There are times when you’re going to want to know the volume of a tank, but you’re not quite sure what it is. When that happens, you’ll need to be able to do some basic math to find out the size of the tank, and we’re here to help!

Let’s get into the basic math you’ll need to figure out that unknown aquarium’s volume!

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## Calculating the Rough Volume of an Aquarium

You’ll need to take some basic measurements to get the volume of the tank. For most of us it will be easier to convert the tank’s size to cubic inches or centimeters before the conversion to gallons or liters.

The three measurements you need for figuring out the volume are the length, width, and depth of the tank. A tape measure is a big help here, unless you’re looking at a particularly tiny tank.

After you’ve got these measurements, you simply multiply them. You’ll work the equation simply from left to right, you don’t even need to worry about PEMDAS in this situation. Just use the following equation:

**Length x Width x Height**

Your final measurement will be in units squared, depending on what measurement of length you used. In any case, the answer is always cubic, so you end up with a volume calculation of in³ or cm³.

For very large tanks, measurements in feet or meters may make sense. Converting them to smaller units afterward is trivial. You’ll just add two zeros for centimeters, or divide by twelve for feet.

### What About Odd Shaped Tanks?

There are a few common varieties of tanks that make it a bit more complicated to calculate their volume.

The main offender here is bow front tanks, where you’d have to calculate a minuscule portion of a larger circle to account for the lightly curved glass or acrylic on the front. **Since these tanks are mass-produced, I recommend just getting the back and side measurements and comparing them to common tank sizes.**

Apart from bow fronts, you’re best off calculating the volume of the bottom shape and then multiplying by the height to receive your volume. The above information is geared for rectangular tanks, but most shapes are easy to calculate.

You just need to determine the area of the base. Common shapes can require a bit more math than most of us use on a daily basis, but you can easily find area calculators that will handle the majority of shapes.

From there you can move on to converting your measurements to your preferred units immediately.

**The only difference for odd-shaped tanks is that you’ll want to calculate the area of the base initially, then multiply it by height.**

### Where to Measure Your Tank

Most of the tanks on the market aren’t measured from the edges of the glass. Instead, the total is put together using the width and height of the bracing.

Since that’s industry standard, you should be able to figure out what size the tank was sold at. An actual gallon one way or the other doesn’t matter much for most applications.

If you *do* want to be ultra-precise, you should measure from the interior of the glass. Glass thickness is another factor that’s often overlooked when tanks are being measured.

**For most people, measuring from the braces or just the edge will do the trick. **If you need a more accurate number then you should measure from the inside of the glass.

## Conversion to Gallons or Liters

Now that you have your volume measurement, you’ll probably want to convert it to something more familiar. 200 in³ doesn’t translate well for most people.

Metric makes everything easier, in my opinion. It turns out that cm³ is equal to a milliliter, or 1/1000th of a liter. You can simply move the decimal three places to the left to convert to liters.

For instance, a tank that ends up being 50,000cm³ will have a 50L capacity.

Of course, a lot of us use imperial measurements as well. I suggest a calculator because it’s a bit more complex.

In this case, it turns out that a gallon is equal to 231 in³. You’ll need to divide your tank’s volume by 231. So, if you have a tank that measures 1155 in³, you have a 5g tank.

The basic equations are easy:

**cm³ to L-****volume/1000****in³ to g- volume/231**

With that, you’ll have the total volume of your tank calculated.

## Converting Gallons to Liters (and Vice Versa)

If you’re an American buying a European tank, or vice versa, you may want to convert the measurements.

If you just need a rough figure, you can divide liters by 4 or multiply gallons by 4. This isn’t 100% accurate, but it’s a quick-and-easy way to get a ballpark figure.

The actual equation is **1 Gallon = 3.78541 Liters**

To convert gallons to liters, you’ll want to multiply the number of gallons by 3.78541, the resulting number is in liters.

To convert liters to gallons, you’ll simply divide the number of liters by 3.78541.

It’s pretty rare that we need ultra-precise conversions for aquarium volume. You can simplify the process by using 3.79 or even 3.8 as the conversion number for simpler math with a slightly less precise result.

## Common Tank Sizes and Dimensions

Most aquarists can eyeball a tank and know what size it is after a few years in the hobby. The vast majority of tanks are mass-produced and sizing doesn’t vary much, if at all, between different brands or manufacturers.

If you’re trying to determine how large of a tank you can stick in any given area, these dimensions are more important than the actual capacity of the tank.

The following are some of the most common sizes.

**10 Gallon-**20” x 10” x 12”**16 Gallon Bowfront-**20.3″ x 12.9″ x 17.8″**20 Gallon High-**24” x 12” x 16”**20 Gallon Long-**30” x 12” x 12”**26 Gallon Bowfront-**24.2″ x 14.8″ x 20.9″**40 Gallon Long-**48” x 12” x 16”**40 Gallon Breeder-**36” x 18” x 16”**55 Gallon-**48” x 13” x 21”**72 Gallon Bowfront-**48.4″ x 18″ x 23″**125 Gallon-**72” x 18” x 21”

In my experience, the above tanks are the most commonly found. Many tanks will still fall outside of these parameters, and if the person selling it doesn’t know the volume… well, you’ll have to calculate it yourself.

You can see that the above tanks aren’t using 100% accurate math either. There’s a bit of rounding involved, and the tank may be measured from the outside edge of the braces.

The good news? **Now you have the tools to calculate the volume of any tank. **It’s a skill most of us never consider, at least until we need it!**The base calculations are simple no matter the shape. **All you have to do is find the area of the bottom, and then multiply it by the height!