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Calcimator

Boat Fuel Efficiency Calculator

GPH and range from engine specs and speed.

About this calculator

Fuel burn here is modeled through specific fuel consumption (SFC) — a standard marine-engineering shortcut that says an engine burns a roughly constant amount of fuel per horsepower-hour at a given load, regardless of boat size. Gasoline engines are assumed to burn about 0.10 gallons per horsepower per hour, diesels about 0.055 — diesel's higher energy density and compression-ignition efficiency roughly halve the rate. Gallons per hour (GPH) is then total fleet horsepower × your engine load percentage × that SFC figure. From GPH, the calculator derives nautical miles per gallon (cruising speed ÷ GPH) and converts to statute miles per gallon using the fixed 1.15078 nm-to-statute-mile ratio.

Range assumes only 90% of tank capacity is usable, holding back a 10% reserve — a standard planning margin, not a hard fuel-gauge cutoff, so don't treat the full tank as available range in practice. Cost per hour and cost per nautical mile fall straight out of GPH and fuel price. The biggest simplification is that load percentage and SFC are treated as constants: real engines are less efficient at low throttle and at wide-open throttle than in their mid-range cruise "sweet spot," and this model doesn't know your boat's actual displacement, hull shape, or prop pitch — all of which shift real-world consumption. Use this for trip-planning fuel budgets and range estimates, but always carry more reserve than the reported range suggests, especially at higher loads or in rough water.

Inputs

%
gal

Results

Fuel burn (GPH)

16.25

Nautical miles per gallon1.54
Statute miles per gallon1.77
Cruising range (NM)138.5
Cruising time (hours)5.5
Fuel cost per hour$89.38
Fuel cost per NM$3.58
How to Use This Calculator
  1. Enter Engine horsepower (HP), Number of engines, and Fuel type (0=gasoline, 1=diesel).
  2. Set Engine load (%), Fuel tank capacity (gal), and Cruising speed (knots).
  3. Adjust Fuel price ($/gal) as needed.
  4. Review the Fuel burn (GPH) result.
  5. Use Nautical miles per gallon and Statute miles per gallon to inform your decision.

How the result changes with Engine horsepower (HP)

Engine horsepower (HP)Fuel burn (GPH)
1258.13
18812.22
37524.38
62540.63

What each input means

Engine horsepower (HP)
Rated horsepower per engine.
Number of engines
Total engines powering the boat.
Fuel type (0=gasoline, 1=diesel)
Gasoline engines burn ~0.10 gal/HP/hr; diesels ~0.055 gal/HP/hr.
Engine load (%)
Average throttle/load percentage. Cruising is typically 60-70%.
Fuel tank capacity (gal)
Total fuel tank capacity in gallons.
Cruising speed (knots)
Speed over water at the given engine load.
Fuel price ($/gal)
Current price per gallon of marine fuel.

What each result means

Fuel burn (GPH)
Gallons per hour consumed at the specified load.
Nautical miles per gallon
Distance in nautical miles per gallon of fuel.
Statute miles per gallon
Distance in statute miles per gallon of fuel.
Cruising range (NM)
Range in nautical miles using 90% of tank (10% reserve).
Cruising time (hours)
Hours of cruising before hitting fuel reserve.
Fuel cost per hour
Dollar cost of fuel per hour of operation.
Fuel cost per NM
Dollar cost of fuel per nautical mile traveled.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Engine horsepower (HP) = 250, Number of engines = 1, Fuel type (0=gasoline, 1=diesel) = 0, Engine load (%) = 65 = 7 input(s) provided
  2. Calculate Fuel burn
    Fuel burn = totalHP * loadFraction * sfc
    16.25 = 16.25
  3. Calculate Nautical miles per gallon
    1.54 = 1.54
  4. Calculate Statute miles per gallon
    Statute miles per gallon = nmpg * 1.15078
    1.77 = 1.77

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does switching from gasoline to diesel roughly halve the fuel burn for the same horsepower?

The calculator applies a fixed specific fuel consumption (SFC) figure per fuel type: 0.10 gal/HP/hr for gasoline versus 0.055 gal/HP/hr for diesel. That difference reflects diesel's higher energy density and more efficient compression-ignition combustion, and since GPH is simply total horsepower × load fraction × SFC, halving the SFC constant directly halves the resulting fuel burn at the same load and horsepower.

Why is my calculated cruising range less than tank capacity times my miles-per-gallon?

Range is computed from usable fuel, which the calculator sets at 90% of total tank capacity — it deliberately holds back a 10% reserve rather than assuming you'll run the tank dry. So rangeNM is 0.9 × tankCapacity × nmpg, not tankCapacity × nmpg, which is why the reported range is always 10% below what a naive full-tank calculation would suggest.

How is cost per nautical mile different from cost per hour, and which should I use for trip budgeting?

Cost per hour is GPH × fuel price and tells you what an hour of running at this load costs regardless of speed. Cost per nautical mile is fuel price ÷ nmpg and tells you what covering distance costs regardless of trip duration. Use cost-per-hour for time-based trips (like a fishing day) and cost-per-mile for distance-based passages (like a delivery run).

Does the calculator adjust efficiency for how hard I'm running the engines?

Only through the load percentage you enter, which scales GPH linearly (higher load = proportionally more fuel burned per hour) — it does not model the fact that real engines are typically less efficient at very low throttle or at wide-open throttle than in their mid-range cruise sweet spot. If your actual cruise setting is well outside 60-70% load, expect real consumption to differ from this straight-line estimate.

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