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How to Find Out Your Boat Gas Tank Size?

Written by Anthony Roberts / Fact checked by Jonathan Larson

how to find out your boat gas tank size

With fuel prices at an all-time high, knowing how to find out your boat gas tank size is crucial. No boater would want to waste even a drop of fuel simply because they overfilled their gas tanks (not to mention the fire risk).

Manufacturers indicate a watercraft’s fuel tank capacity in an Owner’s Manual or similar documents. But what if you misplaced this literature?

Fret no more because we’ll show you two ways to answer the problem, “How big is my gas tank?” Please read on.

What to Prepare


Determining the size of gas tank in any watercraft requires several resources, depending on the method. You can prepare the following materials to measure fuel capacity the old-fashioned way.

  • Personal protective equipment
  • Two 3/8-inch-diameter fuel hoses
  • Primer bulb or fuel move siphon
  • Hose clamps
  • Screwdriver
  • Gas containers with known capacities

A hands-off approach requires an alternative solution using a fuel tank capacity calculator (or a similar device) and a measuring tape.

Step-by Step to Find Out Your Boat Gas Tank Size

Although we promise to show two ways to determine a boat’s gas tank size, you might want to consult a capacity chart for your watercraft’s type, model, and series.

</p> <h3>Method 1. Drain gas or fuel refill</h3> <p>


You have two options for determining how much gas a boat’s fuel tank can hold. We’ll describe the first trick – draining the boat fuel.

Step 1. Operate your boat until the fuel gauge is nearly empty.


Take your boat out and have a pleasant time with friends or family. Keep a watchful eye on the fuel gauge to help you determine when to steer the watercraft back into the dock.

Alternatively, you can keep the engine running while the boat is at the dock. This trick might waste precious fuel, making it less appealing than driving the watercraft.

Pro Tip: Head for the docks if the fuel gauge is about one-eighth full.

Step 2. Retrieve the boat from the water and into your trailer


  • Position your trailer on the boat ramp, engaging the emergency parking brake of the towing vehicle.
  • Ease your watercraft onto the trailer and switch off the engine once the vessel is fully secure.
  • Get on your towing vehicle, carefully drive off the boat ramp, and park in a level and firm spot.

You’re now ready for the next step in determining your watercraft’s bulk tank capacity.

Step 3. Drain the fuel from the boat.


Please wear your goggles, face mask, and gloves before starting.

  • Get the fuel hose and connect one end to the groundwork bulb or fuel move siphon.
  • Secure the connection with the hose clip using a screwdriver.
  • Link the other fuel hose to the siphon or bulb’s opposite side and secure it with a clip.
  • Connect one hose to an empty fuel container (as a drainage compartment).
  • Open the boat’s fuel tank cap and insert the other hose end to the tank bottom.
  • Activate the fuel move siphon to draw gas from the tank and empty it into the fuel container.

Here’s a video for draining a boat’s fuel tank.

Step 4. Drive to the gasoline station and fill the boat’s fuel tank.


  • Close the boat’s fuel tank and place the gas container in a safe place.
  • Drive your towing vehicle to a gas station and fill the watercraft’s fuel tank.
  • Note how many gallons the tank accommodated on the pump gauge, indicating the boat’s tank size.  

Extra tips:

  • You can operate your boat at the dock if you must. This way, you don’t need to watch the fuel gauge and bring the boat to the shore at 1/8 fuel level. Instead, you will continue operating the watercraft until it dies out because of zero fuel.
  • To determine the average boat gas tank size, you may also get fuel into empty containers then transfer the liquid in them to the boat.
  • The last step in this technique requires counting how many fuel containers are empty. For example, suppose you brought five 5-gallon canisters, and four are empty. In that case, you used up 20 gallons (4 x 5 = 20).
  • As for the last fuel container, you can empty the contents into a measuring tool to determine how much gas is left. Suppose there is a gallon left. In that case, the boat has 24 gallons in the tank.
</p> <h3>Method 2. Calculating fuel tank dimensions</h3> <p>

This method hinges on your knowledge of a boat’s gas tank dimensions. It also requires a boat fuel tank size calculator or a similar device.

Step 1. Measure the boat’s fuel tank.


Some boats have rectangular fuel tanks, allowing you to determine the length, width, and height easily. Please ensure accuracy in your measurements to ensure you get the best possible results.

Some gas tanks have a cylindrical design. Hence, you must measure their lengths and diameters. Vertically-oriented gas tanks have height parameters instead of length.

These gas tank measurements are essential to determining the tank’s capacity.

Step 2. Calculate the tank volume.


A dedicated measuring device, like a generator fuel tank size calculator, offers more accurate results than a standard calculator.

For boxy or rectangular fuel tanks, you could multiply the tank’s length, width, and height to determine its volume.

Meanwhile, calculating a cylindrical fuel tank’s volume requires multiplying the Pi value (3.1416) by the tank’s length (or height) and the tank’s squared radius.

Additional Considerations and Tips


Here are several considerations and tips when determining a watercraft’s gas tank size.

  • Factors that May Affect Gas Tank Size Estimation

Fuel density and temperature are two of the most crucial factors affecting gas tank size estimation.

    • Temperature – High environmental temperatures cause gas to expand, making gas tank size estimation faulty during hot summer months.
    • Density – Relative to temperature, fuels expand when heated. The expansion takes up more space while lowering the gas’s density.
  • Tips for Properly Maintaining your Boat’s Gas Tank

Here are some tips to maintain your boat’s fuel tank.

    • Clean the gas tank every six months or after every extreme temperature change. You might also want to clean the tank if the engine isn’t operating “normally” or when taking the watercraft from storage.
    • Schedule an annual “polishing” (deep-cleaning) of the watercraft fuel tank to remove sludge, algae, and other debris.
    • Use only high-quality marine-grade fuel for your boat.


How many gallons of gas does a boat hold?

Watercraft fuel tank capacity depends on the type. For example, a bass boat fuel tank can hold 25 to 50 gallons, while a speedboat can accommodate up to 1000 gallons.

The smallest pontoon boat gas tank size is about five gallons, although the average is between 20 and 35 gallons.

On the other hand, the smallest yacht gas tank size is about 1,300 gallons, while larger vessels can accommodate a maximum of 300,000 gallons of fuel.

The importance of knowing the gas tank size

Knowing a watercraft’s tank size allows you to optimize the boat’s potential, driving it to its operational limits without getting stranded offshore.

How many types of gas tank?

Boats have two types of gas tanks: portable and fixed. Portable units can only accommodate about five gallons of fuel, while permanent boat gas tanks can carry at least ten gallons.


You now know how to find out your boat gas tank size (if you cannot locate your owner’s manual).

The second method is more straightforward, requiring only a tape measure and calculator to determine your watercraft’s fuel capacity. You could always opt for the first trick, especially if you want to take friends and family out for a watery spin.

Knowing a boat’s fuel capacity allows you to push its range to the hilt.

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