Calculating True Aquarium Volume
The Actual Volume of Water in Your Tank is Less Than You Might Think
When purchasing a marine aquarium, you want to know the actual volume of water that will be in the tank, but this calculation isn't always easy to come by.
When you purchase an aquarium for a saltwater set-up, you might expect the advertised volume to be the volume you use when calculating water volume. In fact, it’s usually not. This article will show you how to calculate the real volume of a marine aquarium before you buy it.
Calculating Aquarium Volume
Take a standard 75 gallon rectangle aquarium as an example. The dimensions of this tank are 48 inches long by 15 inches wide (front to back) by 24 inches tall. The equation to determine volume is:
(Length x Width x Height) / 231
Using this calculation, you find the 75-gallon tank is actually a 74.81-gallon tank. Big deal, you say. Truly it’s not a big deal, but there is more to consider.
Adjusting for Interior Dimensions
The 75-gallon tank would be 74.81 gallons IF 48 inches long by 15 inches wide by 24 inches tall were the interior dimensions of the aquarium, not the exterior dimensions. To get a true volume, you need to take into account the thickness of the aquarium walls and subtract that from the dimensions (or just measure the interior of the tank). If the acrylic wall of the 75-gallon tank is 7/18 of an inch, then you lose almost an inch (0.875 of an inch) on each measurement. A tank with those dimensions is actually closer to 73 gallons.
Does this make a tremendous difference, you ask? No. Not really. In fact, most aquarium manufacturers in the United States simply round to the nearest multiple of five when calculating advertised volume. The gallon or two of difference between advertised volume and actual volume is probably not going to be an issue, even when dosing medications.
Adjusting for Water Level
Ah. But there is still more. What many aquarists forget to take into account is that the aquarium is rarely filled to the top. Generally the water level is three-quarters of an inch or more below the top of the aquarium (this depends somewhat on your overflow set-up). The following calculation is now necessary (where “D” equals the difference between the top of your tank and the actual water level):
(Length x Width x D) / 231
Doing this calculation for the 75-gallon tank yields a little more than two gallons, so it is necessary to subtract two gallons. The true volume of the water in the tank is between 70 and 71 gallons. Phew! Still not a big deal, right?
Adjusting for Substrate and Live Rock
But wait! There’s more. If your tank has substrate (like a sandbed), you need to do the above calculation once again but this time with “D” being the depth of your sandbed (note that this is not exact because substrate grain size varies). You will then have to repeat the calculation one last time for the live rock you place in the tank. This last calculation is really little more than a rough guess, as the porosity and shape of live rock varies greatly, but when it’s all said and done, you can clearly see that the 75-gallon aquarium you bought does not hold 75 gallons of water. NOW you can plan accordingly when stocking and medicating your aquarium.
A Few Parting Thoughts
- Don’t purchase equipment (e.g. a protein skimmer) rated for actual volume—purchase equipment rated for advertised volume. The buffer you gain will be good for the system, and you can always scale pumps back if need be.
- Use a sump. This adds water volume to the system.
- Be very conservative when stocking your aquarium. If you are going to stock using a volume calculation, use up to five gallons less than the actual volume in the calculation.
