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Aquarium Tank Calculators
Four aquarium tools: how many gallons the tank holds, the outside dimensions of common sizes (including the 10 gallon length), heater watts, and an inch-per-gallon stocking rule of thumb. Gallons on this site are US gallons unless a result also shows imperial gallons.
Aquarium volume calculator
Rectangle, cylinder, bow-front, hexagon, and corner tanks. US gallons, liters, and water weight.
10 gallon fish tank dimensions
Outside length, width, and height for 10, 20, 29, 55, 75 gallon tanks and more.
Aquarium heater size calculator
Watts from water gallons and the degrees you must hold above the room.
Aquarium stocking calculator
Inch-per-gallon, plus stricter one-inch-per-two and per-three-gallon rules.
Measure water, not the carton
A tank sold as 10 gallons is not a promise that every brand holds exactly 10.00 gallons of water. Glass thickness, the black frame, and the inch of air at the top all change it. If you need the water you actually have, for heater watts or for stocking, measure the water and use the volume calculator. If you need to know whether a 10 gallon tank fits a stand, use the outside dimension chart.
Rules of thumb, labeled as such
Heater watts and fish counts are not laws of physics with one right answer. The heater page states the 0.5 watt per gallon per degree F rule and, beside it, Aqueon's published chart, which is lower for many large covered tanks. The stocking page will not tell you a fish is safe. It only does the inch arithmetic you asked for, and it says when you are over that rule.
Frequently asked questions
How many gallons is my fish tank?
For a rectangle, measure the water in inches and divide length times width times height by 231. That is US gallons. A 20 by 10 by 12 inch water box is 10.39 US gallons. Other shapes use the volume calculator.
What size heater for a 20 gallon tank?
For a 10 F rise, the 0.5 watt per gallon per degree rule is 100 watts, and Aqueon's chart for a covered tank in a 68 to 72 F room is also 100 watts at a 10 F rise. Colder rooms need more.
How many fish can I put in a tank?
The old rule of thumb is one inch of adult fish per gallon. It ignores body shape and waste. The stocking calculator also offers one inch per two gallons and per three gallons, and it labels all three as rules of thumb.
Are the dimension charts inside measurements?
No. The size chart uses Aqueon's 2023 outside dimensions. Outside length times width times height overstates the water, because the glass and the frame are inside that box and you do not fill to the rim.