Aquarium Volume Calculator: How Many Gallons

This aquarium volume calculator works out the water capacity of a rectangular, square, or cylindrical tank in US gallons, liters, and cubic feet. You enter the tank’s dimensions (length, width, height, or diameter and height) and the calculator returns the total volume, plus the actual water volume if you account for the typical 2-inch gap between the water surface and the rim. The result is the volume the tank holds when full, not the weight or the number of fish it can support.

Formula

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

V = L × W × H

Where:

• V = volume in cubic inches (in³)
• L = internal length in inches (in)
• W = internal width in inches (in)
• H = internal height in inches (in)

To convert cubic inches to US gallons, divide by 231 (since 1 US gallon = 231 in³). To convert to liters, multiply cubic inches by 0.0163871 (since 1 in³ = 0.0163871 L). To get cubic feet, divide by 1728 (since 1 ft³ = 1728 in³).

For a cylindrical tank (e.g., a round acrylic tank or a sump), use V = π × (D/2)² × H, where D is the internal diameter in inches and H is the internal height. Then apply the same conversion factors.

Worked Example

Using the default values from the calculator: a tank that is 48 inches long, 12 inches wide, and 20 inches high, with a 2-inch gap from the water surface to the top.

1. Total volume in cubic inches: 48 × 12 × 20 = 11,520 in³
2. Convert to US gallons: 11,520 ÷ 231 = 49.87 gal (the calculator shows 49.9)
3. Water height with gap: 20 − 2 = 18 in
4. Water volume in cubic inches: 48 × 12 × 18 = 10,368 in³
5. Water volume in US gallons: 10,368 ÷ 231 = 44.88 gal (the calculator shows 44.9)
6. Total volume in liters: 11,520 × 0.0163871 = 188.8 L
7. Total volume in cubic feet: 11,520 ÷ 1728 = 6.67 ft³

So a standard 48×12×20-inch tank is commonly sold as a “55-gallon� aquarium, but the actual volume is 49.9 gallons when measured to the rim. The discrepancy comes from the fact that manufacturers measure the outside dimensions and round up to a “nominal� size.

When to Use This Calculator

Use this calculator when you need to know the exact water volume for:

• Dosing medications or water conditioners – most products give doses per gallon, so an accurate volume prevents under- or over-dosing.
• Choosing a heater or filter – equipment is rated for a maximum tank size, usually in gallons.
• Estimating water weight – multiply the water volume in gallons by 8.34 pounds per gallon (freshwater) to get the weight. For a 50-gallon tank, that’s about 417 lb of water alone, not counting the tank, substrate, and stand.
• Calculating CO2 or fertilizer dosing in planted tanks – these are often given per liter or per gallon.

The calculator is also useful when you’re planning a custom tank or a sump, where the volume isn’t a standard retail size.

How to Read the Result

The “total volume� is the capacity when the tank is filled to the very top rim. In practice, you never fill an aquarium that high – you leave a gap of 1 to 2 inches to prevent fish jumping out and to allow for water displacement from decorations and equipment. The “water volume� line accounts for that gap, so it’s the number you should use for dosing and stocking decisions.

Remember that the water volume is still an overestimate if you have thick substrate, large rocks, or many decorations, because those objects displace water. For a heavily decorated tank, subtract 10–20% from the water volume to get a more realistic “actual� water volume.

Common Mistakes and How to Avoid Them

The biggest mistake is measuring the outside of the tank instead of the inside. Glass thickness (typically ¼ to ½ inch for larger tanks) adds up. For a 48-inch tank with ½-inch glass, the inside length is 47 inches, which changes the volume by about 2–3%.

Another common error is mixing units. If you measure in centimeters, you must convert to inches (1 in = 2.54 cm) before using the formula, or use the metric conversion directly: volume in cm³ ÷ 1000 = liters. The calculator here expects inches, so convert first.

Also, be aware of the difference between US gallons and imperial gallons. The calculator uses US gallons (231 in³). Imperial gallons are about 20% larger (277.4 in³). If you’re in the UK or using an older British book, you’ll need to divide by 277.4 instead.

Limits of the Formula

This formula assumes a perfect rectangular or cylindrical shape with straight, parallel sides. It does not account for:

• Bow-front or corner tanks – these have curved or angled fronts, so the true volume is less than a rectangle with the same maximum dimensions. For a bow-front, you can approximate by using the average width (front and back) instead of the maximum width.
• Hexagonal or pentagonal tanks – these require a different geometric formula, usually breaking the shape into triangles.
• Sloped or irregular shapes – such as a “columnâ€� tank that is wider at the bottom than the top.

For those shapes, you’re better off measuring the actual water you pour in with a bucket or a flow meter, or using a more specialized calculator.

Why “Nominal� Sizes Differ from Real Volumes

Retailers sell tanks as “20 gallon,� “55 gallon,� or “75 gallon,� but these are nominal sizes based on the outside dimensions and often rounded to a convenient number. For example, a standard “20-gallon� tank is often 24 × 12 × 16 inches, which gives 24 × 12 × 16 / 231 = 19.9 gallons – close, but not exact. A “55-gallon� is typically 48 × 13 × 21 inches, which is 48 × 13 × 21 / 231 = 56.7 gallons. Always use the actual inside dimensions for dosing and stocking, not the label.

Frequently Asked Questions

How many fish can I keep in a 20-gallon tank?

This calculator doesn’t answer that directly because stocking depends on fish size, activity, filtration, and maintenance. A common rule of thumb is “1 inch of fish per gallon,� but that’s a rough guideline and fails for large or messy fish. For a 20-gallon tank, you might keep a small school of tetras (about 10–15) plus a few bottom dwellers, but always research each species’ adult size and space needs.

Does the calculator include the weight of the water?

No, it only gives volume. To get the weight of freshwater, multiply the water volume in gallons by 8.34 lb/gal. For saltwater, multiply by about 8.5 lb/gal. A 50-gallon freshwater tank holds about 417 lb of water; a 50-gallon saltwater tank holds about 425 lb. Add the weight of the tank, substrate, and stand to get the total floor load.

What if my tank is not rectangular?

For a cylinder, use the formula with diameter and height. For a bow-front, approximate with the average width. For other shapes, the best approach is to measure the water you actually add using a 5-gallon bucket or a flow meter – that gives you the true volume regardless of shape.

Should I use the total volume or the water volume for dosing?

Always use the water volume (with the gap) for dosing medications, conditioners, fertilizers, and CO2. The total volume includes the air space at the top, which doesn’t contain water. Also subtract for substrate and decorations if they take up a lot of space – a heavily planted tank might have 10–20% less water than the tank’s nominal capacity.

Why does my 55-gallon tank only hold 50 gallons of water?

Because the “55� is a nominal size based on outside dimensions. The inside volume is smaller due to glass thickness, and you leave a gap at the top. A typical 48×13×21-inch tank has an inside volume of about 50 gallons when filled to the rim, and about 47 gallons when you leave a 2-inch gap. The label is just a marketing approximation.

Can I use this calculator for a sump or refugium?

Yes, as long as the sump is rectangular or cylindrical. Measure the inside dimensions of the sump chamber, not the whole sump if it has multiple sections. For a sump that runs partly empty, you may want to measure the height of the water level when the sump is running, not the full height of the chamber.

Related

• Acreage Conversion Calculator: Acres to Common Units
• Acreage Calculator by Dimensions: Measure Any Shape
• Above Ground Pool Water Volume Calculator and Formula

in length | Length | in | 48 | 0
in width | Width | in | 12 | 0
in height | Height | in | 20 | 0
in gap | Water gap from top | in | 2 | 0
out gallons | Total volume | US gal | (length * width * height) / 231 | 1
out water_gal | Water volume (with gap) | US gal | ((length * width * (height - gap)) / 231) | 1
out liters | Total volume | L | (length * width * height) * 0.0163871 | 1
out cuft | Total volume | cu ft | (length * width * height) / 1728 | 2