Size does matter when it comes to calculating how much ‘stuff’ you need to put in your tank (‘stuff’ being water treatments and chemicals).  Most chemicals and water treatments have dosage guidelines that are based on a ‘per litre’ measure.   It’s a question we get asked all the time in-store so here’s a simple guide to calculating the volume of various shaped tanks and bowls.

This is a little bit mathematical and is not really an interesting read unless you need to use it!

Rectangle Fish Tank

Square or Rectangular Tanks

Probably the easiest calculation is for square or rectangular tanks.

Multiply Length x Breadth x Height (in cms) and divide the answer by 1000 to get litres.

The height is to where the water level is (or is expected to be when filled) less 2.5cm for the gravel that is likely to be in the bottom.

Triangular Tanks

This is where you might think its get complicated but as long as your triangle tank has a right angle it really isn’t.  Any triangle is just half of a rectangle so work out as for Square or Rectangular Tanks as above, and divide the answer by 2

The length and breadth can be any two of the three sides.  In Centimetres divide the answer by 1000 to get Litres.

Cylindrical Tanks

Measure across the center of the cylinder (Diameter) and then halve it, this is the Radius.

Square the Radius (i.e. Multiply the Radius by the Radius) and then multiply that by pi (or 3.14)

Now multiply that answer by the height of the tank water, and allowing the 2.5cm for the bottom gravel.  In Centimetres divide the answer by 1000 to get Litres.

Bowl TankSpherical Tanks and Bowls

In clever terms the volume of a sphere is 4/3 π r³.  We could use a clever calculation such as this to cube the radius, divide 4 by 3 and multiple the result by pi.  OR for a small goldfish bowl or tank : the easiest way is to fill it once using a calibrated jug etc and make a note of how much it took.

Once you’ve worked it out once just mark the answer down somewhere on the tank, or on the fridge under a magnet, or on your iPhone.  Then all you need to do is remember where you marked it every time you need to know it!