Aquarium Tank Volume Calculator: Gallons & Liters

This calculator works out the water volume of a rectangular, square, or bow-front aquarium in US gallons and liters. You enter the tank’s dimensions (length, width, and height in inches or centimeters) and the glass thickness if you want the inside volume, and it returns the total volume and the actual water capacity when filled to a typical level below the rim.

Formula

For a rectangular tank, the volume is simply the product of its three internal dimensions:

V = L × W × H

Where:

• V = volume in cubic inches (in³) or cubic centimeters (cm³)
• L = internal length of the tank
• W = internal width of the tank
• H = internal height of the tank (water depth, not tank height if you want water volume)

To convert to US gallons, divide cubic inches by 231 (since 1 US gallon = 231 in³). To convert to liters, multiply cubic inches by 0.0163871 (1 in³ = 0.0163871 L). For metric dimensions in centimeters, divide by 1000 to get liters (1 L = 1000 cm³).

For a bow-front (curved) tank, the formula becomes more complex. A common approximation treats it as a rectangular tank plus a cylindrical segment. The exact volume depends on the radius of the curve, which is rarely published. A simpler practical method is to measure the tank’s maximum width at the center and its width at the ends, average them, and use that average in the rectangular formula. This overestimates slightly because the curve is not a straight taper, but for most hobby purposes the error is under 5%.

Worked Example

Using the default values from the calculator: a tank that is 48 inches long, 13 inches wide, 20 inches high, with 0.5-inch glass.

1. Total volume (external): 48 × 13 × 20 = 12,480 in³. Divide by 231 → 54.0 US gallons.
2. Internal dimensions: subtract twice the glass thickness from length and width, and once from height (assuming an open top): length = 48 − 1 = 47 in, width = 13 − 1 = 12 in, height = 20 − 0.5 = 19.5 in.
3. Internal volume: 47 × 12 × 19.5 = 10,998 in³. Divide by 231 → 47.6 US gallons.
4. Water capacity at 90% fill: 47.6 × 0.9 = 42.8 US gallons. This matches the calculator’s second output.
5. Liters: 12,480 in³ × 0.0163871 = 204.5 L (or 47.6 × 3.78541 = 180.2 L for the internal volume).

Notice the difference: the external volume (54 gal) is what manufacturers often advertise, but the actual water you can put in is closer to 43 gallons. That 11-gallon gap matters for medication dosing and fish stocking.

Why the Calculator Gives Two Numbers

The first output, total volume, is the geometric volume of the tank based on its outer dimensions. This is the number you see on the box or in the product name (e.g., a “55-gallon� tank). The second output, water capacity, subtracts the glass thickness and leaves a 10% air gap at the top. This is the realistic volume of water you can actually pour in.

The 10% fill factor is a common rule of thumb, not a physical law. Some aquarists fill to within an inch of the rim, which for a 20-inch-high tank is 95% of the internal height. Others leave more space if they have jumping fish or a hang-on-back filter that displaces water. You can adjust the fill percentage mentally: multiply the internal volume by 0.95 for a nearly full tank, or by 0.85 if you keep a large air gap.

Also note that the calculator does not subtract the volume displaced by substrate, rocks, decorations, or equipment. A tank with 2 inches of gravel and a large piece of driftwood holds noticeably less water than the calculated capacity. For medication dosing, always use the actual water volume, which you can measure by tracking how many gallons you add during a water change.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure length in inches and width in centimeters, the formula gives nonsense. Always convert to the same unit first.
• Using outside dimensions for water volume: Glass thickness typically reduces volume by 5–10% on a standard tank. For a 10-gallon tank with 0.25-inch glass, the internal volume is about 9.3 gallons.
• Confusing US gallons with imperial gallons: A US gallon is 231 in³, while an imperial gallon (used in the UK and Canada) is 277.4 in³. A 50 US-gallon tank is about 41.6 imperial gallons. This calculator uses US gallons.
• Forgetting the substrate displacement: A 1-inch layer of gravel in a 20-gallon tank displaces roughly 1.5–2 gallons of water. For a heavily planted tank with large rocks, the displacement can be 20% or more.
• Measuring water depth instead of tank height: If you only fill the tank halfway, the volume is half. The calculator assumes you fill to the top (minus the 10% gap).

How to Measure Your Tank Correctly

To get an accurate reading, follow these steps:

1. Measure the inside, not the outside. Use a tape measure or ruler placed inside the empty tank, from the bottom glass to the top edge. If you can’t reach inside, measure the outside and subtract twice the glass thickness from length and width, and once from height.
2. Measure at the water line. For water volume, measure from the bottom to the intended water surface, not the top of the glass. Most people leave 1–2 inches of air.
3. For a bow-front tank, take three width measurements: at the left edge, the center (widest point), and the right edge. Average them and use that as the width in the rectangular formula. This is an approximation but works well for typical bow radii.
4. For a hexagonal or cylindrical tank, use the appropriate formula. A cylinder is π × r² × h. A hexagon can be split into triangles or approximated as a rectangle with the average width.

If you have an oddly shaped tank (e.g., a corner unit or a custom acrylic with a curved back), the most reliable method is to fill it with a known volume using a 1-gallon jug or a flow meter. This also accounts for internal braces and overflow boxes that the geometric formula misses.

Why Volume Matters Beyond Sizing

Knowing your tank’s true water volume is not just for choosing the right filter or heater. It is critical for:

• Medication dosing: Most fish medications specify a dose per gallon. Overdosing by 20% can harm fish, especially invertebrates.
• Water change calculations: If you do a 25% weekly water change, you need to know how many gallons to remove and replace. On a 55-gallon tank, that is 13.75 gallons, not 14.
• Stocking levels: The “one inch of fish per gallonâ€� rule is a rough guide, but it assumes the actual water volume, not the advertised tank size. A 55-gallon tank with 2 inches of substrate holds closer to 45 gallons, so you should stock accordingly.
• CO2 and fertilizer dosing in planted tanks: These are also per-gallon. Overdosing CO2 can suffocate fish.
• Choosing a heater: A heater’s wattage rating is based on the water volume and the temperature difference you need. A 100-watt heater is fine for 20 gallons but inadequate for 50.

When in doubt, round down. It is safer to underdose medication and add more if needed than to overdose. For stocking, fewer fish means less waste and a more stable environment.

Frequently Asked Questions

How many gallons is a 48x13x20 tank?

Using the default example above, the external volume is 54 US gallons. The internal water capacity at 90% fill is about 43 gallons. If you see a tank labeled “55 gallon� with those dimensions, it likely refers to the external volume or a slightly taller version (48x13x21). Always check the actual dimensions before buying.

Does the calculator work for a cylindrical tank?

No, this calculator is for rectangular and bow-front tanks. For a cylinder, use the formula V = π × r² × h, where r is the radius (half the diameter) and h is the water height. Convert cubic inches to gallons by dividing by 231, or cubic centimeters to liters by dividing by 1000.

How much water do I need to fill a 20-gallon tank?

If the 20 gallons is the advertised size, you will need about 18–19 gallons of water because of glass thickness and the air gap. If you are adding substrate and decorations, the actual water volume will be less—perhaps 15–16 gallons. Use a measuring bucket to confirm.

What is the difference between a US gallon and an imperial gallon?

A US gallon is 3.785 liters, while an imperial gallon is 4.546 liters. That is a 20% difference. A tank that holds 50 US gallons holds about 41.6 imperial gallons. This calculator uses US gallons, which is the standard for aquarium equipment in North America and most of Asia. If you are in the UK or Australia, divide the US gallon result by 1.2 to get imperial gallons.

Why does my tank hold less water than the label says?

Manufacturers typically list the nominal volume based on external dimensions, ignoring glass thickness, braces, and the fact that you do not fill to the very top. A 55-gallon tank often holds 48–50 gallons of water when filled to 1 inch from the rim. Add substrate and decorations, and the actual water volume drops further. This is normal and expected.

Can I use this calculator for a sump or refugium?

Yes, if the sump is rectangular. Measure the length, width, and water height (not the tank height) inside the sump. Enter those values and set glass thickness to 0 if the sump is made of acrylic or glass with negligible thickness. The result will be the water volume in the sump, which you need for dosing and for ensuring your return pump does not overflow the sump when the main pump is off.

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in len | Length | in | 48 | 0
in wid | Width | in | 13 | 0
in hei | Height | in | 20 | 0
in glass | Glass thickness | in | 0.5 | 0
out vol | Total volume | US gal | (len * wid * hei) / 231 | 1
out water | Water capacity (90% fill) | US gal | ((len - 2 * glass) * (wid - 2 * glass) * (hei - glass) * 0.9) / 231 | 1
out liters | Total volume | L | (len * wid * hei) * 0.0163871 | 1