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Fish Tank Volume Calculator

Aquarium Heater Calculator

Recommended heater

120 W

standard size: 150 W

Setup

1 heater

for even, safe heating

Guide: about 1 watt per litre for a 5 to 6°C rise; more in a cold room. Round up to the next standard size.

Work out the right heater wattage for your aquarium. Enter your water volume and how far you need to warm it above room temperature to get a recommended heater size in watts.

How to size an aquarium heater

Heater wattage depends on how much water you are heating and how far above room temperature you need it. A common guide is about 1 watt per litre for a modest 5 to 6 degree Celsius rise, and more for colder rooms. This calculator uses your volume and target temperature rise, then rounds up to the nearest standard heater size. For tanks over about 200 litres, two smaller heaters are safer and more even than one large one.

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Frequently asked questions

What size heater do I need for my aquarium?
As a guide, allow about 1 watt per litre for a 5 to 6 degree Celsius rise above room temperature, and more in a cold room. Enter your volume and target rise above and the calculator recommends a wattage.
How many watts to heat a 100 litre tank?
For a typical 5 to 6 degree rise, around 100 to 150 watts. In a cold room you may need 200 watts, or two 100 watt heaters.
Is it better to use two heaters?
For tanks over about 200 litres, two smaller heaters give more even heat and keep the tank safe if one fails.
Should I round the wattage up or down?
Round up to the next standard heater size. A slightly oversized heater simply cycles off sooner, while an undersized one runs constantly and may never reach temperature.
Does room temperature affect heater size?
Yes. The colder the room, the larger the temperature rise the heater must maintain, so a cold room needs more watts per litre than a warm one.
What temperature should a tropical tank be?
Most tropical community fish are kept between 24 and 27 degrees Celsius. Set your target rise as this temperature minus the coldest your room reaches.

Related guides

Learn the core ideas behind fish tank volume before or after using the calculator.