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Aquarium Capacity Calculator | US gallons from tank dimensions

Find out exactly how many gallons and litres your aquarium holds from its inside dimensions, with the water volume after substrate and decor accounted for. The number you need for stocking levels, heater sizing and medication dosing.

Last updated: July 2026 · Free · No sign-up required

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Results

Enter values above and click Calculate to see your result instantly.

How the Aquarium Capacity Calculator works

Under the hood, this calculator applies US gallons = (L × W × H in inches) ÷ 231, so the aquarium capacity calculator result you see is genuine math, not a lookup table.

Scroll past the calculator for a quick-reference table, a step-by-step methodology with a fully worked example, and answers to the questions people most often ask about aquarium capacity calculator.

From our guides

Quick reference

Tank inchesUS gallons
10×10×125.2 gal
20×10×1210.4 gal
24×12×1620 gal
36×18×1850.5 gal
US gal divisorin³ ÷ 231

info Aquarium Capacity Calculator

Free math calculator — enter your numbers and get an instant, accurate result.

info Private by design

Everything runs locally in your browser. No uploads, no accounts, no tracking.

info Works everywhere

Fully responsive and mobile-friendly — calculate on any device, any time.

info Educational

Includes the formula and step-by-step explanation so you understand the math, not just the answer.

Interpretation guide

NoteDetail
Use inner dimensionsGlass thickness reduces volume
Substrate & rockDisplace ~10% of water
Fill line below rimActual water < full volume
Litresin³ ÷ 61.02

lightbulb Worked example

Let's say you are using the Aquarium Capacity Calculator. Calculates aquarium capacity using US gallons = (L × W × H in inches) ÷ 231, in real time in your browser. Enter the values that match your situation into the input fields and press calculate — using realistic numbers makes the result directly useful for you.

Result: The calculator instantly applies the formula US gallons = (L × W × H in inches) ÷ 231 and returns the result with appropriate precision.

What this means: Read the result in the context of what you are measuring. The step-by-step breakdown lets you confirm the math and understand which input most affects the outcome.

Formula & methodology

Formula: US gallons = (L × W × H in inches) ÷ 231

How aquarium capacity is calculated

The tool applies US gallons = (L × W × H in inches) ÷ 231, because one US gallon occupies exactly 231 cubic inches. Enter the tank's interior length, width, and height, and the tool returns the capacity in gallons and litres, with the water volume after substrate and decor estimated.

Example: a 48 × 18 × 21 inch tank holds 48 × 18 × 21 ÷ 231 = 78.6 US gallons, about 297.5 litres.

Reading the result: real water volume is 10–20% below the geometric figure once glass thickness, the waterline below the rim, substrate, and decor displace water. Stocking levels, heater sizing, and medication doses should use the filled-water estimate rather than the nominal tank size — the calculator shows both.

Authoritative source: Wolfram MathWorld

Frequently asked questions

How do I calculate aquarium gallons?
Enter the required values into the input fields and press the calculate button. The tool applies the formula US gallons = (L × W × H in inches) ÷ 231 and returns the result with a short explanation of each step, so you can check the working yourself.
How many gallons is my fish tank?
The Aquarium Capacity Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Calculates aquarium capacity using US gallons = (L × W × H in inches) ÷ 231, in real time in your browser. There is no limit on usage, and it works on any device.
How do I convert tank volume to litres?
Multiply the US gallons by 3.78541: a 78.6-gallon tank holds about 297.5 litres. If you measured in metric, compute the volume in cubic centimetres (length × width × height in cm) and divide by 1,000 for litres directly. The calculator shows gallons and litres side by side so you can use whichever your dosing instructions prefer.
Should I use inside or outside dimensions?
Use the inside dimensions for the true water capacity, because glass thickness reduces the interior volume. Outside dimensions overstate the capacity, typically by 5–10% depending on glass thickness. Measure the interior length, width, and height, and subtract the substrate and decor from the water volume once you know them.
Why is my real water volume lower than calculated?
Several factors reduce usable capacity: glass thickness, the waterline below the rim, substrate (gravel) displacing water, decorations and equipment, and the space left at the top for the filter outflow. Real water volume is typically 10–20% below the geometric tank volume. This matters for stocking levels and medication dosing, so use the filled-water estimate rather than the tank's nominal size.
How do I get gallons from my aquarium's dimensions?
Multiply length × width × height in inches and divide by 231 (cubic inches per US gallon). A standard 36" × 18" × 16" tank computes to 44.9 gallons — sold as a '45 gallon'. Actual water volume runs 10–15% lower once you subtract substrate, rock, and the fill line below the rim.
How many fish can my aquarium hold?
The old 'one inch of fish per gallon' rule is only a starting point, and only for slim-bodied fish under ~3 inches — a 20-gallon tank supporting roughly 15 inches of small tetras once cycled. It fails badly for heavy-bodied fish (goldfish need ~20 gallons for the first fish alone) and ignores filtration and territory. Treat the calculator's capacity as a ceiling, stock to about 80% of it, and add fish gradually.
What does the Aquarium Capacity Calculator do?
Calculates aquarium capacity using US gallons = (L × W × H in inches) ÷ 231, in real time in your browser. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.
What formula does the Aquarium Capacity Calculator use?
It uses US gallons = (L × W × H in inches) ÷ 231, because one US gallon occupies 231 cubic inches. Enter the tank's inside length, width, and height in inches and the tool returns capacity in gallons and litres. For example, a 48 × 18 × 21 inch tank holds about 78.6 US gallons before allowing for gravel and decorations.
Is the Aquarium Capacity Calculator free and private?
Yes — the Aquarium Capacity Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
How accurate is the Aquarium Capacity Calculator?
The Aquarium Capacity Calculator applies the standard formula: US gallons = (L × W × H in inches) ÷ 231. It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

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