# Watt per Gallon calculation



## mr_convitbau (Jan 7, 2009)

When we calculate the watt per gallon for the tank, do we use the volume of only water or the volume of the entire tank (substrate + rock + water)?


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## Bert H (Mar 2, 2004)

Keep in mind that the wpg rule is not a hard and fast rule. With small volumes of water (less than 10), a 3wpg tank is not high light, while on a 125 gal tank, 3wpg would be considered high lighting. With that in mind, I personally, do it based on the tank volume size, not the actual amount of water in it. It's a very rough number, so it doesn't make a whole lot of difference.


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