Aquarium Heater Wattage

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 and this calculator estimates the recommended heater wattage using the standard per-gallon guideline, so you can shop with a concrete number instead of guessing.