Aquarium Heater Size Calculator
Find the right heater wattage for your aquarium based on tank volume and the temperature difference between your room and target water temperature.
Choosing the right heater wattage is one of the most overlooked parts of aquarium setup. An undersized heater struggles to maintain temperature in cooler rooms, leading to stress, weakened immune systems, and disease. An oversized heater poses a safety risk - if the thermostat fails, it can overheat the tank rapidly.
The amount of heating power you need depends on two things: how much water the heater needs to warm, and how far above room temperature you need the water to be. A tank in a warm room needs far less wattage than the same tank in a cold garage. This calculator handles the math and recommends a standard heater size that fits your setup.
Quick reference
- Most tropical fish - 24-28°C target
- Goldfish - 18-22°C (may not need a heater)
- Discus, rams - 28-30°C (higher wattage)
- Cold rooms - Size for worst-case winter temperature
- Over 300 litres - Use two heaters for safety
Calculator
Enter your tank volume, room temperature, and target water temperature below.
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Fill in the form and click calculate to see your results here.
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How Heater Wattage Is Calculated
The standard rule of thumb in fishkeeping is 0.5 watts per litre for every degree Celsius the heater needs to raise the water above room temperature. This accounts for the rate of heat loss through the glass and water surface.
Formula
Watts = Litres x 0.5 x (Target°C - Room°C)
The result is then rounded up to the nearest standard heater size available in shops: 25W, 50W, 75W, 100W, 150W, 200W, 250W, 300W, or 400W.
Why room temperature matters
Your heater only needs to make up the difference between the room and the water. If your room stays at 24°C and your fish need 26°C, that is only a 2-degree gap - requiring far less power than a room at 15°C targeting the same water temperature. Always base your calculation on the coldest your room gets, not the average.
When to use two heaters
For tanks requiring more than 300W, or any tank over 300 litres, using two heaters is the standard recommendation. Split the total wattage across two units placed at opposite ends of the tank. This provides more even heating and acts as a failsafe - if one heater breaks, the other keeps the temperature from crashing overnight.
Importance of thermostatic control
A properly sized heater with a reliable thermostat cycles on and off to maintain a stable temperature. Without a thermostat, the heater runs continuously and can overheat the water. Always use a heater with a built-in thermostat, and verify the actual water temperature with a separate thermometer.
Example Calculation
Scenario
You have a 200-litre freshwater community tank. Your room temperature averages 20°C, and your tropical fish need a water temperature of 26°C.
Temperature difference = 26 - 20 = 6°C
Watts = 200 x 0.5 x 6
Watts = 600W
600W exceeds what a single heater can handle. The calculator recommends two 300W heaters, placed at opposite ends of the tank.
Tip: If your room is warmer (say 22°C), the same tank only needs 200 x 0.5 x 4 = 400W. Room temperature has a big impact on heater sizing.
Working with gallons
If you measure your tank in US gallons, the calculator converts to litres automatically (1 US gallon = 3.785 litres) before running the formula. A 55-gallon tank is approximately 208 litres.
Standard heater sizes
Aquarium heaters are sold in fixed wattages. The calculator rounds your result up to the nearest standard size so you know exactly what to buy.
Frequently Asked Questions
Common questions about aquarium heater sizing and temperature control.
Can a heater be too powerful for my aquarium?
Yes. An oversized heater can overshoot the target temperature if its thermostat fails, which can be fatal for fish. A heater that is moderately above what you need is fine - the thermostat will cycle it off - but avoid using a 300W heater on a 40-litre tank. The closer the heater wattage matches the actual demand, the safer the tank is if something goes wrong.
What if my room temperature changes seasonally?
Use the lowest typical room temperature your home reaches during winter when calculating heater size. If your room drops to 15 degrees C in winter but sits at 22 degrees C in summer, size the heater for the 15-degree scenario. During warmer months, the thermostat will simply cycle the heater on less often.
Should I use two smaller heaters instead of one large one?
For tanks over 300 litres, two heaters are generally recommended. They provide more even heat distribution when placed at opposite ends of the tank, and if one fails, the other keeps the temperature from dropping dangerously low. Even for medium tanks (150-300 litres), two heaters offer a useful safety margin.
Do planted tanks need more heating?
Planted tanks do not need more wattage than unplanted tanks of the same size. However, many popular aquarium plants come from tropical regions and do best at 24-28 degrees C, so you need to ensure your heater can maintain that range. The calculation is the same - it is based on tank volume and the difference between room and target temperature.
How accurate are aquarium heaters?
Most aquarium heaters are accurate to within 1-2 degrees C of their set point. Cheaper models may drift more. Always use a separate thermometer to verify the actual water temperature rather than relying solely on the heater dial. Digital thermometers are more reliable than stick-on strip thermometers.
Does tank shape or material affect heater sizing?
Tanks with a larger surface area relative to volume (such as long, shallow tanks) lose heat slightly faster than tall, compact tanks. Glass tanks lose heat faster than acrylic. In most home setups, the standard formula works well, but if your tank is in a drafty location or has an unusually large open top, consider rounding up to the next heater size.
Related Guides
Learn more about temperature management and equipment choices for your aquarium.
Water Temperature Guide
Why stable temperature matters, how to monitor it, and what happens when it fluctuates too much.
Read the guideFish Temperature Compatibility
Match species to temperature ranges so you can stock your tank with fish that thrive in the same conditions.
Read the guideTank Setup Guide
A step-by-step walkthrough of setting up your first aquarium, including equipment placement and cycling.
Read the guide