Aquarium heater size calculator
Heater size depends on how far the water must be lifted above the coldest room temperature, not just on gallons.
How heater size is calculated
A heater replaces the heat the tank loses to the room, and heat loss grows with the difference between water and air temperature. The calculator uses watts = gallons × temperature lift (°F) × 0.3 for a lidded glass tank — about 0.14 W per liter per °C — then adds 25% for an open top (evaporation carries heat away) and takes off 15% for acrylic, which insulates better. The result is rounded up to a standard heater size.
| Tank | Lift 5 °F | Lift 10 °F | Lift 15 °F |
|---|---|---|---|
| 5 gallon (4.7 gal) | 10 W | 25 W | 25 W |
| 10 gallon (9.1 gal) | 25 W | 50 W | 50 W |
| 15 gallon (12.9 gal) | 25 W | 50 W | 75 W |
| 20 gallon high (17.6 gal) | 50 W | 75 W | 100 W |
| 20 gallon long (16.1 gal) | 25 W | 50 W | 75 W |
| 29 gallon (25.1 gal) | 50 W | 100 W | 150 W |
| 40 gallon breeder (39.7 gal) | 75 W | 150 W | 200 W |
| 40 gallon long (34.9 gal) | 75 W | 150 W | 200 W |
| 55 gallon (50.8 gal) | 2 × 50 W | 2 × 100 W | 2 × 150 W |
| 65 gallon (59.6 gal) | 2 × 50 W | 2 × 100 W | 2 × 150 W |
| 75 gallon (69.2 gal) | 2 × 75 W | 2 × 150 W | 2 × 200 W |
| 90 gallon (79.8 gal) | 2 × 75 W | 2 × 150 W | 2 × 200 W |
| 125 gallon (104.4 gal) | 2 × 100 W | 2 × 200 W | 2 × 250 W |
| 150 gallon (138.5 gal) | 2 × 150 W | 2 × 250 W | 3 × 250 W |
Why "5 watts per gallon" is only a starting point
The familiar 3–5 W per gallon rule assumes a room about 10 °F below the tank. In a warm living room a heater rated at half that is plenty; in a cold basement or garage you may need double. Use the coldest temperature the room reaches — overnight in winter — not the daytime average.
One heater or two?
From about 50 gallons, or once you need more than 300 W, the calculator splits the wattage across two heaters. Two smaller heaters at opposite ends warm the tank more evenly, and if one sticks on it cannot cook the tank as quickly as one oversized heater would. If one fails off, the other keeps the water from crashing. Read the heater size guide for placement and safety tips.
Heater size by tank
Worked charts with one- and two-heater options for 3 gallon, 5 gallon, 10 gallon, 20 gallon, 29 gallon, 40 gallon, 55 gallon, 75 gallon tanks, or all of them on one page in the heater size chart.
FAQ
What size heater do I need for a 20-gallon tank?
50–75 W in a room about 10 °F below your target (a 20-gallon tank holds about 16–18 gallons of real water); 100 W if the room gets colder.
What size heater for a 40-gallon tank?
About 150 W when the room is 10 °F colder than the tank. Use 200 W for a cold room.
Can a heater be too big?
A heater with a working thermostat will not overheat the tank because it is too big, but a stuck oversized heater heats the water much faster. That is why large tanks use two smaller heaters.