How much could it hold?


   When trying to calculate the capacity of a water, waste, and/or fuel tank, you need to measure the exterior dimensions of the tank in question and perform the steps below.

   The first step is to get the dimensions of the tank in question.

Example: Let's suppose the above container is...
 height:19", width 21", depth: 14"

First step is to convert inches to feet.

height 19" = 1.583 ft., 
width 21" = 1.75 ft.
depth 14" = 1.166 ft.

Next: multiply length, width, depth to get the cubic footage of the tank.

 [ 1.583 X 1.75 X 1.166 = 3.230 cubic feet ]

Now you're ready to calculate the capacity of your tank. Simply multiply your answer by 7.48051945.

3.230 X 7.48051945 = 24.177178 gallons

Gallons per cubic foot 
1 cubic foot = 7.48051945 US gallons

1 US gallon = 0.133680556 cubic feet

Now... that's all well and good except for a few considerations. 

First: Not all tanks are square. Specialty dimension tanks and hull formed tank bottoms add problems with measurement.

This is where basic geometry, we learned in high school, kicks in to help come up with tank capacity. 

Go to the following web page for a refresher concerning this situation.



You just have to measure carefully. Use the best measurement of the square area of the tank and in most cases the non-measured area won't be a factor. Keep reading for the reasons why.

Second: Since we are using the exterior dimensions; plastic tanks have a thicker  wall than metal tanks and the inside dimensions of a plastic tank will be somewhat smaller than the outside dimensions.

Third: Not all tanks will fill properly without leaving an air bubble at the top of the tank. For example, a long narrow tank with the fill and vent at one end, won't fill completely if the fill/vent end is lower than the non vented end.

However, Not filling completely isn't always a bad thing. Fuel will expand as it gets warmer. The air bubble in the tank allows for fuel expansion without an overboard spill occurring.

Fourth: Usable fuel capacity is less than what the fuel tank can hold. Running the fuel tank completely dry, while under way, can cause a lean run condition of the engine, which is not good for the engine.

In conclusion, always expect the actual usable capacity to be less than your measurement, calculated above. Have the tank as level as possible, when filling. Regularly inspect the tank and hoses for condition and safety. If you are having a problem filling the tank properly, consult professional help to resolve the problem.

Additional reading:

Anti-siphon valves



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