Free Online Calculators
Calculate volume, heater & filter power
The aquarium calculator works out your tank's exact water volume from its length, width, and height, then recommends the right heater wattage and filter flow rate. Volume is calculated as length × width × height (in cm) ÷ 1000 to get litres. It takes the guesswork out of choosing equipment for a healthy, balanced tank.
Multiply length by width by height in inches to get the volume in cubic inches, then divide by the number of cubic inches in a gallon. Measure the inside dimensions and use the actual water height, not the full tank height, if you want the filled volume.
Multiply the three dimensions to get cubic inches and divide by the cubic inches in one gallon; entering the measurements above gives the exact figure. The result is the brim-full capacity, so the real water volume is slightly lower once substrate, rocks and the gap below the rim are counted.
Multiply length, width and height in inches and divide by the cubic inches per gallon to get the capacity. Enter the three measurements in the calculator for the exact number, and subtract a little for substrate and the unfilled top edge.
Compare the adult length of each fish with your tank's water volume and surface area, using the common rule of one inch of fish per gallon as a rough starting point only. Adjust downward for messy or active species, and factor in filtration, swimming space and the minimum tank size each species needs.
No, a one gallon tank is far too small for three guppies. Guppies are active, breed quickly and produce enough waste that ammonia spikes fast in such a small volume, so a properly filtered and cycled tank of several gallons is the minimum.
Multiply the three sides to get cubic inches, then divide by the cubic inches in a gallon; the calculator returns the exact value for these dimensions. That figure is the full-to-the-rim capacity, so plan on somewhat less actual water.
Take length times width times height in inches and divide by the cubic inches per gallon to get the capacity. Enter the dimensions above for the exact result, and remember that substrate, decor and the gap under the rim reduce the real water volume.
Cube the side length to get cubic inches and divide by the cubic inches in one gallon for the total capacity. A cube tank has less surface area per gallon than a long tank of the same volume, which matters for gas exchange and stocking.
Multiply the three measurements in inches and divide by the cubic inches per gallon; the calculator gives the exact figure for these dimensions. Fill height rather than full tank height gives you the true water volume.
Setting up or maintaining an aquarium involves more than just choosing the right fish. You need to know exactly how much water your tank holds, what heater wattage will keep the temperature stable, and how powerful a filter you need to maintain a healthy aquatic environment. Our free aquarium calculator gives you all three answers instantly, based on your tank's dimensions and temperature requirements.
This tool calculates three essential values every aquarium owner needs:
These figures are the foundation for choosing the right equipment, whether you're setting up a small 20-liter desktop tank or a large 600-liter reef aquarium. The calculator works for both freshwater and saltwater setups.
Using the calculator is straightforward. Simply enter the following values:
The calculator automatically applies a standard 10% reduction to account for substrate, decorations, and the air gap at the top of the tank — giving you a realistic net water volume rather than the theoretical maximum. Results appear instantly with no need to refresh the page.
Consider a tank measuring 100 × 40 × 50 cm. The gross volume is 200 liters. After the 10% deduction, the usable water volume is approximately 180 liters. With a target water temperature of 26°C and a room temperature of 20°C, the calculator recommends a heater of around 150–180 watts and a filter with a flow rate of at least 540 liters per hour. These are solid starting points for equipment selection.
The volume of your aquarium affects virtually every aspect of fish keeping. It determines how many fish you can safely house, how quickly water parameters like ammonia, nitrite, and pH can swing, and how much medication or fertilizer to add when treating the tank. Larger volumes are more forgiving — small fluctuations have less impact on fish health. That's why having an accurate aquarium volume calculation is the essential first step before buying any equipment or livestock.
Choosing a heater that's too weak means your water temperature will drop during cold nights, stressing your fish. A heater that's too powerful can overheat the tank if the thermostat malfunctions. The general rule is approximately 1 watt per liter of water when the temperature difference between the room and the tank is small (around 4–6°C). For larger differences or uninsulated tanks, 1.5 to 2 watts per liter is more appropriate. Our calculator adjusts this recommendation dynamically based on the temperatures you enter.
Filtration is arguably the most critical piece of aquarium equipment. A filter handles biological filtration (converting toxic ammonia to nitrate), mechanical filtration (removing particles), and sometimes chemical filtration (activated carbon). The standard recommendation is that your filter should turn over the entire tank volume at least 3 to 5 times per hour. For heavily stocked tanks or predatory fish, this can rise to 8–10 times per hour. The aquarium calculator provides a reliable baseline recommendation.
This tool is invaluable for beginners who are planning their first tank, intermediate hobbyists upgrading to a larger setup, and experienced aquarists who want a quick sanity check on equipment choices. It's also useful for pet store staff, aquarium clubs, and teachers explaining aquarium biology in classrooms.
Multiply the length, width, and height of your tank in centimeters, then divide by 1,000 to get liters. For example, a 100 × 40 × 50 cm tank has a gross volume of 200 liters. Our calculator does this automatically and subtracts 10% for a realistic net volume.
The standard rule is 1 watt per liter. For a 100-liter tank with a 6°C temperature difference, you'd need around 100 watts. Larger differences require more wattage. Our calculator tailors this based on the room and water temperatures you input.
Your filter should circulate the entire tank volume 3 to 5 times per hour at minimum. For a 100-liter tank, look for a filter rated at 300–500 liters per hour. Heavily stocked tanks may need double this flow rate.
Yes. The volume and power calculations apply to saltwater tanks as well. Note that reef aquariums often require additional equipment like protein skimmers and higher flow rates than freshwater setups.
Absolutely. The calculator on simple-calculator.online is 100% free, requires no registration, and can be used as many times as you need.