Aquarium Heater Size Calculator

Wattage sized to the gap between your room and your tank, rather than a flat watts-per-gallon figure that was written for somebody else's house.

Last reviewed August 2026 · Free · Nothing you enter is stored

Wattage follows the temperature gap, not the gallons

A heater does one job: it replaces the heat the tank loses to the room. How much heat that is depends almost entirely on the difference between the water temperature and the air temperature around it. A tank sitting in a room at 76 degrees and held at 78 barely loses anything and needs very little heater. The same tank in an unheated conservatory at 55 degrees is losing heat constantly, and needs several times the wattage to hold the same 78.

This is exactly why a single watts-per-gallon figure cannot be right for everyone. Every such figure carries an unstated assumption about the room, and the room is the thing that varies most from one keeper to the next. The table below is what this calculator uses instead, scaling the watts per gallon with the lift required.

Read it against your own coldest night rather than your daytime average. A heater sized for the mild part of the year will simply run continuously in January and still lose ground.

What the manufacturers actually publish

The two most commonly cited guides in the hobby are Aqueon's wattage chart and Fluval's T-series capacity ratings. Both are useful, and both share the same limitation: they give one flat figure per tank size and publish no adjustment for temperature differential at all. Neither states the room temperature their figures assume.

Used as a sanity check they are fine, and if your answer here falls a long way outside the manufacturer's band it is worth asking why. In a warm house you will usually find you need less than the chart suggests; in a cold one, more.

Wattage scales with the gap between room temperature and target tank temperature, which is why a single watts-per-gallon number cannot be right for every room. The manufacturer columns are Aqueon's published wattage guide and Fluval's T-series capacity ratings, both of which assume an unstated ordinary room temperature. Splitting the load across two heaters is hobby practice rather than a manufacturer instruction, but the reasoning is sound: a single heater stuck on can cook a tank, while one half-sized heater stuck on usually cannot.

Two smaller heaters beat one large one

Above roughly 40 gallons, splitting the wattage across two heaters at opposite ends of the tank is worth doing. The argument is not about efficiency, which is identical either way, but about what happens when a heater fails.

Aquarium heaters fail in two directions. A heater that fails off lets the tank drift down to room temperature, which is unpleasant but rarely fatal and usually noticed within a day. A heater that fails on is the dangerous one, because the thermostat sticks closed and the element runs continuously. A single heater sized for the whole tank has enough power to cook it. One half-sized heater running flat out generally cannot reach a lethal temperature in a tank of that volume, and the second heater keeps the tank warm in the meantime if the first one dies quietly.

This is hobby practice rather than a manufacturer instruction, and it is worth being clear about the distinction. No heater manufacturer tells you to buy two of their products and run them at half load. The reasoning stands on its own merits.

Placement, guards and controllers

A heater needs moving water around it. The thermostat inside is reading the water immediately next to the element, so a heater tucked into a dead corner behind the hardscape will read a warm pocket, switch off early, and leave the rest of the tank cool. Position it near the filter outflow or intake where there is real flow. Horizontal placement low in the tank works well because the warmed water rises through the whole column on its own.

A heater guard is worth fitting if you keep large fish, plecos that will rest against anything warm, or a cichlid curious enough to worry at a glass tube. It also stops a fish becoming trapped between the heater and the back glass, which happens more often than people expect. Guards cost a few dollars and the failure they prevent is unpleasant.

For valuable livestock, the real answer is a separate controller or external thermostat. It cuts power at a temperature you choose, independently of the heater's own thermostat, which means a stuck heater is caught by a second device rather than by you noticing the fish behaving oddly. It is the single most effective thing you can add to a heated tank, and it matters more than which brand of heater sits underneath it.

Target temperatures by species group

Before sizing anything, settle on the temperature you are aiming for. These are the ranges used across our fish data:

Mixing groups is where problems start. A tank run at 84 for discus is a tank running most community tetras at the top of their tolerance permanently, and a goldfish kept at 78 lives faster and shorter than one kept cool. Our stocking calculator lists the range for each species alongside the space it needs.

When a heater is the wrong answer

If your room is already warmer than your target, no heater will help, and adding one simply means it never switches on. Cooling is the harder problem. A small fan blowing across the water surface will pull the temperature down a few degrees through evaporation, at the cost of topping the tank up more often. Beyond that you are into chiller territory, which is expensive and noisy.

The cheapest fix is usually to stop fighting the room. A house that sits at 80 all summer is a good home for species that want that, and an ongoing argument for anything that does not. Choosing livestock around the temperature you actually have is less work than air-conditioning a room for a tank.

Frequently asked questions