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Aquarium Heater Size Calculator
Aquarium Heater Size Calculator — What Wattage Do You Need?
Choosing the right heater size for a reef aquarium is critical — undersized heaters struggle to maintain temperature during cold weather, while oversized heaters risk cooking your tank if the thermostat fails. Reef corals have a very narrow temperature tolerance (typically 76–80°F / 24–27°C), and a temperature swing of even 2–3°F per day stresses corals and can cause bleaching. The required heater wattage depends primarily on the temperature differential between your desired tank temperature and the ambient room temperature, and on the tank's volume. This calculator uses a standard formula based on the heat transfer coefficient of glass aquariums. The result is the minimum recommended wattage — it's best practice to use two heaters at 60–70% of total required wattage each, so one heater alone cannot overheat the tank if it sticks "on", and if one fails the other maintains temperature.
How to Use This Calculator
1
Enter your tank volume in liters. Include the sump volume if applicable.
2
Enter your target water temperature — most reef tanks run 76–80°F (24–27°C).
3
Enter the coldest room temperature you'll ever experience (winter night with HVAC off). This gives a safe margin.
4
The calculator shows minimum wattage. Split across two heaters — each at 60–70% of the total required wattage.
5
Place heaters in high-flow areas (return section of sump, near pump). Never let a heater run dry.
Setting up a new reef?
Browse our WYSIWYG corals — every specimen photographed individually so you know exactly what you're getting.
Frequently Asked Questions
What temperature should a reef tank be?
Most coral species thrive at 76–80°F (24.5–26.5°C). SPS corals can be sensitive to anything above 82°F. LPS and softies tolerate slightly higher temperatures. Stability is more important than the exact number — daily swings above 1–2°F cause more stress than a constant temperature 1°F above ideal.
Why use two heaters instead of one?
A heater that sticks 'on' in the full-on position will rapidly overheat a tank — a single 300W heater can raise a 100-gallon tank to lethal temperatures within hours. With two heaters each sized at 60–70% of required wattage, if one sticks on it cannot overheat the tank alone. This redundancy is standard practice for reef keeping.
How do I reduce heater usage in summer?
Reef tank chillers run in summer; heaters in winter. If you live in a warm climate and have a chiller, you may not need a heater at all year-round. Some reef keepers use a smart controller (Apex, GHL) that monitors temperature and cuts power if it reads a runaway temperature.
Is a 50W heater enough for a 10-gallon tank?
For a room that stays above 68°F, yes. But for a room that drops to 60°F in winter, a 10-gallon tank with a 6°F differential needs about 12W minimum — so 50W is more than adequate and provides a good safety margin.
