How do I calculate aquarium gallons?

Measure the length, width, and height of the tank in inches. Multiply them together to get cubic inches (L × W × H). Then divide by 231 (since 1 gallon = 231 cubic inches). For example, a 24" × 12" × 16" tank is 4,608 cubic inches, which equals roughly 20 gallons.

How many fish can I put in my tank?

The general '1 inch of fish per gallon' rule is a good starting point for small, slender community fish (like tetras). However, it's flawed for larger or messy fish (like goldfish or cichlids). Goldfish need 20+ gallons for the first fish and 10+ for each additional. Always consider the adult size of the fish and their bioload.

Does substrate reduce tank volume?

Yes. Gravel, sand, and decorations displace water. A 2-inch layer of gravel in a 20-gallon tank can reduce the actual water volume by 2-3 gallons. This is important for dosing medications or water conditioners, where you need to know the actual water volume, not just the tank size.

What is the weight of a filled aquarium?

Water weighs about 8.34 lbs per gallon. Glass and substrate add significant weight. A standard 55-gallon tank filled with water, gravel, and decorations can weigh over 600 lbs. Ensure your floor and stand can support this weight.

🐠 Aquarium Size Calculator

Calculate volume, weight, and stocking capacity

Tank Shape

Length (inches)

Width (inches)

Height (inches)

Substrate Depth (inches)

Gravel/sand layer thickness

📏 How to Measure Your Tank

For the most accurate volume calculation, measure the internal dimensions of your tank. If you measure the outside, subtract the glass thickness (usually 1/4" to 1/2") from each measurement.

Common Standard Sizes

10 Gallon: 20" L x 10" W x 12" H
20 Gallon Long: 30" L x 12" W x 12" H
29 Gallon: 30" L x 12" W x 18" H
55 Gallon: 48" L x 13" W x 21" H
75 Gallon: 48" L x 18" W x 21" H

💡 From Sarah Jenkins, Aquarist (12 years):

"Don't forget about displacement! A '55-gallon' tank rarely holds 55 gallons of water. Once you add 50 lbs of gravel, 30 lbs of rock, and leave an inch of space at the top, you might only have 40-45 gallons of actual water. This matters when you're dosing medication—always dose for the actual water volume, not the tank size, to avoid overdosing."

🐟 Stocking Guidelines

The "1 inch per gallon" rule is a popular but simplified guideline. It works okay for small, slim fish like Neon Tetras, but fails for larger fish.

Better Stocking Rules:

Surface Area Rule: Fish need oxygen exchange at the surface. A wide, shallow tank supports more fish than a tall, narrow one of the same volume.
The "Adult Size" Rule: Always calculate based on the fish's adult size, not the size they are at the store. That 1-inch Oscar will grow to 12 inches!
Filtration Matters: A tank with a powerful canister filter can support a slightly higher bioload than one with a simple sponge filter.

⚠️ Common Mistakes

1. Overstocking immediately: New tanks need to "cycle" (build beneficial bacteria). Add only 1-2 hardy fish at first, then wait a few weeks.

2. Ignoring weight: Water is heavy (8.34 lbs/gal). A 55-gallon tank weighs over 600 lbs filled. Make sure your floor and stand can handle it.

3. Not accounting for substrate: If you want a deep sand bed (4+ inches), you lose significant water volume.

Sarah Jenkins

Professional Aquarist, 12 years experience

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