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Aquarium Heater Wattage

Find the recommended heater wattage for your fish tank size.

How it is calculated

The tool applies one fixed rule of thumb: watts = tank volume (gallons) × 5. The result is rounded to a whole number of watts. Tank volume is entered in gallons (one US gallon is about 3.8 litres, so a 150 litre tank is roughly 40 gallons). A volume of zero or less gives no result. The calculation does not ask about room temperature, how many degrees you need to raise the water, lid or insulation, or the species kept. It always uses the same 5 W per gallon, which suits a typical lift above room temperature. In a cold room or for a larger temperature rise the wattage needed can be higher. Heaters are sold in fixed sizes, so you normally pick the nearest standard unit.

Worked example

You have a 40 gallon aquarium. Multiply the volume by 5 watts per gallon: 40 × 5 = 200. The tool shows 200 W as the recommended total heater power. Because the heating element is a single point of failure, many keepers would meet this with two 100 W heaters at opposite ends of the tank rather than one 200 W unit. For a 10 gallon tank the same rule would give 50 W.

Try this example →

Common mistakes

  • Entering litres in the gallons field. A 100 litre tank is about 26 gallons, and typing 100 gives a heater far too big.
  • Treating the result as exact. Heaters come in standard sizes, so round to the nearest available unit and avoid a much larger one.
  • Ignoring room temperature. A tank in a cold garage needs more power than the fixed 5 W per gallon assumes.
  • Using the full tank size when the water level is much lower. The heater has to warm the water actually present, not the glass volume.
  • Putting one large heater in a big tank. If it sticks on, the water can overheat, which is why two smaller ones are often used.

About this calculator

Choosing the right aquarium heater wattage comes down to tank volume and how far you need to raise the water above room temperature: the widely used aquarium-hobby rule of thumb is about 3 to 5 watts per gallon (roughly 0.8 to 1.3 watts per liter) for a typical temperature lift of 5 to 10°F above ambient, with larger, cooler rooms or bigger swings needing wattage toward the higher end of that range.

Undersized heaters struggle to hold a stable temperature during cold snaps or in a chilly room, causing swings that stress fish and can trigger disease outbreaks in sensitive species like discus or bettas, while oversized heaters risk overshooting and cooking a tank if the thermostat fails — which is why experienced keepers often split the total wattage across two smaller heaters instead of running a single large unit. Reptile and tropical fish retailers print wattage charts based on exactly this per-gallon guideline when recommending a heater for a given tank size.

Enter your tank volume in gallons and this calculator multiplies it by 5 watts per gallon, the top of that range, so you can shop with a concrete number instead of guessing. That figure suits a cool room or a large temperature lift; for a small lift in a warm room, about 3 watts per gallon is enough, and for a tank measured in liters, divide by 3.785 to get gallons first.

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