Setup & Sizing
Heater Size Calculator
Work out what wattage aquarium heater you need. The calculator factors in your tank volume, the temperature you want to hold and how warm the room already is.
Heater Size Calculator
The lowest the room reaches in winter, not the average.
Typical tropicals 24-26 C, discus 28-30 C, coldwater 18-22 C.
Recommended wattage
500W
- Closest stocked heater
- 300 W
- Temperature lift
- 7 C
- Watts per litre
- 5 W/L
How heater wattage is worked out
An aquarium heater has to do two jobs: lift the water from room temperature to the target temperature, and then hold it there against room heat loss. Bigger tanks need more heat input, and the colder the room, the harder the heater works.
We start from a baseline of 1.5 W per litre (plenty for a warm room and modest lift) and add 0.5 W per litre for every extra degree Celsius you need to lift. The result is capped at 5 W per litre, which is more than enough for any domestic tank.
Rule of thumb: round up to the next stocked heater size, not down. An oversized heater costs nothing extra in electricity (it only runs when needed) and recovers faster after a water change.
Worked example
A 200 L community tank in a 17 C garage room, aimed at 25 C: lift is 8 C. Base 1.5 + (0.5 x 8) = 5.5 (capped at 5) W/L. 200 L x 5 W/L = 1000 W, so run two 500 W heaters, or a single 300 W plus a 200 W backup. This dual-heater pattern is safer than a single 1000 W unit.
Safety notes
Always unplug the heater before a water change and let it cool in the water before switching it back on. A hot glass heater suddenly exposed to air can crack; the reverse is even more dramatic. Modern titanium or shatter-proof heaters avoid that risk.
Frequently asked questions
How many watts per litre does an aquarium heater need?
The UK shop-floor rule is 2-3 W per litre for a tropical tank in a heated room. If the room is cool or unheated (below 18C), go up to 4-5 W per litre. If the tank sits in a very warm room, 1.5 W per litre can be enough.
Should I use one heater or two?
On tanks above 200 litres, two smaller heaters are safer than one big one. A stuck-on heater of half the wattage will only push the temperature up slowly, giving you time to notice; a single 300 W unit can poach a tank before you notice. Two heaters also give you backup if one fails cold.
Do I need a heater if my room is always warm?
In most UK homes the answer is still yes. Rooms drop a few degrees overnight and in winter, and tropical fish really do not like rapid swings. A heater with a thermostat prevents those dips; it will simply sit idle on warm days.
What is the difference between heater wattage and heater power?
Wattage is the maximum heat output when the heater is switched on. Most modern aquarium heaters have an internal thermostat that cycles the element on and off - so a 200 W heater does not constantly use 200 W of electricity, only when it needs to lift the temperature.
Are heater guards necessary?
Strongly recommended for tanks with goldfish, plecos, cichlids or any fish big enough to rest against the glass tube. Burns from a hot heater surface can be fatal. A clip-on plastic guard costs a few pounds and prevents a very expensive mistake.
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