Aquarium Size Calculator: Tank Volume in Gallons

Calculate your fish tank volume in gallons and liters from its dimensions.

How tank volume is calculated

Volume in gallons = (length × width × height in inches) ÷ 231. This is the full tank volume; actual water volume is a bit less once you add substrate and decor, usually around 85 to 90 percent.

Why tank size matters

Tank size determines how many fish you can safely keep, how stable the water stays, and what heater and filter you need. Bigger tanks are actually easier for beginners because water conditions change more slowly.

Quick guidance

• Heater: about 5 watts per gallon for tropical tanks.
• Filter: choose one rated for your gallons or higher.
• Stocking: a common starting rule is 1 inch of adult fish per gallon, adjusted for species.

Frequently asked questions

How do I calculate aquarium gallons?

Multiply length, width, and height in inches, then divide by 231. This tool does it for you.

Why is actual water less than tank volume?

Substrate, rocks, and decor displace water, so plan for about 85 to 90 percent of the full volume.

What size heater do I need?

Around 5 watts per gallon for a tropical tank, more in cold rooms.

Is a bigger tank easier?

Yes, larger volumes keep water parameters more stable, which is more forgiving for beginners.

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General guidance only, not a substitute for a vet or exotics specialist.