Aquarium Volume Calculator by Dimensions

This calculator works out the water volume of a rectangular, square, or cylindrical aquarium from its dimensions. You enter the length, width, and height (or diameter and height for a cylinder), choose your unit of measurement, and it returns the volume in gallons and liters, along with the approximate water weight. The result is the total volume of the tank, not the actual amount of water you will put in, because you will leave space for substrate, decorations, and the waterline below the rim.

Formula

For a rectangular aquarium, the volume is:

Volume = Length × Width × Height

Where length, width, and height are in the same unit. If you measure in centimeters, divide by 1000 to get liters. To convert liters to US gallons, multiply by 0.264172; to UK gallons, multiply by 0.219969.

For a cylindrical tank (like a barrel or a bow-front with a circular cross-section), the formula is:

Volume = π × (diameter/2)² × Height

The calculator uses the same length, width, and height fields for a cylinder, treating length as the diameter. The shape selector applies the factor π/4 (about 0.7854) to the rectangular formula, which converts length×width into the area of a circle with that diameter.

Worked Example

Using the default values: a rectangular tank 60 cm long, 30 cm wide, and 40 cm high.

1. Calculate the volume in cubic centimeters: 60 × 30 × 40 = 72,000 cm³.
2. Convert to liters: 72,000 ÷ 1000 = 72.0 L.
3. Convert to US gallons: 72.0 × 0.264172 = 19.0 US gal.
4. Convert to UK gallons: 72.0 × 0.219969 = 15.8 UK gal.
5. Water weight: 72.0 kg (since 1 L of water weighs 1 kg), which is 72.0 × 2.20462 = 158.7 lb.

The calculator rounds the liter output to one decimal, giving 72.0 L and 19.0 US gal.

How to Measure Your Aquarium Correctly

To get an accurate volume, measure the internal dimensions of the tank, not the outside. Glass thickness can add a centimeter or more to each side, which overestimates the true water capacity. Use a tape measure or ruler and measure from the inside edge to the inside edge. For height, measure from the bottom glass to the top edge, but keep in mind that the actual water level will be lower than the rim—usually 1 to 2 cm below the top—so the real water volume is slightly less than the calculated volume.

For a bow-front tank, the shape is not a perfect rectangle. A common approximation is to treat it as a rectangle plus a half-cylinder. You can measure the maximum width at the center and the width at the ends, then average them for a rough width. The calculator’s rectangle mode will give a reasonable estimate, but for exact volume, use a bucket and measure water as you fill.

Why Volume Matters

Knowing your aquarium volume is essential for several practical reasons:

• Dosing medications and water conditioners: Most treatments specify a dose per gallon or liter. An overestimate can harm fish; an underestimate can be ineffective.
• Choosing a heater: Heater wattage is rated for tank size. A general rule is 3 to 5 watts per gallon, but this varies with room temperature.
• Stocking fish: Many aquarists use the “one inch of fish per gallonâ€� rule, though modern practice considers adult size, behavior, and filtration. Volume is the starting point.
• Filter sizing: Filters are rated for a maximum tank volume. Your filter should turn over the tank volume at least 4–5 times per hour.
• Water changes: If you change 25% of a 20-gallon tank, you need to prepare 5 gallons of new water.

Common Mistakes and Misconceptions

A frequent error is mixing units. If you measure length in inches and width in feet, the formula fails. Always convert all dimensions to the same unit before multiplying. Another common mistake is using the outside dimensions, which adds glass thickness and overestimates volume. Also, many people confuse US gallons with UK gallons. A US gallon is 3.785 liters, while a UK gallon is 4.546 liters—about 20% larger. The calculator shows both to avoid this.

Another misconception is that the volume you calculate is the amount of water you can pour in. In reality, substrate (gravel or sand), rocks, driftwood, and equipment like filters and heaters displace water. A tank filled with 2 inches of gravel will hold less water than the empty volume. Also, you never fill the tank to the brim; you leave an air gap at the top. So the actual water volume is typically 10–20% less than the calculated volume.

How to Use the Result

The volume you get is the gross volume. For practical use, subtract the displacement of substrate and decorations. As a rough guide, each inch of substrate depth reduces water volume by about 1–2% of the tank volume, depending on grain size. For dosing, many aquarists use the gross volume for safety, but for water changes, it is better to measure the water you actually remove with a bucket.

If you are setting up a new tank, use the volume to choose a filter and heater. For example, a 72 L (19 US gal) tank would need a filter rated for at least 72 L/h (19 gph) but preferably 4–5 times that, so look for a filter rated for 100–150 L/h. A heater of 50–75 watts would be adequate for a 19-gallon tank in a typical room.

Limits of the Formula

The formula assumes a perfectly rectangular or cylindrical tank with flat, parallel sides. Real tanks have rounded corners, sloping fronts, or irregular shapes. For these, the formula gives an approximation. The only exact method is to fill the tank with a known volume of water (using a measuring jug or bucket) and record the amount. Also, the formula does not account for the thickness of the glass, the substrate, or the air gap, so it overestimates the true water capacity.

For a bow-front or hexagonal tank, you can approximate by breaking the shape into simpler geometric figures. For example, a hexagon can be divided into rectangles and triangles. But for most hobbyists, the rectangle approximation is close enough.

Frequently Asked Questions

Do I measure the outside or inside of the tank?
Measure the inside, because that is the space water actually occupies. The glass thickness adds to the outside dimensions and would make the volume too large.

How much water will I actually put in?
Less than the calculated volume. You leave a gap at the top (usually 1–2 cm), and substrate, rocks, and equipment displace water. A typical 20-gallon tank might hold 16–18 gallons of actual water.

What is the difference between US and UK gallons?
A US gallon is 3.785 liters, while a UK (imperial) gallon is 4.546 liters. So a tank that holds 20 US gallons holds about 16.6 UK gallons. Always check which gallon your medication or filter rating uses.

Can I use this calculator for a cylindrical or bow-front tank?
Yes, the cylinder option works for a true cylinder (like a barrel). For a bow-front, the rectangle mode with an average width gives a rough estimate, but it is not exact.

Why is my calculated volume different from the manufacturer’s rating?
Manufacturers often list the nominal or “trade� size, which may be based on outside dimensions or rounded to a standard size. Your internal measurements give a more accurate number.

How do I convert liters to gallons?
Multiply liters by 0.264172 to get US gallons, or by 0.219969 to get UK gallons. The calculator does this automatically.

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in l | Length | cm | 60 | 0
in w | Width | cm | 30 | 0
in h | Height | cm | 40 | 0
sel shape | Shape | Rectangle=1, Cylinder=0.785398
out vol_l | Water volume | L | (l * w * h * shape) / 1000 | 1
out vol_us | Water volume | US gal | ((l * w * h * shape) / 1000) * 0.264172 | 1
out vol_uk | Water volume | UK gal | ((l * w * h * shape) / 1000) * 0.219969 | 1
out weight_kg | Water weight | kg | (l * w * h * shape) / 1000 | 1
out weight_lb | Water weight | lb | ((l * w * h * shape) / 1000) * 2.20462 | 1