Use this free aquarium calculator to find your tank’s volume in gallons and liters, get a recommended heater wattage, and estimate a rough stocking limit. Enter your tank’s inside dimensions in inches and the results update instantly.
Stocking is a rough guide only — filtration capacity, species temperament, adult size, and bioload matter more than any formula. Always research each species.
How to use this aquarium calculator
Measure the inside length, width (front-to-back depth), and height of your tank in inches and type each value above. The tool multiplies the three dimensions and divides by 231 to get US gallons, then converts to liters. Subtract roughly 10% if you want “net” water volume after substrate, rocks, and the gap below the rim — these displace water that fish never swim in.
Heater size and stocking guidelines
A common rule of thumb is about 4 watts of heater per gallon for a typical heated room; cold rooms or large temperature swings may need more, so we round to the nearest standard heater size (25–300 W). For stocking, the old “one inch of adult fish per gallon” guideline gives a quick ceiling, but it ignores body mass, waste output, and swimming space. Treat the inches figure as an upper bound, stock slowly, and let your filtration and water-test results be the final word.
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