Aquarium Heater Size Calculator
Heater Size Calculator recommends the right wattage for your tank based on temperature differential — more accurate than the standard 1W/litre rule of thumb.
Calculator
Volume unit
Temperature
Most tropical fish thrive at 24–26°C
Use the coldest your room gets in winter, not the average
Enter your values above to see results
Heaters worth considering
Based on a typical planted tank setup.
Connects to canister filter outlet — heats water outside the tank for a clean, uncluttered aquascape
Precise thermostat to ±0.5°C — safety cutoff prevents overheating if stuck on
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
- How many watts per litre does an aquarium heater need?
- The answer depends on how much you need to raise the water above your room temperature. For small differentials (0–5°C), roughly 1.5W per litre is sufficient. For moderate differentials (6–10°C), around 2.5W per litre is needed. High differentials (11–15°C) require about 4W per litre, and extreme cases above 15°C need 5W per litre. For example, a 100-litre tank in a room that drops to 20°C in winter, heated to 26°C (6°C differential), needs 100 × 2.5 = 250W — well above the '1W per litre' starting estimate.
- Should I use one heater or two?
- Two heaters are required when the load exceeds 500W — the largest standard single unit. 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, 300W, and 500W. For most home aquariums (50–200 litres in a centrally heated room), a 100W or 200W heater is sufficient. Very small tanks (nano tanks, 20–30 litres) typically use 25W or 50W. The correct size depends primarily on tank volume and how cold your room gets in winter — this calculator gives you an accurate recommendation based on 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?
- Dramatically. A 100-litre tank needs only 50W if your room stays at 23°C year-round, but 250W if your room drops to 15°C in winter — a fivefold difference. This is why the 1W/litre rule is often inadequate: it was designed for moderately heated rooms and breaks down in cold climates or unheated spaces. 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.