Nominal Capacity vs Actual Capacity


In order to help our customers get an accurate quote for the right tank size they require, we have listed the differences between nominal capacity vs actual capacity. Our tank size chart will also help customers pick the right capacity water tank.

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The Different Water Capacity Types

Nominal Capacity – the volume of water in the tank when it is full to the brim.

Actual Capacity – the water volume in the tank from the base to the overflow pipe

Actual Useable Capacity – the volume of water from above the outlet to the overflow pipe.

Nominal capacity vs actual capacity

Differences Between Nominal and Actual Capacity

Image illustrating nominal capacity
Image illustrating actual capacity
image illustrating actual useable capacity

Nominal capacity

The nominal capacity of a water storage tank is the volume of water in the tank when it is filled to the brim. You calculate the nominal capacity by multiplying the length x width x height. Use our tank size calculator to find the nominal capacity for your water storage tank.

Actual Capacity

In order to get the actual capacity of your water tank, you need to factor in the volume of water lost when an overflow and warning pipe is fitted. This will give you the top water level. If the top level water lost was 200mm, this is deducted from the overall height. A 3m x 2m x 2m sectional tank would have an actual capacity of 10,800 litres because it will be calculated as a 3m x 2m x 1.8m tank after considering the 200mm volume of water lost.

Image illustrating actual capacity

What is Actual Useable Capacity

Actual useable capacity is the volume of water that can be physically used before air is taken into the system. You calculate actual useable capacity between two data points: the top water level (see actual capacity) and the dimension of the top of the outlet or low-level switch (pump protection). This water is known as dead water because it cannot be used.

How to calculate actual useable water tank capacity?

Suppose the area of water was 300mm. In that case, this is added to the top water level and subtracted from the overall height, e.g. the actual useable capacity of a 3m x 2m x 2m sectional tank is calculated as follows:

  • 200mm (top water level) + 300mm (Dead water) = 500mm
  • 2000mm (2m) – 500mm = 1500mm
  • 3m x 2m x 1.5m = an actual useable capacity of 9,000 litres

Effective Capacity

The effective capacity is the physical volume of water available to use and is calculated the same as the actual useable capacity. The term ‘effective capacity’ is used by sprinkler companies to size their sprinkler tanks.

Image illustrating nominal capacity
image illustrating actual useable capacity
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