Fish Tank Volume Calculator Litres: How to Use It

This calculator works out the water volume of a rectangular, square, or cylindrical fish tank in litres. You enter the tank’s dimensions (length, width, height, or diameter and height) and the calculator returns the total volume in litres, along with the volume in gallons and the weight of the water in kilograms. It is a practical tool for sizing filters, heaters, and fish stock levels, and for dosing water treatments accurately.

Formula

The volume of a rectangular tank is calculated by multiplying its length, width, and height, then converting cubic centimetres to litres. The formula is:

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

• Length (l) – the longest horizontal dimension of the tank, in centimetres (cm).
• Width (w) – the shorter horizontal dimension, in centimetres (cm).
• Height (h) – the vertical dimension from the bottom to the top of the tank, in centimetres (cm).
• Volume (L) – the total capacity in litres (L).

For a cylindrical tank (e.g., a round bowl or a cylindrical aquarium), the formula is:

Volume (L) = π × (Diameter/2)² × Height / 1000

Where π ≈ 3.14159, and diameter and height are in centimetres. The division by 1000 converts cubic centimetres to litres because 1 litre = 1000 cm³.

Worked Example

Using the default values from the calculator: length = 60 cm, width = 30 cm, height = 40 cm.

1. Multiply the dimensions: 60 × 30 × 40 = 72,000 cm³.
2. Divide by 1000 to get litres: 72,000 / 1000 = 72.0 L.
3. Convert to US gallons: 72.0 × 0.264172 = 19.0 gal (rounded to one decimal).
4. Water weight: 72.0 L × 1 kg/L = 72.0 kg (fresh water).

The calculator returns 72.0 L, 19.0 gal, and 72.0 kg for the default inputs.

Why Tank Volume Matters

Knowing the exact water volume of your aquarium is not just a fun fact – it drives several practical decisions:

• Filter sizing: Most filters are rated for a maximum tank volume (e.g., “for tanks up to 100 L�). An undersized filter can’t handle the biological load.
• Heater wattage: A general rule is 3–5 watts per litre of water to maintain a stable temperature in a typical room.
• Fish stocking: Overcrowding leads to poor water quality. The “one inch of fish per gallon� rule (about 2.5 cm per 4 L) is a rough starting point, but it varies by species.
• Water treatments: Dechlorinators, medications, and fertilizers are dosed per litre. An accurate volume prevents under- or over-dosing.

How to Measure Your Tank Correctly

Measure the inside dimensions of the tank, not the outside. The glass thickness adds a few millimetres that can throw off the calculation, especially for smaller tanks. Use a tape measure or ruler and record the dimensions in centimetres for direct input.

If your tank is not a perfect rectangle (e.g., a bow-front or hexagonal tank), the formula becomes more complex. For a bow-front, you can approximate by treating it as a rectangle using the widest point, but this overestimates volume. For a hexagon, divide it into triangles or use a specialized calculator. The rectangle formula is accurate for the vast majority of standard aquariums.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure in inches, you must convert to centimetres (1 inch = 2.54 cm) before using the formula. Entering inches directly will give a wildly wrong volume.
• Using outside dimensions: As mentioned, inside dimensions are what matter for water capacity.
• Forgetting the substrate and decor: The actual water volume is less than the tank volume because substrate, rocks, and decorations displace water. For dosing treatments, you should use the actual water volume, which is typically 10–20% less. You can measure this by noting how many litres of water you add when filling the tank.
• Rounding too early: If you round intermediate results, the final volume can be off. Keep full precision until the end.

Limitations of the Calculator

This calculator assumes a perfectly rectangular, empty tank. It does not account for:

• Substrate (gravel, sand) – reduces water volume.
• Decorations (rocks, driftwood) – reduce water volume.
• Water level – if you don’t fill to the brim, the actual volume is lower. Measure the height of the water, not the tank, if you want the true water volume.
• Glass thickness – the inside dimensions are slightly smaller than outside.

For a more accurate “actual water volume,� measure the height of the water level (not the tank height) and subtract an estimate for substrate and decor (usually 10–15% for a lightly decorated tank).

Frequently Asked Questions

How many litres is a 60x30x40 cm tank?

Using the formula: (60 × 30 × 40) / 1000 = 72 litres. This is a common “20-gallon� tank size (actually 19 US gallons).

How do I convert gallons to litres?

1 US gallon = 3.78541 litres. Multiply the number of gallons by 3.78541 to get litres. For UK gallons (imperial), 1 UK gallon = 4.54609 litres. Most aquarium equipment uses US gallons, but check your product manual.

Does the volume include the gravel?

No, the calculator gives the total volume of the empty tank. Gravel, rocks, and decorations displace water, so the actual water volume is less. A typical rule of thumb is to subtract 10–20% for substrate and decor.

What if my tank is not rectangular?

For a cylinder, use the formula π × (diameter/2)² × height / 1000. For other shapes (bow-front, hexagon, L-shaped), break the tank into simpler shapes, calculate each volume, and add them together. For a bow-front, approximating as a rectangle using the widest point is common but overestimates by a few percent.

Why is the water weight important?

Water weighs about 1 kg per litre for fresh water (saltwater is slightly heavier, about 1.025 kg/L). Knowing the total weight helps you ensure your floor and stand can support the tank. A 72 L tank with substrate and glass can easily weigh over 100 kg.

What is the difference between US and UK gallons?

A US gallon is 3.785 litres, while a UK (imperial) gallon is 4.546 litres. The difference is about 20%. Most aquarium equipment, especially from North America, uses US gallons. Always check which gallon your product refers to.

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in l | Length | cm | 60 | 0
in w | Width | cm | 30 | 0
in h | Height | cm | 40 | 0
out vol_l | Volume (litres) | L | (l * w * h) / 1000 | 1
out vol_gal | Volume (US gallons) | gal | ((l * w * h) / 1000) * 0.264172 | 1
out water_kg | Water weight | kg | (l * w * h) / 1000 | 1