Aquarium Volume Calculator
This aquarium volume calculator works out exactly how much water your fish tank holds from its shape and dimensions, then converts the result into liters, US gallons, imperial gallons, and cubic feet. Knowing the true volume is the starting point for safe stocking, the right dose of water conditioner or medication, and a filter that matches your tank.
It supports five common tank shapes — Rectangular, Cube, Cylinder, Bow-front, and Hexagonal — and measures in centimeters or inches. Beyond raw capacity, it estimates the actual usable water volume after substrate and equipment, calculates water weight, and offers a freshwater fish stocking guide so you can plan responsibly.
How to Use the Calculator
Select your tank shape
Choose Rectangular, Cube, Cylinder, Bow-front, or Hexagonal. The dimension fields and tank illustration update automatically to match the shape you pick.
Choose your measurement unit
Toggle between centimeters (cm) and inches (in). If you switch units after entering values, the numbers are converted for you, so you never have to do the math by hand.
Enter dimensions
Fill in the measurements your shape needs — for example length, width, and height for a rectangular tank, or diameter and height for a cylinder. Optionally add the Substrate Depth if you have gravel or sand on the bottom.
Calculate and read the results
Press Calculate (or hit Enter) to see total volume in four units, water weight, the actual usable water volume, surface area, and a fish stocking guide.
Understanding your results
Total Volume
The full geometric capacity of the tank, shown in liters with conversions to US gallons, imperial gallons, and cubic feet.
Water Weight
The approximate weight of the water in kilograms and pounds — useful for checking that your stand and floor can support a full tank.
Actual Water Volume
Usable volume after subtracting substrate displacement and an estimated 10% for equipment such as filters, heaters, and decorations.
Surface Area
The water surface area in cm² and in², which drives gas exchange and is a key factor when planning how many fish a tank can hold.
Fish Stocking Guide
Suggested counts of small, medium, and large freshwater fish, based on the actual water volume rather than the empty-tank figure.
Features
Five Tank Shapes
Calculate volume for rectangular, cube, cylinder, bow-front, and hexagonal tanks, each with its own formula and dimension inputs.
cm and Inch Conversion
Switch between centimeters and inches at any time and every value you have entered converts automatically.
Four Volume Units
See the result in liters, US gallons, imperial gallons, and cubic feet at the same time.
Substrate Displacement
Enter your gravel or sand depth to subtract its volume and reveal the actual usable water volume.
Water Weight Estimate
Get the water weight in both kilograms and pounds so you can confirm your stand can take the load.
Surface Area
View the water surface area in cm² and in² — an important factor for oxygen exchange and stocking.
Fish Stocking Guide
Suggested numbers of small, medium, and large freshwater fish using the common one-inch-per-gallon guideline.
Dynamic Fields & Illustration
Dimension inputs and a side/top tank illustration adjust to the shape you choose, so you only enter what is relevant.
Frequently Asked Questions
How do I calculate my fish tank volume?
Pick your tank shape, enter its dimensions in centimeters or inches, and press Calculate. Each shape uses its own geometric formula: a rectangular tank is Length × Width × Height, while a cylinder is π × radius² × height. All measurements are converted to cubic centimeters, then divided by 1,000 to give liters.
How many gallons is my aquarium?
Enter your dimensions and the calculator shows the volume in US gallons and imperial gallons alongside liters and cubic feet. As a guide, one liter is about 0.264 US gallons or 0.220 imperial gallons, so the tool handles the conversion for you in one step.
What is the difference between total volume and actual water volume?
Total volume is the full geometric capacity of the tank. Actual water volume subtracts the space taken up by substrate (gravel or sand) plus an estimated 10% for equipment like filters, heaters, and decorations. The actual figure is the more realistic number to use for fish stocking and water-treatment dosing.
How do I calculate volume for a bow-front or cylinder tank?
Select the matching shape and the right fields appear. A cylinder asks for diameter and height. A bow-front asks for length, width, height, and bow depth — the bow depth is how far the curved front panel extends beyond the straight sides, and the calculator adds that extra curved volume on top of the rectangular base.
Does substrate reduce the real water volume?
Yes. Gravel, sand, and soil sit at the bottom and take up space that water would otherwise fill. Enter the substrate depth and the calculator subtracts its volume, then also factors in roughly 10% displacement from equipment and decorations to give you the actual usable water volume.
Why does surface area matter for aquariums?
The water surface is where gas exchange happens — oxygen enters and carbon dioxide escapes. A larger surface area lets in more oxygen, which is important for fish health. Tanks with greater surface area relative to their volume can generally support more fish than tall, narrow tanks of the same capacity.
How accurate is the fish stocking guide?
It uses the general one-inch-of-fish-per-gallon rule for freshwater aquariums, which is a simplified starting point. Real stocking capacity depends on species behavior, filtration strength, water parameters, and tank dimensions, so always research the specific needs of the fish you plan to keep before adding them.
How much does aquarium water weigh?
Fresh water weighs about 1 kg per liter, or roughly 8.34 pounds per US gallon. The calculator bases the weight on the actual water volume (after substrate and equipment) and shows it in kilograms and pounds, so you can confirm your stand and floor can safely support the full tank.
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