Aquarium Tank Size Calculator: Gallons & Liters

This calculator works out the water volume of a rectangular aquarium in US gallons and liters. You enter the tank’s length, width, and height in inches (or centimeters), and it returns the total volume, accounting for the fact that the tank is not filled to the brim. The result is the volume of water the tank holds when filled to a typical level, which is what you need for stocking decisions, medication dosing, and filter sizing.

Formula

The standard formula for a rectangular aquarium volume is:

Volume (cubic inches) = Length × Width × Height

Then convert cubic inches to gallons: 1 US gallon = 231 cubic inches. So:

Gallons = (L × W × H) / 231

For liters, use the conversion 1 cubic inch = 0.0163871 liters, or work directly in centimeters: Volume (liters) = (L × W × H in cm) / 1000.

In this calculator, we multiply the raw geometric volume by 0.95 to account for the water line being below the rim. Most tanks are filled to about 1–2 inches from the top, and decorations, substrate, and equipment displace additional water. The 5% reduction is a practical average; a bare tank filled to the very top would be 100% of the geometric volume, but that is not how tanks are run.

Worked Example

Using the default values: Length = 48 in, Width = 13 in, Height = 20 in.

1. Compute cubic inches: 48 × 13 × 20 = 12,480 in³
2. Convert to gallons: 12,480 / 231 = 54.03 gallons (geometric)
3. Apply the 95% fill factor: 54.03 × 0.95 = 51.3 gallons

For liters: 12,480 × 0.016387 = 204.5 L (geometric), then × 0.95 = 194.3 L.

This matches the calculator output: 51.3 gal and 194.3 L. A 48×13×20 inch tank is commonly sold as a “55-gallon� aquarium; the difference between the advertised size and the actual water volume is exactly why this calculator uses the fill factor.

How to Measure Your Tank Correctly

Measuring a tank sounds trivial, but small errors change the result. Follow these rules:

• Measure inside dimensions, not outside. Glass thickness (typically ¼ to ½ inch on larger tanks) adds several gallons if you measure the outside. Use the interior width and length.
• Measure height from the bottom glass to the top rim. Do not include the plastic frame; the water line sits below it.
• Use the same unit for all three dimensions. Mixing inches and centimeters is the most common error. If you measure in cm, the result is liters directly; if in inches, you get gallons.
• For bow-front or hexagonal tanks, this rectangular formula will overestimate. Use a different shape calculator or approximate the average width.

Why the “Advertised� Size Differs

Many tanks are sold by a nominal size that does not match the true water volume. A “20-gallon high� is often 24×12×16 inches, which is 19.9 gallons geometric and about 18.9 gallons actual. A “75-gallon� is usually 48×18×21 inches, giving 78.5 gallons geometric and 74.6 gallons actual. The difference comes from marketing rounding and from the fact that the tank dimensions are exterior, not interior. When you calculate stocking levels, always use the actual water volume, not the tank’s brand name.

Common Mistakes and Misconceptions

• Using the full height. You do not fill a tank to the rim. The 5% factor is a good default, but if you keep the water level 2 inches below the top on a 12-inch-high tank, that is a 16% reduction, not 5%.
• Confusing US and UK gallons. A UK gallon is 277.4 cubic inches, about 20% larger than a US gallon. Most aquarium equipment and fish stocking guidelines use US gallons. This calculator outputs US gallons.
• Forgetting displacement. Substrate, rocks, and decorations can displace 10–20% of the water volume. A heavily aquascaped tank holds significantly less water than the empty tank volume. For medication dosing, you must subtract the displacement of hardscape and substrate.
• Ignoring the sump. If you have a sump filter, the total system volume includes the sump’s water. Add the sump volume separately; the main tank calculator only gives the display tank.

How to Use the Result

Once you know the actual water volume, you can make several decisions:

• Stocking fish: A common rule of thumb is 1 inch of adult fish per gallon, but this is crude. Use the actual gallons, not the advertised size, and research each species’ specific needs.
• Medication and additives: Most treatments give a dose per gallon. Using the wrong volume can underdose (ineffective) or overdose (harmful). Always use the true water volume.
• Filter sizing: Filters are rated for a tank size in gallons. Choose a filter rated at least 2–3 times your actual water volume per hour for good turnover.
• Heater wattage: A general guideline is 3–5 watts per gallon. A 50-gallon tank needs a 150–250W heater, depending on room temperature.

Limitations of the Formula

This calculator is accurate for rectangular, flat-bottom tanks. It does not account for:

• Substrate displacement (gravel, sand, soil)
• Decorations and hardscape
• Water level below the rim (we assume 5% below)
• Bowed fronts, hexagonal, cylindrical, or custom shapes
• Water displacement from fish themselves (negligible for most aquariums)

For a more precise volume, fill the tank with buckets and measure how much water you actually add. That is the only way to know the true volume for medication dosing, and it automatically includes all displacements.

Frequently Asked Questions

How many fish can I keep in a 55-gallon tank? Using the 1-inch-per-gallon rule, you could theoretically keep 55 inches of adult fish, but that rule is outdated. A better approach is to research each species’ minimum tank size and adult size, and aim for no more than 80% of the rule’s capacity. For a 55-gallon tank, a community of small tetras, corydoras, and a pair of dwarf cichlids is typical, but always check compatibility.

Does the 5% fill factor apply to all tanks? No. It is a default for a typical tank filled to about an inch below the rim. If you keep the water level lower (to reduce jumping or for a specific aquascape), the actual volume is lower. For a 12-inch-high tank with water 2 inches below the rim, the factor is about 0.83, not 0.95. Adjust the factor based on your actual water height.

What is the difference between US and UK gallons for aquariums? A US gallon is 231 cubic inches (3.785 liters). A UK (imperial) gallon is 277.4 cubic inches (4.546 liters). Most aquarium literature, equipment, and fish stocking guidelines use US gallons, so always confirm which unit is being referenced. If you see a tank listed in “gallons� from a UK source, it is likely imperial gallons.

How do I calculate the volume of a bow-front tank? Bow-front tanks are not rectangular. A rough approximation is to treat them as a rectangle using the average width (the width at the middle of the bow). Multiply the average width by length and height, then apply the same conversion. This is less accurate than a rectangle; for exact volume, measure water added with buckets.

Should I include the sump volume in my tank size? Yes, for medication and total system volume, you should add the sump’s water volume to the display tank volume. The sump typically holds 10–30% of the display volume, depending on its size and water level. For stocking, however, the display tank’s footprint is what matters, not the total water volume, because fish need horizontal swimming space.

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in l | Length | in | 48 | 0
in w | Width | in | 13 | 0
in h | Height | in | 20 | 0
out gal | Water volume (US gallons) | gal | (l * w * h * 0.004329) * 0.95 | 1
out lit | Water volume (liters) | L | (l * w * h * 0.016387) * 0.95 | 1