Aquarium Calculator

How Does the Formula Work?

The aquarium calculator determines the water volume of a rectangular fish tank from its internal dimensions and provides essential setup recommendations. Whether you are setting up a nano tank on a desktop or a large reef system in your living room, knowing the exact volume is critical for proper filtration, heating, medication dosing, and fish stocking. The tool also estimates substrate requirements for planted tanks, recommends heater wattage based on tank volume, and provides a fish stocking guideline using the widely accepted one-inch-per-gallon rule.

All measurements should be internal dimensions — the space available for water, not the outer glass dimensions. Standard aquarium glass is 6 mm thick for tanks under 20 gallons, 8 mm for 20–55 gallons, and 10–12 mm for tanks over 55 gallons. If you are measuring the outside of your tank, subtract twice the glass thickness from each dimension.

Standard Tank Sizes in the United States

Aquarium manufacturers in the US use gallon-based sizing. The most popular sizes at Petco and PetSmart are: 10 gallon (51×25×30 cm, about 38 liters), 20 gallon long (76×31×31 cm, about 75 liters), 29 gallon (76×31×47 cm, about 110 liters), 40 gallon breeder (91×46×41 cm, about 151 liters), 55 gallon (122×33×51 cm, about 208 liters), 75 gallon (122×46×51 cm, about 284 liters), and 125 gallon (183×46×56 cm, about 473 liters). Brands like Aqueon, Marineland, and Fluval dominate the US market. Custom acrylic tanks from companies like Clear for Life and Envision Acrylics offer non-standard dimensions for unique installations.

Filtration Sizing

Knowing your exact tank volume determines the filter you need. The general recommendation is a filter rated for 4 to 6 times the tank volume per hour. A 55-gallon (208 L) tank needs a filter moving 800 to 1,200 liters per hour. Hang-on-back (HOB) filters like the AquaClear 70 and Fluval C4 are popular for tanks up to 75 gallons. Canister filters from Fluval (FX series), Eheim (Classic and Pro series), and Penn-Plax (Cascade) handle larger tanks. Sponge filters powered by air pumps are ideal for breeding tanks and shrimp aquariums. Proper filtration is the most important factor in fish health — an undersized filter leads to ammonia spikes and fish loss.

Heater Selection

Most tropical fish thrive at 76–80 °F (24–27 °C). The calculator recommends heater wattage based on the assumption that you need to raise the water temperature about 10 °F (6 °C) above room temperature. The rule of thumb is 3 to 5 watts per gallon or approximately 1 watt per liter. For a 20-gallon tank you need a 75–100 W heater. For a 55-gallon tank 200 W is standard. In very cold rooms or for larger temperature differentials, consider two smaller heaters instead of one large one — this provides redundancy and more even heating. Top-rated aquarium heaters in the US include the Fluval E-series, Eheim Jager, Cobalt Neo-Therm, and Aqueon Pro. Always use a heater with a built-in thermostat and consider an external temperature controller like the Inkbird ITC-306A for added safety.

Substrate for Planted Tanks

The calculator estimates substrate volume based on a default 5 cm (2 inch) depth across the tank floor. For planted aquariums, 5 to 7 cm is recommended to provide adequate root anchorage. Popular substrates in the US market include Fluval Stratum, ADA Aqua Soil Amazonia, Seachem Flourite, and CaribSea Eco-Complete. A 60 cm tank needs about 9 liters (approximately 20 pounds) of substrate. Substrate is sold by the pound or by the bag — Fluval Stratum comes in 4.4 and 8.8-pound bags, ADA Aqua Soil in 3 and 9-liter bags. For non-planted tanks, gravel at 2.5 cm depth is sufficient — roughly half the planted tank amount. Prices range from 15 dollars for basic gravel to 30–50 dollars for premium planted substrates per bag at Petco, PetSmart, and specialty stores like Aquarium Co-Op.

Fish Stocking Guidelines

The one-inch-per-gallon rule (approximately 1 cm per liter) is a starting point for community fish. A 20-gallon tank can hold about 20 inches of adult fish — for example, 10 neon tetras (1 inch each) plus 4 corydoras catfish (2.5 inches each). However, this rule has limitations: it works for slender-bodied fish like tetras and rasboras but underestimates waste production for heavy-bodied fish like goldfish and oscars. Goldfish need 20 gallons for the first fish plus 10 gallons for each additional one. Cichlids require more space per fish due to territorial behavior. Shrimp and snails have a much smaller bioload than fish. For accurate stocking, use resources like AqAdvisor.com or consult experienced fishkeepers on forums like r/Aquariums and PlantedTank.net. Always cycle your aquarium (establish the nitrogen cycle) for 4 to 8 weeks before adding fish.

Medication and Water Treatment

Accurate volume is essential for dosing medications and water treatments. Ich (white spot disease) treatment with API Super Ick Cure requires one packet per 10 gallons. Seachem Prime (the most popular water conditioner in the US) doses at 5 mL per 50 gallons. Overdosing can stress or kill fish, while underdosing is ineffective. This calculator gives you exact liters and gallons so you can dose with confidence. Always remove activated carbon from your filter before medicating and follow the specific medication instructions for water changes during treatment.