The gallon or liter figure printed on a tank's box is a manufacturing spec, not the amount of water actually in your tank once it's set up. This calculator does the straightforward geometry, length times width times height, and gives you a real starting figure in both liters and US gallons.
Nominal Size vs. Actual Water Volume
A tank's marketed size ("20 gallon," "60-liter") is calculated from its outer glass dimensions, the full box the water could theoretically fill to the brim. In practice, real water volume runs meaningfully lower than that nominal figure. Substrate (gravel or sand) displaces water on the bottom, decor, rockwork, and equipment displace more, and no one fills a tank completely to the rim, both to leave room for filter flow and to stop water sloshing out. Between those factors, actual water volume typically lands 10 to 15% below the tank's marketed size. A tank sold as 20 gallons might realistically hold closer to 17 to 18 gallons of actual water once it's properly set up, worth knowing before you plug that nominal number into a stocking calculation.
A Worked Example
A common small tank measures 60cm long, 30cm wide, and 35cm tall. Multiply those together: 60 × 30 × 35 = 63,000 cubic centimeters. Divide by 1,000 to convert to liters: 63 liters. Convert to US gallons by multiplying by 0.264172: about 16.6 gallons. That is the tank's full nominal capacity, before subtracting substrate, decor, and the gap below the rim, the real working water volume would land closer to 54 to 57 liters, about 14 to 15 gallons.
US Gallons vs. UK Gallons: Not the Same Thing
This calculator outputs US gallons (3.78541 liters each), the standard used by most aquarium retailers and hobbyist forums. If you're shopping or reading care guides from a UK source, watch out: the UK (imperial) gallon is a genuinely different, larger unit, 4.546 liters, about 20% bigger than a US gallon. A tank described as "20 gallons" means something meaningfully different depending on which side of that line the source is from, roughly 76 liters if US gallons, roughly 91 liters if UK gallons. When in doubt, convert to liters, the one unit that means the same thing everywhere.
How to Use This Calculator
Enter your tank's inner length, width, and height in centimeters (or toggle to inches). The calculator returns the full nominal volume in both liters and US gallons. Once you have that figure, plug it into the Fish Tank Stocking Calculator to get a rough sense of how many fish it can support.
Frequently Asked Questions
Should I measure the outside or inside of the glass?+
Inside dimensions give a more accurate water-volume figure, since glass thickness on larger tanks can meaningfully shrink the usable interior compared to the outer measurements printed on the box.
Why does my tank hold less water than the number on the box?+
Substrate, decor, equipment, and not filling to the rim all reduce real usable water volume, typically by 10 to 15% below the tank's nominal marketed size.
Know your real number before you stock it.
Nominal tank size is a useful shorthand for shopping, but the actual liters of water sitting in your tank is what should drive stocking decisions. Measure your real dimensions and knock 10 to 15% off for substrate and decor.
Have your liters figure? Head to the Fish Tank Stocking Calculator next.