Aquarium Volume Calculator: Rectangular Tank Size

This calculator works out the water volume of a rectangular aquarium in litres and US gallons. You enter the tank’s length, width, and height in either centimetres or inches, and it returns the volume in both units, accounting for the fact that you will not fill the tank to the very brim. The first result shown is the total volume in litres, followed by the same volume in US gallons, so you can size a heater, filter, or water-change schedule correctly.

Formula

The volume of a rectangular prism is simply the product of its three internal dimensions:

V = L × W × H

Where:

• V = volume in cubic centimetres (cm³)
• L = internal length in centimetres (cm)
• W = internal width in centimetres (cm)
• H = internal height in centimetres (cm)

To convert cubic centimetres to litres, divide by 1,000 (since 1 L = 1,000 cm³). To convert litres to US gallons, multiply by 0.264172 (since 1 US gallon ≈ 3.78541 L). If you measure in inches, multiply each dimension by 2.54 to get centimetres first, or use the direct conversion: 1 cubic inch = 16.3871 cm³.

Worked Example

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

1. Compute the cubic volume: 60 × 30 × 40 = 72,000 cm³.
2. Convert to litres: 72,000 ÷ 1,000 = 72.0 L.
3. Convert to US gallons: 72.0 × 0.264172 = 19.0 gal (rounded to one decimal).

The calculator shows 72.0 L and 19.0 gal for these defaults. Note that this is the total volume when filled to the top edge. In practice you leave a few centimetres of air gap, so the actual water volume will be slightly lower.

Why Internal Dimensions Matter

Aquarium manufacturers usually list the external dimensions on the box. The glass walls, bottom pane, and any plastic trim take up space, so the internal volume is always less than the external volume. For a typical glass tank with 6 mm glass, the difference is small (roughly 2–4% of the total), but it becomes significant for accurate medication dosing or when you need to know exactly how much water a tank holds for a water change. If you want the true water volume, measure the inside from glass surface to glass surface, not the outside.

Also, the height you enter should be the height of the water, not the height of the tank. Most aquarists fill the tank to about 2–3 cm below the top rim to prevent fish from jumping out and to allow for water evaporation. If you enter the full tank height, you will overestimate the water volume by roughly 5–10%, which can matter when adding salt or treatments.

How to Read the Result

The calculator gives two numbers: litres and US gallons. Litres are the standard unit in most of the world; US gallons are common in the United States. Note that a US gallon is smaller than a UK (imperial) gallon (1 UK gal ≈ 4.546 L). If you need imperial gallons, multiply the litres by 0.219969. The result is the theoretical volume of a perfectly rectangular box. Real tanks have rounded corners, sloping front panels (bow-front), or curved glass, all of which reduce the actual volume. For a standard rectangular tank with flat glass, the error is usually under 2%.

When you use this number to choose equipment, remember that the total water volume includes the water in your filter, sump, or external canister. If you have a large sump, add its volume to the tank volume for an accurate total. Conversely, if you are dosing medication, you usually treat only the display tank volume, not the sump, unless the instructions say otherwise.

Common Mistakes and How to Avoid Them

• Mixing units: Do not enter length in centimetres and width in inches. Convert everything to the same unit first. The calculator assumes all three inputs are in the same unit (cm).
• Using external dimensions: As noted, external dimensions overestimate volume. Measure the inside of the tank.
• Including the substrate: Gravel, sand, rocks, and decorations displace water. If you have a thick substrate layer, the actual water volume is lower. A rough rule: each 1 cm of substrate across the entire bottom reduces the water volume by about 10% of the tank’s height.
• Forgetting the water line: The tank is not filled to the brim. Subtract 2–3 cm from the height for a realistic water volume.
• Rounding too early: If you round the cubic volume to the nearest litre before converting to gallons, you may get a slightly different answer. Keep full precision until the final step.

Practical Uses for the Volume

Knowing your aquarium’s water volume is essential for several routine tasks:

• Heater sizing: A common guideline is 3–5 watts per litre (or 10–20 watts per US gallon) to maintain a tropical temperature. A 72 L tank would need roughly 216–360 W, though in practice a single 200 W heater often suffices for a room-temperature room.
• Filter flow rate: Most filter manufacturers recommend a flow rate of 4–6 times the tank volume per hour for freshwater. For 72 L, that means 288–432 L/h.
• Water changes: If you change 25% weekly, you need to prepare 18 L (72 × 0.25) of treated water each time.
• Medication and additives: Dosing instructions are usually per litre or per gallon. Using the wrong volume can under-dose (ineffective) or over-dose (harmful to fish).
• Stocking levels: A common (but rough) rule is 1 cm of adult fish per litre of water, or 1 inch per US gallon. This is a starting point, not a guarantee.

Limitations of the Rectangular Formula

This formula works only for tanks with straight sides and a flat bottom. If your tank is a bow-front, hexagon, cylinder, or has a curved corner, the rectangular formula will overestimate the volume. For those shapes, you need a different geometric formula (e.g., for a cylinder, V = πr²h). Also, the formula assumes the tank is filled to the top edge. As discussed, the actual water volume is lower because of the air gap and any hardscape. For a precise measurement, you can physically measure the water you add when filling the tank for the first time—using a bucket or a flow meter—and note that number for future reference.

Frequently Asked Questions

How many fish can I keep in a rectangular tank?

There is no universal answer because fish size, behaviour, and filtration matter more than volume alone. A very rough guideline is 1 cm of adult fish length per litre of water (or 1 inch per US gallon), but this ignores the fact that some fish need more swimming space or produce more waste. Always research the specific species before stocking.

Should I measure the tank from the outside or inside?

Measure from the inside, because the glass thickness reduces the internal volume. If you only have the external dimensions, subtract twice the glass thickness from the length and width, and subtract the bottom glass thickness from the height (plus any trim). For a 6 mm glass tank, that means subtracting 12 mm from length and width.

Does the substrate reduce the water volume?

Yes. Gravel, sand, and rocks take up space that would otherwise be water. A 5 cm layer of gravel across a 60×30 cm base displaces roughly 9 litres (60×30×5 / 1000). If you have a deep substrate, reduce the height you enter in the calculator accordingly.

How much water do I need for a 50% water change?

Multiply the tank’s water volume by 0.5. For a 72 L tank, that is 36 L. If you have a sump, include its volume in the total. Always treat the new water with a dechlorinator before adding it to the tank.

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 calculator gives US gallons. To convert to UK gallons, multiply the litre result by 0.219969. Many aquarium products sold in the UK use litres, so the difference rarely matters in practice.

Can I use this calculator for a sump or refugium?

Yes, if the sump is rectangular. Measure its internal dimensions and enter them. The calculator will give the volume of the sump alone. To get the total system volume, add the sump volume to the display tank volume.

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in length | Length | cm | 60 | 0
in width | Width | cm | 30 | 0
in height | Height | cm | 40 | 0
out litres | Volume (litres) | L | (length * width * height) / 1000 | 1
out gallons | Volume (US gal) | gal | ((length * width * height) / 1000) * 0.264172 | 1