Calculate your tank size capacity in gallons (and weight)

Don’t remember how many gallons your tank or pond holds? No worries, this little code below will calculate the amount of gallons the container can hold based on its dimensions you enter. Besides, knowing your actual water volume helps you determine what 10%, 30% water change means in gallons, and precise measurements of additives (conditioner, fertilizer, medicine, calcium, magnesium, etc.) you need to dose as per your needs.

You can enter fractions (e.g. 0.5 or 12.75, etc.) for any of the inputs. The final estimate in gallons will show with up to 1 decimal point precision. Remember, the inputs for this little app should be in inches, and the output is in gallons as well as in liters (USA) or litres (UK). Also note that this is for rectangular tanks only at this time (other shapes require slightly different calculations but not harder, I may add them in the future). The app also lets you enter the thickness of substrate or gravel in the tank as they displace water thereby reducing the actual capacity of water, so it’ll accomodate for that when giving you the final answer.

Example I/O session:

Enter the frontal length of the tank (in inches): 48
Enter the width or side length of the tank (in inches): 12
Enter the height of the tank (in inches): 19
Enter the thickness of substrate/gravel in the tank (in inches). Enter 0 for none: 0.5
The tanks capacity in gallons is ≈ 46.1

NOTE: This is an estimate. The exact amount vary slightly depending on the thickness of the tank walls, internal components (decors, filter, heater, etc.). This calculation assumes a rectangular shaped tank. Curved or hexagon or irregular shaped tank's volume will vary.

You can enter your own inputs below get the answer. Click in the widget below to start.

Example of how to figure out different capacities: In the image below, we have a tank that’s 19″ (inch) tall, 24″ in length, and 12″ wide, and it has a substrate/gravel layer that’s 2″ thick from it bottom. If you want to calculate the full capacity of the tank (how many gallons it’ll hold if filled to the brim), then you would enter the height as 19 and the width and length as 12 with substrate thickness as 2 in the ap when prompted. However, if there was no gravel, you would enter 0 for the thickness of substrate.

But if you wanted to calculate the effective capacity (how many gallons are actually in it) when you fill the tank to a certain level, then you would enter the height of the top water level instead of the entire height of the tank, e.g. 16 in this example. You get the idea.

So, in the above example tank diagram, the inputs and output will look like this:

Enter the frontal length of the tank (in inches): 24
Enter the width or side length of the tank (in inches): 12
Enter the height of the tank (in inches): 16
Enter the thickness of substrate/gravel in the tank (in inches). Enter 0 for none: 2
The tanks capacity is ≈ 17.5 gallons | 66.1L

This brings us to the next related question, how much will my tank weigh when filled in with water? This is a critical and necessary question as if you do not have a sufficiently sturdy stand, it will be complete disaster. So, always get a stand that can stand weight more than the approximate weight as shown below.

To get weight of the water in the tank, it depends on the temperature (and of course decors and substrate inside the tank), because weight depends on density and density depends on temperature.
We can get an estimate on this free online too: https://www.thecalculatorsite.com/conversions/common/water-weight.php
(it support Gallons, Liters, and other units in both US and UK standards).

However, you can easily do the calculation yourself. Just use an approximate value of 8.33 pounds per gallons of water at room temperature. So, for a 55 gallon tank, it’d be: 55*8.33lb/gallon = 458.15 lbs (pounds)

But wait! Neither of the above will give you the total weight because they don’t account for the frame, 5 glass walls (glass is heavy!) required for a tank, lids, substrate or gravel, other heavy decors like stones or rocks (if any) and you need to also account for that to get the total weight exerted by your tank.
As a general rule, 1 pound of substrate per gallon is used to get a 1 inch deep bed (or 2 pounds per gallon, for a 2 inch deep bed).
So, if you have a 1″ layer of gravel in a 55 gallon, you’ll need to add another 55 pounds to the weight of tank and water. For a 2″ layer, that’s 119LB.

Ok, but how much does the tank itself weigh when empty (no water, substrate, decor…just the glass and framing)? Here’s a general guideline (based on typical thickness of walls used in aquariums in the USA):

It’s of paramount importance that you select the right location (floor/foundation-wise), right leveling (crooked or uneven placement will cause cracks in the tank with subsequent danger of explosion and serious damage), and the proper stand.
For example, for a 55 gallon tank with about 2″ gravel (and not counting any heavy rocks in it), the total weight when filled with water and gravel the minimum weight you need to support is a whopping 655 pounds minimum. See calculation below:
Weight of the empty tank (78 lbs) + 2″ layer gravel weight (119 lbs) + room-temp water weight (458 lbs) = 655 lbs (pounds)

Hope it helps. Wish you a successuful fish and/or shrimp-keeping 🙂

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