Aquarium Math: Volume, Water Weight, Heater Size, and Water Changes
October 29, 2026
Setting up an aquarium requires a handful of calculations: how many gallons the tank holds, how much it weighs when filled, what size heater and filter to buy, and how much water to change. The basic formula is volume in gallons = length × width × height in inches ÷ 231, because one US gallon is 231 cubic inches.
A standard 20-gallon tank measures 24 × 12 × 16 inches, so the volume is 4,608 ÷ 231 = 19.95 gallons. Water weighs 8.34 pounds per gallon, so the water alone weighs 166 pounds. Add glass, substrate, and decorations, and the total is roughly 200 pounds.
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The listed capacity is a bit higher than the usable volume, because substrate, rocks, and the gap below the rim take up space; assume 10% to 15% less water. The weight is the problem for furniture: the water in a 55-gallon tank weighs 473 pounds, and the full setup around 570 pounds, a load that needs a purpose-built stand on a floor that can bear it, ideally over joists. Calculate weight per square foot: a 48 x 13 inch footprint is 4.3 square feet, so about 130 pounds per square foot, much higher than typical furniture.
| Tank | Dimensions (in) | Volume (gal) | Water weight (lb) |
|---|---|---|---|
| 10 gallon | 20 x 10 x 12 | 10.4 | 87 |
| 20 gallon | 24 x 12 x 16 | 19.9 | 166 |
| 55 gallon | 48 x 13 x 21 | 56.7 | 473 |
| 75 gallon | 48 x 18 x 20 | 74.8 | 624 |
Heater and Filter Sizing
A common rule is 3 to 5 watts of heater power per gallon, depending on how much warmer the tank must be than the room. For a 20-gallon tank, 60 to 100 watts is appropriate, so a 75-watt heater is a good choice. A heater cycles on and off; at a 50% duty cycle, 75 watts uses 37.5 W × 720 hours = 27 kWh a month, which costs about $4.32 at $0.16 per kWh. Filter flow should turn over the tank volume about 4 times an hour, or 80 gallons per hour for a 20-gallon tank, more for heavily stocked or messy fish. Remember that flow ratings are for a clean filter with no media in it.
Water Changes
Regular partial water changes dilute nitrates and replace minerals. A typical schedule is 25% weekly. For the 20-gallon tank, that is 5 gallons, about 42 pounds of water, which is two 2.5-gallon buckets. A 55-gallon tank needs about 14 gallons weekly. Use a dechlorinator and match the temperature within a couple of degrees. To estimate how a change dilutes nitrate, if a tank has 40 ppm and you change 25% with water with 0 ppm, the result is 30 ppm. After two weekly changes with no more input, it would be 22.5 ppm.
Stocking Guidelines
The old rule of 1 inch of fish per gallon is a rough starting point that works for small, slim fish and fails for large or messy ones. For the 20-gallon tank, that would be 20 inches of adult fish, but a single oscar could exceed that. Research each species's adult size, schedule, and compatibility, and add fish gradually while the biological filter develops (the nitrogen cycle takes 4 to 6 weeks). Test ammonia, nitrite, and nitrate during the first weeks.
Frequently Asked Questions
How do I convert tank dimensions to gallons?
Multiply length, width, and height in inches and divide by 231.
How much does a gallon of water weigh?
About 8.34 pounds.
What heater size do I need?
Roughly 3 to 5 watts per gallon.
How much water should I change weekly?
Commonly about 25% of the tank volume.
Is the 1 inch per gallon rule reliable?
Only as a rough guide for small fish.