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Aquarium Chiller Sizing Calculator

Size an aquarium chiller from your tank volume and the temperature drop you need.

How it is calculated

The tool treats the tank as a body of water and works out the heat that has to be removed. Water weighs about 8.34 lb per US gallon and needs 1 BTU to change one pound by 1 °F, so: BTU total = gallons × 8.34 × temperature drop (°F). The capacity is then BTU/hr = BTU total ÷ hours. Watts are BTU/hr × 0.29307107, and the nominal HP figure is BTU/hr ÷ 12,000, the trade rule of thumb that one ton of cooling is about one compressor horsepower. It is not mechanical horsepower (2,545 BTU/hr), so compare chillers by their rated BTU/hr. All three inputs must be above zero, otherwise no result is shown. Volume is in US gallons, the drop is in °F (not °C) and hours is the time you allow for the pull-down. The result is pure water cooling: it does not add heat from lights, pumps or the room, so leave yourself a margin when choosing a unit.

Worked example

A 75-gallon tank must be cooled by 8 °F in 6 hours. Heat to remove: 75 × 8.34 = 625.5 lb of water per degree, times 8 °F = 5,004.00 BTU in total. Spread over 6 hours that is 5,004 ÷ 6 = 834.00 BTU/hr. In watts, 834 × 0.29307107 = 244.42 W, and 834 ÷ 12,000 = 0.070 nominal HP. A chiller rated comfortably above 834.00 BTU/hr covers this pull-down.

Try this example →

Common mistakes

  • Entering the temperature drop in °C. The formula expects Fahrenheit degrees, so an 8 °C drop is about 14.4 °F and needs almost twice the capacity.
  • Typing the tank size in litres. Volume is in US gallons (1 gallon is about 3.785 litres), so 300 litres is roughly 79 gallons.
  • Entering the target temperature instead of the drop. If the tank is at 82 °F and you want 78 °F, the drop is 4, not 78.
  • Using the full tank size when a sump, rock and substrate take up space. Use the real water volume, not the glass dimensions.
  • Setting a very short pull-down time. Halving the hours doubles the BTU/hr; a longer period gives a much smaller, cheaper chiller.

About this calculator

Aquarium chiller sizing calculates the cooling capacity in BTU/hr needed to lower a tank's water temperature by a specific number of degrees, based on the tank's water volume (in US gallons), the desired temperature drop (in °F), and how quickly the chiller needs to achieve it. Larger volumes and bigger temperature drops both require proportionally more cooling capacity. Ambient room temperature and tank insulation are not part of the calculation, even though heat constantly migrates back into the tank from the surrounding air, so a warm room calls for a chiller sized above the result.

Reef keepers and tropical/coldwater fish keepers size a chiller carefully because many sensitive species — particularly corals and coldwater fish like goldfish or certain trout — suffer stress or death outside a narrow temperature band, and an undersized chiller can run constantly without ever reaching the target temperature during hot weather, while lighting and pump equipment in the tank add heat load that the calculator does not include, so leave some headroom for it.

Tank volume, how many degrees you need to drop and how many hours you allow for it set the heat to remove; you get that total in BTU and the chiller capacity to buy in BTU/hr, watts and horsepower.

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