Aquarium Heater Size Calculator
Plan on about 3 to 5 watts per gallon: roughly 100 W for a 20-gallon tank and 250 W for a 55-gallon when the tank sits about 8°F above room temperature. Colder rooms need more. Enter your tank volume and temperatures below for an exact size and whether to split it across two heaters.
Updated 2026 · free, no sign-up
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Aquarium Heater Size Chart
A heater needs about 3 to 5 watts per gallon: roughly 100 W for a 20-gallon tank and 250 W for a 55-gallon at an 8°F rise over room temperature.
Heater wattage by tank size and how far above room temperature you heat, using this calculator's own formula (about 4 W per gallon at an 8°F rise).
| Tank | 8°F above room | 12°F above room | 16°F above room |
|---|---|---|---|
| 5 gal | 25 W | 50 W | 50 W |
| 10 gal | 50 W | 75 W | 100 W |
| 20 gal | 100 W | 150 W | 200 W |
| 29 gal | 150 W | 200 W | 250 W |
| 40 gal | 200 W | 250 W | 500 W |
| 55 gal | 250 W | 500 W | 500 W |
| 75 gal | 300 W | 500 W | 800 W |
| 125 gal | 500 W | 800 W | 1000 W |
Sources: wattage formula is this site's calculator (about 4 watts per gallon at an 8°F rise, scaled by the rise); manufacturer sizing charts from heater makers such as Eheim and Fluval use similar 3 to 5 W per gallon guidance. Researched from published data, not professional advice. Updated 2026.
How Aquarium Heater Wattage Works
Heater sizing depends on two things: how much water you are heating and how far above room temperature you need to keep it. The standard guideline of 3 to 5 watts per gallon works for an average home, but it breaks down in cold rooms or for large temperature jumps. The calculator above scales the wattage with your temperature rise, so a tank in a chilly basement is sized higher than the same tank in a warm living room.
One Heater or Two?
For tanks over 40 gallons, splitting the required wattage across two heaters at opposite ends gives more even heat, protects against a single heater failing on or off, and warms the water faster. A separate digital thermometer, placed away from the heater, is essential for verifying the real temperature, since built-in dials are often inaccurate.
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Frequently Asked Questions
What size heater do I need for my aquarium?
A good rule of thumb is 3 to 5 watts per gallon, with more watts needed when the room is cold or the temperature you want is far above room temperature. A 20-gallon tank in a normal room usually needs a 50 to 75 watt heater, a 40-gallon needs about 150 watts, and a 75-gallon often needs 250 to 300 watts. This calculator factors in your actual room temperature and target temperature for a more precise figure.
Should I use one heater or two?
For tanks above about 40 gallons, many keepers split the wattage across two smaller heaters placed at opposite ends. This gives more even heating, keeps the tank from overheating if one heater sticks on, and keeps the tank warm enough if one fails off. If the calculator recommends 300 watts, two 150-watt heaters is often a smarter setup than a single 300.
How many degrees should a heater raise the temperature?
Heaters are sized by the difference between your room temperature and your target tank temperature, called the temperature rise. A small rise of 5 to 8 degrees needs fewer watts, while heating a tank 15 or more degrees above a cold room needs significantly more. If your house gets very cold at night, size up so the heater is not running at full power constantly.
What temperature should a tropical aquarium be?
Most community tropical fish do well between 76 and 80 degrees Fahrenheit. Bettas prefer the warmer end, around 78 to 80, while coldwater fish like goldfish actually prefer cooler water in the upper 60s to low 70s and often need no heater at all in a heated home. Always confirm the ideal range for your specific species.
How many watts per gallon does an aquarium heater need?
About 3 to 5 watts per gallon. Use the low end in a warm room and the high end if the room is much colder than the tank. A 20-gallon tank typically needs about 100 W and a 40-gallon about 150 to 200 W.
Can a heater be too powerful for a tank?
An oversized heater is generally safe with a working thermostat, but if it sticks on it can overheat a small tank fast, which is dangerous. The bigger risk is an undersized heater that runs constantly and still cannot hold temperature. Matching wattage to your tank size and room conditions, and using a separate thermometer to verify, is the safest approach.
