How to Figure the Number of Gallons Water in a Tank


This article covers three methods for determining the number of gallons of water in a tank. The first method is the fastest, but requires the most calculation. The second method is the simplest, but could take a long time on larger tanks. The final method discussed is fairly simple but can be physically taxing when large tanks are involved.

Things You'll Need

  • Measuring tape or ruler
  • Scale
  • 1/2-gallon or 1-gallon jug
  • Calculator

By Measuring Sides

Measure the tank. If the tank has rectangular sides, measure and record the width, height, and depth of the tank in feet. If the tank is a cylinder, measure the diameter of the base and the height of the tank.

Calculate the volume of the tank. If the tank is rectangular, multiply the width by the height and depth. If the tank is a cylinder, multiply the height by the area of the base which is pi* (diameter/2) ^2. These calculations will give you the volume in cubic feet.

Convert from cubic feet to gallons. 1 cubic foot = 7.48 U.S. gallons, so multiply the cubic feet of your tank by 7.48 to learn how many gallons it will hold.

By Filling with Known Container

Find a container of known volume, like a 1-gallon milk jug.

Fill the container of known volume and pour it into the tank.

Repeat Step 2 until the tank is full. Multiply the number of times you filled the container by its volume to get the volume of the tank.

By Weighing

Weigh the tank when it is empty and record the weight.

Fill the tank with water and weigh it again. If the tank is large, this step could be difficult and you might want to consider trying one of the above methods instead.

Subtract the weight of the empty tank from the weight of the full tank. This will give you the weight of the water you added.

Divide the weight of the water by 8.34 if you measured the weight in pounds or by 3.79 if you measured in kilograms. The answer is the number of gallons your tank holds.

Tips & Warnings

  • Water can get heavy. Be careful when lifting a tank with more than a few gallons of water in it.

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