Fish Tank Volume Calculator: Gallons Made Easy

This calculator works out the water volume of a rectangular fish tank in US gallons and litres. You enter the tank’s length, width, and height in inches (or centimetres), and it returns the volume in gallons, litres, and cubic inches. The result is the total tank capacity; the actual water volume will be slightly less because of substrate, decorations, and the space left at the top.

Formula

The volume of a rectangular tank is length × width × height. That gives you cubic inches (in³) when all three are in inches. To convert cubic inches to US gallons, divide by 231 (because 1 US gallon = 231 in³). To convert to litres, multiply the cubic-inch figure by 0.0163871 (because 1 in³ = 0.0163871 L).

In symbols: V = L × W × H, where V is volume, L is length, W is width, and H is height (all in inches). Then Gallons = V ÷ 231 and Litres = V × 0.0163871.

Worked Example

Using the default values from the calculator (L = 24 in, W = 12 in, H = 16 in):

• Cubic inches: 24 × 12 × 16 = 4,608 in³
• US gallons: 4,608 ÷ 231 = 19.95 gallons (rounded to 19.95)
• Litres: 4,608 × 0.0163871 = 75.51 L (rounded to 75.51)

So the tank holds about 20 US gallons or 75.5 litres. That matches the calculator’s output.

When to Use This Calculator

Use it when you need to know the capacity of a rectangular aquarium:

• To size a heater or filter (they are rated for tank volume).
• To calculate how much water conditioner or medication to add (always read the product label – never rely on memory).
• To estimate how many fish you can keep (a common rule is 1 inch of adult fish per gallon, but that is a rough guideline, not a law).
• To figure out how much substrate or how many plants you need.

For non-rectangular tanks (bow-front, hexagon, cylinder), use a different formula – this one only works for straight-sided rectangles.

How to Read the Result

The number you get is the total volume when the tank is filled to the brim. In practice, you leave about an inch of air at the top (to prevent fish jumping out and to allow oxygen exchange), and you have substrate, rocks, and equipment that displace water. So the real water volume is typically 10–20% lower than the calculated value. If you need a precise dose of medication or conditioner, measure the actual water you put in (e.g., with a bucket) rather than relying on the tank’s nominal volume.

Common Mistakes and Misconceptions

• Mixing units: If you measure length in inches but width in centimetres, the result will be wrong. Keep all three in the same unit.
• Imperial vs. US gallons: The calculator gives US gallons. A UK (imperial) gallon is about 4.546 litres, while a US gallon is 3.785 litres. The difference is about 20% – significant if you use the wrong one.
• Forgetting the glass thickness: The calculator uses outside dimensions. If you measure the outside, the inside volume is slightly less (typically a few percent for larger tanks). For exact medication dosing, measure the inside or use the actual water volume.
• Overestimating fish capacity: A 20-gallon tank does not hold 20 gallons of water once you add gravel and decor. Also, the “inch per gallon� rule ignores body shape and waste production – research each species.

Alternatives and Other Shapes

If your tank is not a rectangle, use these formulas:

• Cylinder (round): π × r² × h (r = radius, h = height). Convert to gallons by dividing by 231.
• Bow-front: Approximate as a rectangle plus a half-cylinder, or use a tank-specific calculator.
• Hexagon: Divide into triangles or use a dedicated hexagonal tank calculator.

Cost and Safety Considerations

Knowing the volume helps you choose the right equipment and avoid overstocking, which can lead to poor water quality and stress for fish. A larger tank is generally more stable in temperature and water chemistry than a small one. Always place the tank on a stand that can support the weight: water weighs about 8.34 pounds per gallon (1 kg per litre), so a 20-gallon tank with water alone weighs over 160 lb (72 kg) – plus the glass and substrate. Never place a large tank on an unbraced shelf or table.

Frequently Asked Questions

How many gallons is a 20-inch by 10-inch by 12-inch tank?

Multiply 20 × 10 × 12 = 2,400 in³. Divide by 231 to get 10.39 US gallons. That is the total capacity; actual water volume will be less.

What is the difference between US and UK gallons?

A US gallon is 231 in³ (3.785 L). A UK (imperial) gallon is 277.42 in³ (4.546 L). So a UK gallon is about 20% larger. If you are in the UK, use the litre output or multiply the US gallon result by 0.833 to get imperial gallons.

How much water is actually in my tank after adding gravel and decorations?

It varies, but a good rule is to subtract 10–20% from the total volume. For a 20-gallon tank, that means roughly 16–18 gallons of water. For precise dosing, measure the water you add with a bucket.

Does the calculator work for a hexagon or bow-front tank?

No, this calculator assumes a rectangular tank with straight sides. For other shapes, use a different formula or a dedicated calculator that accounts for the shape.

Can I use this to figure out how many fish I can keep?

You can use the volume as a starting point, but fish stocking depends on species, adult size, filtration, and behavior. The “inch per gallon� rule is a rough guide, not a guarantee. Always research the specific fish.

Why do I need to leave space at the top?

Fish need oxygen at the surface, and some species jump. Leaving an inch or two of air also prevents the water from overflowing when you add your hand or equipment. It also gives a buffer for evaporation.

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in length | Length | in | 24 | 0
in width | Width | in | 12 | 0
in height | Height | in | 16 | 0
out gallons | US Gallons | gal | (length * width * height) / 231 | 2
out litres | Litres | L | (length * width * height) * 0.0163871 | 2
out cubic_inches | Cubic Inches | in³ | length * width * height | 0