Aquarium calculator: volume, fish, heater, and salt
One tool with four essential calculations for anyone setting up or maintaining an aquarium: volume in liters, how many fish fit, heater wattage, and maintenance salt dosage. All the rules used here are general and approximate — every fish species has its own specific needs.
Tank volume
63.0 liters
The volume actually available to the fish is a bit smaller: substrate, decorations, and equipment usually take up 5% to 10% of that total.
How the calculation works
Volume: this is a purely geometric calculation. We multiply the internal length, width, and height of the tank (in centimeters) and divide by 1000, since 1 liter equals 1000 cm³. The actual "usable" volume for the fish tends to be a bit smaller, since substrate, decorations, filters, and other equipment take up space.
How many fish fit: we use the most commonly cited metric conversion of the classic "1 inch of fish per gallon of water" rule — roughly 1 cm of adult fish length per liter of water. This is a GENERAL rule, meant for small to medium tropical freshwater fish in well-filtered community tanks. It does not apply to cold-water fish, marine fish, large fish, or territorial species, which need much more space per individual. The surface-area rule (based on the water surface exposed to gas exchange) is considered even more precise by the aquarium community, but it requires the tank's floor area separately from its height.
Heater: we apply the common aquarium rule of thumb of roughly 1 watt per liter of water to raise the temperature up to 10°C above the room's ambient temperature, scaling the wattage proportionally for larger differences. On top of that value, we add a 20% safety margin to compensate for heat loss (open lid, drafts, tank near a window, etc.).
Salt: we calculate the general MAINTENANCE dosage (not a disease-treatment dose), using the common ratio of roughly 1 tablespoon (≈15 g) of aquarium-grade salt per 19 liters (5 gallons) of water. Treatments for specific diseases require higher concentrations and should always follow the product manufacturer's instructions.
Frequently asked questions
How do I calculate the volume of an aquarium in liters?
Measure the internal length, width, and height of the tank in centimeters, multiply the three values, and divide by 1000 (since 1 liter = 1000 cm³). For example, a 60 × 30 × 35 cm tank holds 63 liters. Keep in mind that the real "usable" volume tends to be 5% to 10% smaller, since substrate, decorations, and equipment take up space.
How many fish can fit in my aquarium?
This calculator uses the metric conversion of the classic "1 inch of fish per gallon of water" stocking rule (today popularly referred to as "2 inches per gallon ≈ 1 cm per liter"), meaning roughly 1 cm of adult fish length per liter of water. It is a GENERAL rule for small to medium tropical freshwater fish in well-filtered aquariums — it does not apply to cold-water fish, marine fish, large fish, or highly territorial species. The surface-area rule for the water's surface (more commonly used by experienced aquarists) is considered even more precise, but it requires the tank's floor area separately from its height.
Is the "1 cm of fish per liter" rule always reliable?
No. It is just a simple starting point. Factors such as territorial behavior, waste production (bioload), oxygenation, filtration type, and swimming habits vary a lot between species and are not captured by fish length alone. Always research the specific needs of each species before deciding the final stocking of your aquarium.
How do I choose the right aquarium heater wattage?
The most common rule of thumb is about 1 watt per liter of water to raise the temperature up to 10°C above the room's ambient temperature, scaling proportionally for larger differences (up to ~2 W per liter in very cold climates). This calculator already applies a 20% safety margin on top of that value. Large tanks (over 200-300 liters) or very cold rooms often benefit from using two smaller heaters instead of one, to distribute heat better and have redundancy in case one fails.
Can I use regular table salt in my aquarium?
No. Use only salt made specifically for aquariums (made from evaporated seawater, without iodine or anti-caking agents), since regular table salt can contain additives that are harmful to fish. The dosage calculated here is for general maintenance (~1 tablespoon, about 15 g, per 19 liters / 5 gallons) and does not replace manufacturer instructions for specific disease treatments, which use higher concentrations.
Is the salt dose for treating disease the same as general maintenance?
No. The dosage calculated here is for general maintenance only (reducing stress, supporting gill function). Treatments for specific diseases use much higher salt concentrations for a set period of time — always follow the product instructions or the guidance of an aquarium specialist, and never use salt in tanks with species sensitive to salinity (such as certain scaleless fish) or with plants, without first researching their tolerance.
Important notice
This calculator is for educational purposes and does not replace a veterinary evaluation. When in doubt, consult a veterinarian.