Aquarium Gallon Size Calculator: Tank Volume

This aquarium gallon size calculator works out the water volume of a rectangular, square, or bow-front tank in US gallons and liters. You enter the tank’s dimensions (length, width, and height) in inches or centimeters, and it returns the total volume, accounting for glass thickness if you choose to subtract it. The result is the volume you need for stocking decisions, water-change calculations, and dosing treatments.

Formula

The calculator uses the standard rectangular volume formula: Volume = Length × Width × Height. For a tank with glass walls, the interior dimensions are reduced by the glass thickness on each side. The formula in the calculator is:

Interior volume (in³) = (L − 2t) × (W − 2t) × (H − t)

Where:

• L = external length of the tank (inches)
• W = external width (inches)
• H = external height (inches)
• t = glass thickness (inches)

To convert cubic inches to US gallons, divide by 231 (since 1 US gallon = 231 in³). To convert to liters, multiply cubic inches by 16.3871 (since 1 L = 61.024 in³). The calculator subtracts the glass thickness from length and width (two sides each) and from height (bottom only, assuming an open top). If you enter 0 for glass thickness, it gives the external volume, which is a slight overestimate for actual water capacity.

Worked Example

Using the default values: length = 48 in, width = 13 in, height = 20 in, glass thickness = 0.5 in.

1. Interior length = 48 − 2×0.5 = 47 in
2. Interior width = 13 − 2×0.5 = 12 in
3. Interior height = 20 − 0.5 = 19.5 in
4. Interior volume = 47 × 12 × 19.5 = 10,998 in³
5. US gallons = 10,998 / 231 = 47.6 gal
6. Liters = 10,998 × 0.0163871 = 180.2 L

The calculator rounds to one decimal place, so it shows 47.6 gal and 180.2 L. This matches the default output.

How to Choose the Right Tank Size

Choosing a tank size is about more than just the volume number. Consider these factors:

• Fish adult size: A common rule is 1 inch of adult fish per gallon, but this is a rough guide. Larger fish produce more waste and need more space per inch.
• Swimming behavior: Active swimmers like danios or rainbowfish need longer tanks, not just taller ones. A 55-gallon long tank is better than a 40-gallon tall for them.
• Aquascaping: Substrate, rocks, and driftwood displace water. A tank advertised as 20 gallons may hold only 16 gallons of water once decorated.
• Filtration and heating: Larger tanks are more stable in temperature and water chemistry, which is why beginners are often advised to start with at least a 20-gallon tank.

Common Mistakes When Calculating Gallons

People often make these errors:

• Using external dimensions for water volume: Glass thickness takes up space. For a 48×13×20 tank with 0.5-inch glass, the difference is about 2.4 gallons (from 54 to 47.6).
• Mixing units: If you measure in inches but use the liter conversion for centimeters, you’ll be off by a factor of 16.4. Always keep the same unit system.
• Forgetting the substrate and decor: A 20-gallon tank with 2 inches of gravel and a large rock may only hold 15 gallons of water. This matters for medication dosing.
• Assuming a 55-gallon drum is the same as a 55-gallon tank: Tank volumes are nominal; actual water capacity is less due to glass and the fact that water doesn’t reach the very top rim.

Rectangular vs. Bow-Front vs. Custom Shapes

This calculator works for rectangular and square tanks. Bow-front tanks (curved front glass) have a slightly different volume. For a bow-front, the volume is roughly the rectangular volume plus a segment of a cylinder. A common approximation is to multiply the rectangular volume by a factor between 1.05 and 1.15, depending on the curve depth. For example, a 48×13×20 bow-front might hold about 50-52 gallons instead of 47.6. For custom shapes (hexagonal, cylindrical, or irregular), use a different formula. A cylindrical tank uses Ï€ × radius² × height. For a hexagon, divide it into triangles. If you’re unsure, fill the tank with buckets and measure the water you add—that’s the most accurate method.

Why Tank Volume Matters for Fishkeeping

Knowing your tank’s true water volume is critical for several reasons:

• Stocking levels: Overcrowding leads to stress, aggression, and poor water quality. A 47.6-gallon tank can safely hold fewer fish than a 54-gallon external volume suggests.
• Medication and treatments: Most fish medications specify dosage per gallon. Overdosing can kill fish; underdosing is ineffective. Use the actual water volume, not the tank’s advertised size.
• Water changes: If you do a 25% water change, you need to know the volume to mix the right amount of dechlorinator and to match temperature.
• Heater sizing: A general rule is 3-5 watts per gallon. For a 47.6-gallon tank, you’d need a 150-250 watt heater, depending on room temperature.

What About the ‘Stocking Rule’?

The “1 inch per gallon” rule is widely quoted but often misapplied. It works for small, slender fish like neon tetras, but fails for fish with high body mass or territorial behavior. A 6-inch goldfish produces far more waste than six 1-inch tetras. A better approach is to research each species’ minimum tank size and adult size, then plan for a community that doesn’t exceed the tank’s biological load. Also, the rule assumes a filter that handles the bioload; a heavily stocked tank needs more filtration and more frequent water changes.

Frequently Asked Questions

How many gallons is a 48x13x20 tank?
With 0.5-inch glass, the interior volume is about 47.6 US gallons. Without subtracting glass, it’s 54 gallons. Most manufacturers label this as a 50-gallon tank, but the actual water volume is closer to 47-48 gallons.

Does the calculator work for saltwater tanks?
Yes. Volume is volume. The only difference is that saltwater weighs more per gallon (about 8.5 lbs vs. 8.34 lbs for freshwater), but the gallon count is the same.

Should I measure from the inside or outside?
For most purposes, measure the inside dimensions if you can. If you only have outside dimensions, subtract the glass thickness (usually 0.25 to 0.5 inches for standard tanks). This calculator lets you enter thickness to do that automatically.

How do I find the volume of a cylindrical tank?
Use the formula π × r² × h, where r is the radius and h is the height. Convert cubic inches to gallons by dividing by 231, or cubic centimeters to liters by dividing by 1000.

Why does my tank’s label say 20 gallons but it only holds 16?
Labels usually refer to the nominal volume based on external dimensions. The actual water volume is less because of glass thickness, substrate, decorations, and the fact that water level is typically below the rim. This is normal.

Can I use this for a sump or refugium?
Yes, as long as the shape is rectangular. For a sump, measure the section that actually holds water, not the entire cabinet. The same volume formula applies.

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in len | Length | in | 48 | 0
in wid | Width | in | 13 | 0
in hei | Height | in | 20 | 0
in thick | Glass thickness | in | 0.5 | 0
out gal | Water volume (US gal) | gal | ((len - 2*thick) * (wid - 2*thick) * (hei - thick)) / 231 | 1
out lit | Water volume (L) | L | ((len - 2*thick) * (wid - 2*thick) * (hei - thick)) * 0.0163871 | 1