Aquarium Volume Calculator in Liters

This calculator works out the water volume of a rectangular aquarium in liters when you enter its length, width, and height. You get the total volume in liters, plus the same volume in US gallons and the weight of the water in kilograms. The calculation uses the standard geometric formula for the volume of a rectangular prism, with a small adjustment for the fact that the tank is not filled to the very brim.

Formula

The volume of a rectangular aquarium is calculated by multiplying the three internal dimensions, then converting cubic centimeters to liters:

Volume (L) = (Length × Width × Height) / 1000

Where:

• Length (cm) – the longest horizontal dimension of the tank, measured inside the glass.
• Width (cm) – the shorter horizontal dimension, measured inside the glass.
• Height (cm) – the vertical dimension from the bottom glass to the top rim, measured inside.
• 1000 – the number of cubic centimeters in one liter.

Because a working aquarium is never filled to the rim (you leave a gap of a few centimeters for the lid, filter, and to prevent fish from jumping out), the calculator multiplies the raw volume by 0.95 to estimate the actual water volume at about 95% of the full capacity. This is a common rule of thumb used by aquarists.

Worked Example

Using the default values from the calculator (Length = 80 cm, Width = 35 cm, Height = 40 cm):

1. Calculate the raw volume in cubic centimeters: 80 × 35 × 40 = 112,000 cm³.
2. Convert to liters: 112,000 / 1000 = 112 L (full capacity).
3. Apply the 95% fill factor: 112 × 0.95 = 106.4 L.
4. Convert to US gallons: 106.4 × 0.264172 = 28.1 gal.
5. Water weight (since 1 L of fresh water weighs 1 kg): 106.4 kg.

The calculator displays the same results: 106.4 L, 28.1 gal, and 106.4 kg.

Why Volume Matters in an Aquarium

Knowing the exact water volume is not just a curiosity – it drives several practical decisions:

• Dosing medications and water conditioners: Most treatments specify a dose per liter or per gallon. Overdosing can harm fish, and underdosing is ineffective.
• Stocking levels: A common guideline is “one inch of fish per gallonâ€� (or about 1 cm per 2 liters), though this varies with fish species and filtration. An accurate volume helps you avoid overcrowding.
• Heater sizing: A general rule is 3–5 watts per gallon (about 1 watt per liter). A 100 L tank needs a 100–150 W heater.
• Filter capacity: Filters are rated for a maximum tank volume. Choosing a filter rated for your actual water volume (or slightly higher) ensures adequate biological filtration.
• Water changes: Many aquarists change 10–20% of the water weekly. Knowing the volume lets you measure the right amount.

How to Measure Your Tank Correctly

The most common mistake is measuring the outside of the tank. Glass thickness (usually 4–10 mm) adds to the dimensions, so the internal volume is smaller than the external dimensions suggest. To get accurate numbers:

1. Use a metal tape measure or a ruler.
2. Measure the inside of the tank: from inner glass to inner glass for length and width, and from the bottom glass to the top edge for height.
3. If you already have substrate, rocks, and decorations in the tank, they displace water. The actual water volume will be less than the calculated empty-tank volume. You can estimate the displacement by noting how much water you add when filling the tank after a full cleaning.
4. For oddly shaped tanks (bow-front, hexagonal, cylindrical), the rectangular formula does not apply. Use a formula specific to that shape, or measure the water you actually pour in with a bucket.

Common Mistakes and Misconceptions

• Mixing units: If you measure in inches, you must convert to centimeters (1 inch = 2.54 cm) before using the formula, or use a different conversion factor. The calculator expects centimeters.
• Forgetting the fill factor: A tank that is 100 L “capacityâ€� does not hold 100 L of water when in use. The 5% reduction is a realistic estimate, but the exact gap depends on your setup.
• Confusing US and UK gallons: The calculator outputs US gallons. A US gallon is 3.785 L, while a UK (imperial) gallon is 4.546 L. If you need UK gallons, divide the liters by 4.546.
• Assuming 1 L of water weighs 1 kg: This is true for fresh water at room temperature. Saltwater is denser (about 1.025 kg/L), so a saltwater tank of the same volume weighs more.
• Ignoring the weight of the tank itself: The water weight is only part of the load on your stand or floor. Glass, substrate, and equipment add significant weight. A 100 L tank with substrate and glass can easily weigh 150 kg.

Alternatives to a Rectangular Tank

If your aquarium is not a rectangular box, you need a different formula:

• Cylindrical tank: Volume (L) = Ï€ × (radius in cm)² × height in cm / 1000.
• Bow-front tank: Approximate the volume by treating it as a rectangular tank with the average of the front and back widths, or use a more complex integral if you need high accuracy.
• Hexagonal tank: Volume (L) = (3√3/2) × side² × height / 1000, where side is the length of one of the six equal sides.
• Irregular or custom tanks: The most reliable method is to fill the tank with water using a container of known volume (e.g., a 10 L bucket) and count how many buckets it takes.

Frequently Asked Questions

How many liters is a standard 20-gallon tank?

If “20 gallons� refers to US gallons, then 20 × 3.785 = 75.7 L. If it is a UK gallon tank, it would be 20 × 4.546 = 90.9 L. Most aquarium equipment and tanks sold in the US use US gallons, but always check the label.

Does the volume include the gravel and decorations?

No. The calculator gives the volume of the empty tank. Substrate, rocks, driftwood, and decorations displace water, so the actual water volume is lower. A heavy substrate layer can reduce the water volume by 10–20% in a planted tank.

How much water do I need for a 10% water change?

If your tank holds 106.4 L (as in the example), a 10% water change is 10.64 L. Round to 10–11 L. Always measure the water you remove, not the tank volume, to be precise.

Why is my tank heavier than the water volume suggests?

Because the water is not the only weight. The glass, substrate, filter, and other equipment add up. A 100 L tank with a sand substrate and a glass lid can easily weigh 150 kg. Ensure your stand and floor can support the total weight.

Can I use the same formula for a saltwater tank?

Yes, the volume calculation is the same. However, the weight is different because saltwater is denser. Multiply the liters by 1.025 to get the weight in kilograms for saltwater.

What if my tank has a curved front?

For a bow-front tank, measure the width at the narrowest point (the sides) and at the widest point (the front bulge). Use the average of the two widths in the rectangular formula. This gives a good approximation, but for exact volume, measure the water you pour in.

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in l | Length | cm | 80 | 0
in w | Width | cm | 35 | 0
in h | Height | cm | 40 | 0
out vol | Water volume | L | (l * w * h / 1000) * 0.95 | 1
out gal | US gallons | gal | (l * w * h / 1000) * 0.95 * 0.264172 | 1
out kg | Water weight | kg | (l * w * h / 1000) * 0.95 | 1