Sizing assumes a covered tank indoors. Always run a separate thermometer, and consider an external temperature controller as a fail-safe against stuck-on heaters.
Heater wattage chart by tank size
Recommended heater wattage by tank size and how far your room temperature sits below the target. Larger tanks and bigger temperature gaps are shown as two heaters — more even heat, and redundancy if one fails.
| Tank | +5°F | +10°F | +15°F | +20°F |
|---|---|---|---|---|
| 5 gal | 25W | 25W | 25W | 25W |
| 10 gal | 50W | 50W | 50W | 50W |
| 20 gal | 75W | 100W | 100W | 100W |
| 29 gal | 100W | 150W | 150W | 150W |
| 40 gal | 150W | 200W | 200W | 200W |
| 55 gal | 2 × 100W | 2 × 125W | 2 × 125W | 2 × 150W |
| 75 gal | 2 × 125W | 2 × 150W | 2 × 150W | 2 × 150W |
| 90 gal | 2 × 150W | 2 × 150W | 2 × 150W | 2 × 150W |
| 125 gal | 2 × 150W | 2 × 150W | 2 × 150W | 2 × 150W |
How heater sizing works
The working rule is 3–5 watts per gallon, scaled by the gap between room and target temperature: about 3 W/gal when the room sits within 9°F of the target, 4 W/gal for a 10–15°F gap, and 5 W/gal beyond that. Round up to the next standard heater size. Oversizing slightly is safe with a working thermostat — undersizing means the heater runs constantly and still loses ground on cold nights.
Pair the heater with reliable circulation and monitoring: see our filter & heater combo picks, canister filters for larger tanks, and water change schedule guide.
Frequently Asked Questions
What size heater do I need for my aquarium?
Use 3–5 watts per gallon depending on how far your room temperature sits below the target: about 3 W/gal for a gap under 10°F, 4 W/gal for 10–15°F, and 5 W/gal beyond that — then round up to the next standard heater size.
Is one big heater or two small heaters better?
For tanks of 55 gallons and up — or any setup needing more than 200W — two half-size heaters at opposite ends heat more evenly, and if one fails stuck-on or stuck-off, the other limits how far the temperature can swing.
Can an aquarium heater be too powerful?
A correctly working thermostat makes moderate oversizing safe — the heater just cycles less often. The real risk is a stuck-on failure, which a bigger heater turns into a faster disaster; an external temperature controller is the standard fail-safe.