🌡️ Aquarium Heater Size Calculator
Enter your tank size, room temperature and target temperature to get the recommended heater wattage.
Suggested heater
75 W
calc ≈ 70 W, rounded to a stock size
Temperature rise
10 °F above room
A common rule of thumb is 2.5–5 watts per gallon, scaled by how far above room temperature you need to heat: about 2.5 W/gal for a small rise, 5 W/gal for a large one in a cold room. For tanks over ~40 gallons, two smaller heaters give more even heat and a safety margin if one fails. Always use a separate thermometer to confirm. 🔒 In your browser.
How the aquarium heater size calculator works
A heater has to add enough energy to hold the tank above room temperature and offset heat loss. The common hobby guideline is 2.5–5 watts per gallon, scaled by how far above the room you need to heat: about 2.5 W/gallon for a small rise, 3.5 for moderate, and 5 for a large rise in a cool room. The tool works out the temperature rise from your room and target, applies the right figure, and rounds up to the nearest stock heater size.
For tanks larger than about 40 gallons, two smaller heaters are better than one big one — they heat the water more evenly and, if one sticks off, the tank cools slowly rather than crashing (and if one sticks on, it can't cook the whole tank). Always run a separate thermometer to verify, and pick a heater with a reliable thermostat.
Frequently asked questions
What size heater do I need for my aquarium?
As a guide, 2.5–5 watts per gallon depending on how far above room temperature you're heating. A 20-gallon tank in a normal room needs roughly 50–100 watts. Enter your numbers for a specific recommendation rounded to a stock size.
How many watts per gallon for an aquarium heater?
About 2.5 watts per gallon for a small temperature rise, 3.5 for a moderate one, and up to 5 watts per gallon if the room is cold and you need a large rise. Colder rooms need more wattage for the same tank.
Should I use one heater or two?
For tanks over ~40 gallons, two heaters splitting the wattage give more even heating and a safety margin: if one fails off the other limits the drop, and if one sticks on it can't overheat the whole tank. Small tanks are fine with one.
Does room temperature matter for heater size?
Yes — the heater only has to make up the difference between room and target temperature. A tank in a warm room needs far less wattage than the same tank in a cold basement, which is why this calculator asks for both.
Can a heater be too powerful?
An oversized heater can overheat the tank quickly if its thermostat sticks on, which is risky for fish. Match the wattage to the tank rather than massively oversizing, use a reliable thermostatic heater, and always confirm with a separate thermometer.