Aquarium Heater Wattage Calculator

Aquarium heater sizing depends on three main factors: tank volume, the difference between room temperature and desired water temperature, and whether the tank is in a drafty location.

General rule: Watts needed = Tank Volume (gallons) × Temperature Difference (°F) × 0.5

The metric form is the same rule, not a different one. A gallon is 3.785 litres and a Fahrenheit degree is 5/9 of a Celsius degree, so 0.5 × 1.8 ÷ 3.785 gives 0.238, which most charts round to 0.24:

Watts needed = Tank Volume (litres) × Temperature Difference (°C) × 0.238

That is where the familiar “3 to 5 watts per gallon” comes from: a 10°F lift needs exactly 5 W per gallon, a gentler 6°F lift needs 3, and most rooms sit somewhere between. If a metric formula ever hands you four times the wattage the imperial one does, one of them has the conversion wrong.

Sizing up does not cost more to run

Worth being clear about, because it puts people off buying the bigger heater. The thermostat decides how long the element is energised, and it stops as soon as the water reaches temperature. A 300 W heater in a tank losing 200 W runs two-thirds of the time; a 200 W heater in the same tank runs continuously. Both deliver 200 W of heat and both cost the same to run. What the larger one buys is recovery speed after a water change and margin on a cold night, which is why the tip at the bottom of this page says to size up rather than down.

Standard heater sizing chart. These are heater sizes you can actually buy, rounded up from the formula.

Common fish temperature requirements:

• Tropical freshwater (tetras, guppies, angelfish): 75–80°F (24–27°C)
• Goldfish: 65–72°F (18–22°C), and in a heated room they often need no heater at all
• Bettas: 76–82°F (24–28°C)
• Discus: 82–86°F (28–30°C)
• Marine/reef: 75–80°F (24–27°C)

Important considerations:

• For tanks over 40 gallons, consider using two smaller heaters instead of one large one. This provides backup if one fails.
• Submersible heaters are more efficient than hang-on heaters.
• Place the heater near a water flow source (filter output) for even heat distribution.
• Always use a separate thermometer to verify temperature, not just the heater’s built-in dial.

Tip: If your room temperature fluctuates widely (e.g., drops at night), size up to the next wattage level for consistent water temperature. A slightly oversized heater with a good thermostat is better than an undersized one running constantly.