cruise Rate — US / Knots mode: Cruise Rate (Gal/hr) = Cruise Base Rate
- Cruise Rate — Metric mode: Cruise Rate (L/hr) = Cruise Base Rate × 3.78541
Fuel Economy
- Cruising Efficiency = Speed ÷ Cruise Rate (result: MPG, NMPG, or km/L)
- WOT Economy = Speed ÷ WOT Rate
Time to Empty
- Time to Empty (hrs) = Capacity ÷ Cruise Rate
- WOT Time to Empty (hrs) = Capacity ÷ WOT Rate
Reserve Fuel
- Reserve Fuel = Capacity × 0.10
Range
- Operating Range = Capacity × Cruising Efficiency
- Safe Range = Operating Range × 0.90
- One-Way Limit = Operating Range ÷ 3
Alternative Measures — US / Knots Mode
- Flow Rate (L/hr) = Cruise Base Rate (Gal/hr) × 3.78541
- Flow Rate (Imp Gal/hr) = Cruise Base Rate (Gal/hr) × 0.832674
- Speed (KPH) from MPH = Speed (MPH) × 1.60934
- Speed (KPH) from Knots = Speed (Knots) × 1.852
- Metric Efficiency (km/L) = Speed (KPH) ÷ Flow Rate (L/hr)
Alternative Measures — Metric Mode
- Flow Rate (US Gal/hr) = Cruise Rate (L/hr) ÷ 3.78541
- Flow Rate (Imp Gal/hr) = Flow Rate (US Gal/hr) × 0.832674
- Speed (MPH) = Speed (KPH) ÷ 1.60934
- US Efficiency (MPG) = Speed (MPH) ÷ Flow Rate (US Gal/hr)
Validation
- HP, Speed, and Capacity must each be a positive number greater than zero. Any zero or blank input clears all outputs.
How It Works
Every calculation in this tool flows from a single starting point: the engine’s rated horsepower multiplied by an established fuel-burn coefficient for that engine type. The 4-stroke gasoline figure of 0.10 gallons per hour per horsepower, the 2-stroke figure of 0.11, and the marine diesel figure of 0.055 represent the wide-open-throttle (WOT) fuel demand — the maximum rate at which that engine will consume fuel. This is the raw WOT Base Rate in US gallons per hour regardless of which measurement mode you select; the conversion to litres per hour happens afterward for metric display.
Cruising doesn’t happen at 100% throttle. The calculator applies a 65% factor to the WOT Base Rate to arrive at the Cruising Fuel Rate, reflecting steady-state consumption at a typical cruising RPM range. This is what you see as the hero output — Cruising Fuel Burn. Everything downstream depends on this figure.
Dividing your input speed by the Cruising Fuel Rate gives Cruising Efficiency (MPG, NMPG, or km/L depending on mode). Dividing tank capacity by the Cruising Fuel Rate gives Time to Empty. Multiplying capacity by efficiency gives Operating Range — the theoretical maximum distance at cruise pace with a full tank. Safe Range pulls that back by 10%, and the One-Way Limit divides the full Operating Range by three, marking the farthest you can go on one-third of your fuel in keeping with the rule of thirds.
The WOT figures in the Peak Performance card run the same logic without the 65% cruise adjustment, showing how quickly the tank drains if you push the throttle to the wall. The Alternative Measures card converts the cruising burn rate into the other two measurement systems — litres per hour, imperial gallons per hour, and the corresponding efficiency figure — so you can compare across regions or fuel dock readouts without separate math.
Switching the Measurement Protocol between US Customary, Nautical, and Metric also switches the fuel capacity input between gallons and litres, and updates all range units accordingly (Miles, Nautical Miles, or km). The mode change also resets the three numeric inputs to representative defaults for that system so the initial output is always a coherent example.
Where the Rule-of-Thumb Estimate Breaks Down
The multipliers baked into this calculator — 0.10 for 4-stroke gasoline, 0.11 for 2-stroke, 0.055 for diesel — are well-established rough-cut figures, not manufacturer fuel maps. Real consumption deviates from these baselines for predictable reasons that the calculator cannot capture:
- Hull load and drag. A heavily loaded boat sits deeper in the water, increasing resistance. The engine works harder at the same throttle position, so actual burn runs higher than the estimate predicts.
- Sea state and headwind. Chop and wind resistance can add 20–30% to fuel burn on the same route compared to flat-water conditions. The calculator has no weather input.
- Engine age and tune. Worn injectors, fouled fuel systems, and prop pitch mismatches all shift consumption away from the rated multiplier. A well-maintained engine will often beat the estimate; a neglected one may fall well short.
- Multi-engine installations. If you have twin engines, enter combined horsepower. The calculator does not know how many engines share that HP figure, and combined-HP input is the only correct approach.
- Speed vs. throttle position. The WOT rate is tied only to horsepower, not to the actual speed the boat reaches at full throttle. If your boat planes out at 40 MPH at WOT but you entered 25 MPH as your cruise speed, the WOT Economy figure will look artificially low — the speed input only flows into the economy and range math, not into the WOT burn rate itself.
For passage planning where the consequences of running dry are serious, add your own margin on top of the Safe Range output. The 10% buffer built into Safe Range is a widely-used baseline; extended offshore passages typically operate on a one-third planning rule with an additional personal buffer, not the Safe Range figure alone.
Worked Example: Single 150HP 4-Stroke, Day Trip Planning
Setup: 150HP gasoline 4-stroke outboard, 22 MPH cruise speed, 40-gallon usable tank. US Customary (MPH) mode selected.
The WOT Base Rate works out to 15.00 Gal/hr (150 × 0.10). Applying the 65% cruise factor gives a Cruising Fuel Burn of 9.75 Gal/hr — the hero output shown at the top of the results. The Cruising Efficiency card reports 2.26 MPG (22 ÷ 9.75), Time to Empty of 4.10 hours (40 ÷ 9.75), and a Reserve Fuel figure of 4.00 gallons (40 × 0.10), leaving 36 gallons as the effective planning capacity.
The Operating Range card shows 90.26 Miles (40 × 2.26), with a Safe Range of 81.23 Miles and a One-Way Limit of 30.09 Miles. That One-Way Limit is the key planning number here: staying inside 30 miles from the launch ramp means 1/3 of the fuel gets you there, 1/3 gets you back, and the remaining third (including the explicit 4-gallon reserve) covers weather delays and unexpected detours.
The Peak Performance card shows WOT burn at 15.00 Gal/hr, WOT Economy of 1.47 MPG, and a WOT Time to Empty of 2.67 hours. That last number is a useful sanity check: if something goes wrong and you need to run at full throttle, the tank lasts under three hours — a meaningful constraint for offshore planning. The Alternative Measures card converts the 9.75 Gal/hr cruise rate to 36.91 L/hr and 8.12 Imp Gal/hr, with a Metric Efficiency of 0.96 km/L, useful when fueling at a marina that bills in litres.
Frequently Asked Questions
Why do my HP, speed, and capacity values reset whenever I change the Measurement Protocol?
This is intentional. A speed of 25 in MPH mode equals about 21.7 knots and 40.2 KPH — very different operational scenarios. Rather than silently carry a value that would produce a misleading result in the new unit system, the calculator loads a representative default for each mode so the first output you see is always numerically coherent. If you switch modes and then re-enter your actual values, the calculation will run on what you typed.
My WOT fuel rate doesn’t change when I adjust cruising speed. Is that correct?
Yes. The WOT rate in the Peak Performance card is HP × multiplier — it depends only on engine horsepower and engine type, not on speed. Speed only enters the economy and range math (speed ÷ fuel rate). This reflects how marine WOT rules of thumb work: the fuel consumption estimate is tied to rated output, not to how fast the hull happens to be moving. If your boat tops out at a different speed than you entered, the WOT Economy figure will shift accordingly, but the raw WOT Gal/hr figure will not.
What is the actual difference between Operating Range and Safe Range?
Operating Range is the theoretical maximum distance using the entire tank at cruise pace. Safe Range subtracts exactly 10% from that figure (Operating Range × 0.90), representing a standard marine buffer for fuel gauge inaccuracy, consumption variation, and the like. Neither figure should be treated as a hard stop on a real-world passage — the One-Way Limit (Operating Range ÷ 3) is the more conservative planning boundary for a round trip, and it is always shorter than the Safe Range.
The diesel result looks roughly half the gasoline output. Is the 0.055 multiplier accurate?
The diesel multiplier (0.055 gal/hr per HP) is approximately half the 4-stroke gasoline multiplier (0.10), which correctly reflects the thermodynamic efficiency advantage of a compression-ignition engine. Marine diesel engines genuinely do burn significantly less fuel per horsepower than gasoline equivalents at similar loads. That said, the same rule-of-thumb caveats apply: the actual advantage at light cruise loads can be even greater than this ratio suggests, while heavily loaded diesels may approach the multiplier more closely.
What happens if I enter zero or leave a field blank?
All three numeric fields — HP, Speed, and Capacity — must be positive numbers above zero. If any field is blank, contains a zero, or holds a negative value, the calculator clears all output fields and shows a warning. No partial result is displayed. Enter valid positive values in all three fields to restore the full output.
In Metric mode the capacity input label changes to Liters. How does that affect the calculation?
When you select Metric (KPH), the capacity field accepts litres, not gallons. The internal math converts the HP-based gallon-per-hour rates to litres per hour (× 3.78541) and uses those directly against the litre capacity input. Economy is expressed in km/L and range in kilometres. If you enter your tank in gallons while in metric mode, the results will be significantly understated — a 50-gallon tank is about 189 litres, and entering 50 in litre mode treats it as a 13-gallon tank.