Square Tank Water Volume Calculator

This calculator works out the water volume of a square or rectangular tank in litres and gallons. You enter the length, width, and height of the tank in your preferred unit (metres, centimetres, feet, or inches), and it returns the volume in litres, US gallons, and imperial gallons. The calculation is the standard geometric formula for the volume of a rectangular prism: length × width × height.

Formula

The volume of a square or rectangular tank is calculated by multiplying its three internal dimensions:

V = L × W × H

Where:

• V = volume (cubic metres, cubic feet, etc.)
• L = internal length of the tank
• W = internal width of the tank
• H = internal height (or depth) of the tank

If you measure in metres, the result is in cubic metres (m³). To convert to litres, multiply by 1000 (since 1 m³ = 1000 L). To convert to US gallons, multiply by 264.172. To convert to imperial gallons, multiply by 219.969. If you measure in centimetres, the volume in cubic centimetres (cm³) equals the volume in millilitres, so divide by 1000 to get litres.

Worked Example

Using the default values from the calculator above: length = 1 m, width = 0.5 m, height = 0.5 m.

1. Calculate the volume in cubic metres: 1 × 0.5 × 0.5 = 0.25 m³
2. Convert to litres: 0.25 × 1000 = 250 litres
3. Convert to US gallons: 0.25 × 264.172 = 66.0 US gallons
4. Convert to imperial gallons: 0.25 × 219.969 = 55.0 imperial gallons

So a tank that is 1 metre long, 0.5 metres wide, and 0.5 metres tall holds exactly 250 litres of water when filled to the brim.

Why Measure Inside Dimensions?

A common mistake is measuring the outside of the tank. The wall thickness of a glass aquarium, plastic storage container, or steel tank reduces the usable internal volume. For example, a glass aquarium with 10 mm thick walls will have an internal length about 20 mm shorter than its external length. For a 1-metre tank, that is a 2% reduction in volume, which matters when dosing chemicals or calculating fish stocking levels. Always measure from the inside edge to the inside edge.

Also, the water level is rarely at the very top. If you leave a 2–3 cm gap between the water surface and the rim (which is standard for aquariums to prevent fish jumping out), the actual water volume is slightly less than the full tank volume. To account for this, measure the height from the bottom to the intended water line, not to the top edge.

How to Use the Calculator for Different Units

The calculator above uses metres as the default, but you can easily adapt it. If you have measurements in centimetres, divide each by 100 before entering them (e.g., 50 cm = 0.5 m). If you have measurements in feet, multiply each by 0.3048 to get metres. If you have measurements in inches, multiply each by 0.0254.

Alternatively, you can use the calculator in a different way: enter your measurements in metres but think of the result in cubic metres, then convert manually. One cubic metre is a large volume—roughly the size of a standard bathtub. Most household aquariums are measured in litres, so the conversion to litres (×1000) is the most useful.

Common Mistakes and How to Avoid Them

• Mixing units: Do not enter length in metres and width in centimetres. Convert everything to the same unit first.
• Using outside dimensions: As mentioned, this overestimates volume. Measure inside.
• Forgetting the water line: The tank may be 50 cm tall, but if you only fill it to 45 cm, use 45 cm as the height.
• Confusing US and imperial gallons: A US gallon is 3.785 litres; an imperial gallon is 4.546 litres. They differ by about 20%, so always check which one your pump or chemical instructions use.
• Assuming a square tank is a cube: A square tank means the base is square (length = width), but the height can be different. The formula still applies.

When This Formula Does Not Apply

This formula only works for tanks with straight sides and a rectangular or square base. It does not apply to:

• Cylindrical tanks (use π × radius² × height)
• Conical or tapered tanks (use the average radius)
• Spherical tanks (use 4/3 × π × radius³)
• Tanks with sloped bottoms or irregular shapes

For a hexagonal or octagonal tank, you need the area of the polygon base multiplied by the height. For a fish tank with a curved front (bowfront), the volume is often estimated by treating it as a rectangle and subtracting a small percentage (typically 10–15%), but the exact value depends on the curvature.

Practical Applications

Knowing the exact water volume is essential for several reasons:

• Aquarium stocking: Fish need a minimum volume per fish. Overstocking leads to poor water quality and stress.
• Water treatment: Dechlorinators, medications, and fertilizers are dosed per litre. An incorrect volume means an incorrect dose.
• Heater sizing: A general rule is 3–5 watts per litre for aquariums, depending on room temperature.
• Filter selection: Filters are rated for tank size; an undersized filter cannot handle the biological load.
• Water changes: If you do a 20% weekly water change, you need to know how many litres that is.

Frequently Asked Questions

How many litres is a 4×2×2 foot tank?

Convert feet to metres: 4 ft = 1.219 m, 2 ft = 0.610 m, 2 ft = 0.610 m. Volume = 1.219 × 0.610 × 0.610 = 0.453 m³. Multiply by 1000 to get 453 litres. In US gallons, that is about 120 gallons. This is a common large aquarium size.

Does the calculator account for the glass thickness?

No. The calculator assumes the dimensions you enter are the internal dimensions. If you enter external dimensions, the result will be slightly too high. Measure the inside of the tank for the most accurate answer.

What is the difference between a square tank and a rectangular tank?

In everyday usage, a square tank has a square base (length equals width), while a rectangular tank has a rectangular base (length and width differ). The volume formula is identical for both; only the numbers differ.

How do I calculate the volume if my tank is not full?

Measure the height of the water from the bottom of the tank to the water surface, and use that as the height in the formula. The length and width stay the same. For example, a 1 m × 0.5 m × 0.5 m tank filled to 0.4 m depth holds 1 × 0.5 × 0.4 × 1000 = 200 litres.

Is a litre the same as a kilogram of water?

At 4°C, one litre of pure water has a mass of exactly one kilogram. At room temperature (20–25°C), one litre of water weighs approximately 0.998 kg, which is close enough for practical purposes. This means the volume in litres is numerically almost equal to the weight in kilograms, which is useful when calculating the total weight of a filled tank.

Why does my tank hold less water than the label says?

Manufacturers often state the nominal or external volume. The actual water capacity is lower because of glass thickness, the water line being below the rim, and internal equipment (substrate, decorations, filter) displacing water. It is normal for the usable volume to be 10–20% less than the advertised size.

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in length | Length | m | 1 | 0
in width | Width | m | 0.5 | 0
in height | Height | m | 0.5 | 0
out litres | Volume in litres | L | length * width * height * 1000 | 1
out us_gallons | Volume in US gallons | gal | length * width * height * 264.172 | 1
out imperial_gallons | Volume in imperial gallons | gal | length * width * height * 219.969 | 1