Aquarium Heater Size Calculator
Enter your tank size and the coldest your room gets to find the heater wattage you need, or check the chart below for common tank sizes in litres and gallons.
Calculator
Volume unit
Temperature
L
°C
Most tropical fish thrive at 24–26°C
°C
Use the coldest your room gets in winter, not the average
Enter your values above to see results
Aquarium heater size chart (litres and gallons)
Recommended heater wattage for a tank kept at 25°C / 77°F. Each column is a room at a different coldest temperature: warm room 22°C / 72°F, cool room 18°C / 64°F, unheated room 13°C / 55°F.
| Tank size | Warm room22°C / 72°F | Cool room18°C / 64°F | Unheated room13°C / 55°F |
|---|---|---|---|
| 5 gal (19 L) | 25W | 50W | 75W |
| 20 L (5 gal) | 25W | 50W | 75W |
| 10 gal (38 L) | 50W | 75W | 150W |
| 40 L (11 gal) | 50W | 75W | 150W |
| 50 L (13 gal) | 50W | 100W | 150W |
| 60 L (16 gal) | 50W | 100W | 150W |
| 20 gal (76 L) | 50W | 150W | 200W |
| 80 L (21 gal) | 50W | 150W | 200W |
| 100 L (26 gal) | 75W | 150W | 250W |
| 29 gal (110 L) | 75W | 150W | 250W |
| 125 L (33 gal) | 75W | 150W | 250W |
| 150 L (40 gal) | 75W | 200W | 300W |
| 40 gal (151 L) | 75W | 200W | 300W |
| 200 L (53 gal) | 100W | 200W | 2 × 200W |
| 55 gal (208 L) | 100W | 200W | 2 × 200W |
| 250 L (66 gal) | 100W | 250W | 2 × 200W |
| 75 gal (284 L) | 150W | 250W | 2 × 250W |
| 300 L (79 gal) | 150W | 300W | 2 × 250W |
| 100 gal (379 L) | 150W | 300W | 2 × 300W |
| 400 L (106 gal) | 150W | 2 × 200W | 2 × 300W |
Bigger tanks need fewer watts per gallon, because heat escapes through the glass and the water surface, not through the volume. In a cool room, a 10 gallon tank needs about 6.3W per gallon but a 75 gallon tank only about 3.2W.
Heaters worth considering
Based on a typical planted tank setup.
Inline External HeaterWhat I use
Connects to canister filter outlet — heats water outside the tank for a clean, uncluttered aquascape
In the US: DaToo 300W Inline Heater
Fluval Electronic Heater
Precise thermostat to ±0.5°C — safety cutoff prevents overheating if stuck on
Zacro Aquarium Thermometer
Always verify temperature independently — don't rely on the heater's dial or display alone
I’ve used everything listed in my own tanks. If you buy through a link, I earn a small commission — at no extra cost to you, and it never affects what I recommend.
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Frequently Asked Questions
- What size heater do I need for a 55 gallon tank?
- For a 55 gallon (208 litre) tank kept at 25°C / 77°F, you need a 100W heater if the room stays around 22°C / 72°F, a 200W heater if it drops to 18°C / 64°F, and two 200W heaters (2 × 200W) in an unheated room at 13°C / 55°F. Use the coldest temperature your room reaches in winter, not the average, and enter your own numbers in the calculator above for an exact figure.
- What size heater for a 100 litre tank?
- For a 100 litre (26 gallon) tank kept at 25°C / 77°F, you need a 75W heater if the room stays around 22°C / 72°F, a 150W heater if it drops to 18°C / 64°F, and a 250W heater in an unheated room at 13°C / 55°F. Use the coldest temperature your room reaches in winter, not the average, and enter your own numbers in the calculator above for an exact figure.
- What size tank is a 50W heater for?
- Assuming a 25°C / 77°F tank, a 50W heater covers tanks up to about 95 litres (25 gallons) if the room stays around 22°C / 72°F, 26 litres (7 gallons) if it drops to 18°C / 64°F, and 11 litres (3 gallons) in an unheated room at 13°C / 55°F. A 25W heater is for nano tanks: up to about 33 litres (9 gallons) in a warm room, or 9 litres (2 gallons) in a cool one. Go by the coldest your room gets, not the average.
- How many watts per litre does an aquarium heater need?
- There is no single watts-per-litre figure, because heat escapes through the tank's glass and water surface, not through its volume. A bigger tank has less surface for each litre of water, so it needs fewer watts per litre. For a 25°C / 77°F tank in a room that falls to 20°C / 68°F, a 20 litre tank needs about 29W (1.4W per litre), a 100 litre tank needs about 86W (0.9W per litre) and a 300 litre tank needs about 179W (0.6W per litre). The gap between your tank and your room matters just as much: double the gap and you double the wattage.
- Should I use one heater or two?
- This calculator recommends two heaters once the load is above 300W, the largest size EHEIM and Fluval make, with each heater covering half the load. For example, a 200 litre tank at 25°C / 77°F in an unheated room at 13°C / 55°F needs about 328W, so two 200W heaters (2 × 200W). Below that, a single heater can handle the job, but a backup unit is still a smart precaution for large tanks (over 200 litres). The main advantage of two heaters is redundancy: if one fails (sticks off), the other maintains temperature long enough for you to notice and replace it. A heater stuck in the on position will heat the tank dangerously regardless of how many you have, which is why a reliable thermometer is essential.
- What are the most common aquarium heater sizes?
- Standard heater sizes are 25W, 50W, 75W, 100W, 150W, 200W, 250W and 300W; 300W is the largest size EHEIM and Fluval make. For a 25°C / 77°F tank in a cool room at 18°C / 64°F, a 20 litre nano tank needs a 50W heater, a 60 litre tank a 100W heater and a 200 litre tank a 200W heater. The right size depends on tank volume and how cold your room gets in winter, which is why this calculator asks for both.
- How do I know if my heater is too small?
- Signs of an undersized heater include: the heater runs continuously without cycling off; the tank temperature is consistently 1–2°C below your setpoint in cold weather; the heater body feels warm throughout the day rather than cycling; or your temperature drops noticeably overnight when room heating switches off. A heater that runs constantly is also at higher risk of early failure. If you see these signs, check the wattage against this calculator using your actual room temperature in winter.
- Does room temperature really affect heater size?
- A lot. A 100 litre tank kept at 25°C / 77°F needs about 34W (a 50W heater) if your room stays at 23°C / 73°F, but about 172W (a 200W heater) if it drops to 15°C / 59°F: roughly 5 times as much. Heat loss rises in step with the gap between the tank and the room, so a flat watts-per-litre rule breaks down in cold rooms. Always size your heater for the coldest conditions your room will reach, not the average. It is better to have a slightly oversized heater running on a thermostat than an undersized one running at full capacity.