Aquarium Calculator
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What Your Result Means
- Tank Volume: the water capacity in gallons and liters — actual usable volume is typically 10-15% less due to substrate, decor, and equipment.
- Filled Weight: water alone at 8.34 lbs/gallon plus the tank itself — a 55-gallon tank weighs over 500 lbs filled, so floor support matters.
- Gravel: based on 1.5 lbs per gallon for a standard 2-inch substrate bed — planted tanks may need more.
- Heater Wattage: the 4-watts-per-gallon rule works for rooms around 70°F — colder rooms need more.
- Fish Capacity: the inch-per-gallon rule is a rough starting point — active or messy species need more space per inch.
How This Calculator Works
Tank volume is calculated from length, width, and height in inches, then converted to gallons and liters. The calculator derives all other results as multipliers: water weight uses 8.34 lbs/gal; gravel uses 1.5 lbs/gal for a 2-inch bed; heater sizing applies 4 watts per gallon; filter flow assumes 3 GPH per gallon; fish capacity follows the 1 inch of fish per gallon rule as a starting baseline.
Quick Questions
Is the inch-per-gallon rule accurate?
The 1 inch of fish per gallon is a rough guideline and varies widely. Small, active fish like tetras need less space per inch, while large territorial fish need more. Goldfish and plecos are exceptions — they need 20+ gallons each.
How much does a full tank weigh?
Water weighs 8.34 lbs per gallon. Add the tank glass, stand, substrate, rocks, and decorations. A 55-gallon tank typically weighs 500+ lbs when fully set up. Always verify your floor can handle the weight.
Do I need a heater for a cold-water tank?
Many species like goldfish, plecos, and tetras prefer 72-76°F. Even "cold-water" tanks often need a heater to maintain stable temperature. The 4 watts-per-gallon rule assumes tropical (75-78°F) conditions.
How often should I run the filter?
Run the filter 24/7. The GPH calculation assumes continuous operation for biological filtration. Turning it off harms the beneficial bacteria colony that removes ammonia and nitrite.
Can I use this for saltwater tanks?
Yes for volume and basic dimensions, but saltwater requires higher-flow filtration (often 5-10x turnover per hour vs. 3x) and additional equipment like protein skimmers. Adjust the filter GPH upward for marine setups.
Sources
Method & review
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.