Aquarium Volume & Stocking Calculator
Enter your aquarium's inside dimensions in inches and this calculator returns its volume in gallons and litres, a rough stocking capacity using the inch-per-gallon guideline, and a suggested heater wattage. It's a quick planning tool for setting up a tank — with the honest caveat that stocking depends on far more than volume alone.
Tank size (inches)
The inch-per-gallon rule is a rough guide — research each species' real needs.
Tank volume
19.95 gal
75.50 litres
Stocking (in fish)
≈ 20
Heater
≈ 100 W
At a glance
Compare scenarios
Run the same calculation with two or three input sets side by side. Differences are highlighted; every number comes from the same tested formula as the calculator above.
| Input | Scenario A | Scenario B |
|---|---|---|
| Length In | ||
| Width In | ||
| Height In | ||
| Inches Per Gallon |
How it works
Volume comes from the box formula: length × width × height gives cubic inches, and dividing by 231 (the number of cubic inches in a US gallon) converts to gallons. A 24 × 12 × 16 inch tank holds 4,608 cubic inches, or about 20 gallons. Multiply gallons by 3.785 for litres. Use the water dimensions, not the outside glass — substrate, rock, and the gap below the rim mean the actual water volume is a bit less than the full box.
The stocking estimate uses the classic 'one inch of adult fish per gallon' beginner guideline: a 20-gallon tank supports roughly 20 inches of fish. This is deliberately rough. It ignores body mass (a 3-inch chunky fish produces far more waste than three 1-inch slim fish), filtration capacity, and species behaviour and territory needs. Treat it as a ceiling to stay well under, not a target to fill.
Heater sizing follows a rule of thumb of about 5 watts per gallon for a typical room, so a 20-gallon tank wants roughly a 100-watt heater. Colder rooms or larger temperature swings need more; nano tanks less. As with stocking, this is a starting point — actual heater choice depends on your room temperature, the species' preferred range, and whether you want redundancy with two smaller heaters. Always cycle a new tank before adding fish, regardless of what the volume math says.
Frequently asked questions
Is the one-inch-per-gallon rule reliable?+
It's a rough beginner guideline, not a hard rule, and it's easy to misuse. It works passably for small, slim community fish but breaks down for larger or heavy-bodied species, which produce far more waste per inch than the rule assumes. It also ignores filtration, swimming space, territory, and social needs — a single fish that needs a 4-foot tank can't live in a 20-gallon just because the inches 'fit'. Use it to stay conservatively under capacity, research each species' real requirements, and let your water-test results, not a formula, tell you if the tank is overstocked.
Should I use inside or outside tank dimensions?+
Measure the inside water dimensions for the most accurate volume, and expect the real filled volume to be a little less than even that. Glass thickness, substrate, decorations and rock, and the inch or two of freeboard left below the rim all displace or exclude water. A tank sold as '20 gallons' typically holds somewhat less water once it's set up. For stocking and dosing decisions, erring toward the lower actual volume is the safer choice than assuming the full box measurement.
Do I always need a heater?+
Only for tropical species, which most aquarium fish are — they need stable temperatures typically in the mid-to-upper 70s Fahrenheit, so a heater is standard. Cold-water species like goldfish and white cloud minnows can live at room temperature and may not need one, provided your room stays stable. The ~5 watts-per-gallon estimate here is a starting point for tropical setups in an average room; a colder room, a bigger tank, or a species needing warmer water will push the wattage higher. Match the heater to the species and always pair it with a thermometer.