What size heater do I need for my aquarium?

It depends on the water volume and how far above room temperature you need to keep it — not volume alone. As a starting point this calculator uses about 0.2 watts per liter for each °C of rise, so a 100-liter tank kept 5 °C above the room needs roughly a 100 W heater. Enter your numbers above for a size rounded to a real heater.

Is the ‘watts per gallon’ rule reliable?

Only loosely. Rules like ‘5 watts per gallon’ assume an average room and temperature gap. A tank in a cold garage needs far more per gallon than one in a heated living room. Sizing from the actual temperature rise, as this tool does, is more accurate.

Should I use one heater or two?

On tanks over about 200 liters, two smaller heaters that add up to the required wattage are safer than one large heater. If one fails on, the other can't overheat the tank as fast; if one fails off, the other limits how far the temperature drops. They also spread heat more evenly.

Can a heater be too powerful?

An oversized heater cycles in short bursts and, if its thermostat sticks on, can overheat the tank quickly. Match the wattage to the tank rather than buying the biggest one. This calculator rounds up only to the next standard size, not far beyond your need.

Where should I measure room temperature?

Use the coldest the room realistically gets — winter nights or when the heating is off — because that's when the heater works hardest. Sizing for the warmest part of the day would leave the tank cold when it matters.

Free aquarium calculator

Aquarium Heater Wattage Calculator

Find the right heater size for your tank. Enter the water volume and the temperatures you're working between, and get a recommended wattage rounded to a real, off-the-shelf heater — instantly and shareable.

Water volume

Use your net volume from the volume calculator for the most accurate sizing.

Target temperature

Coldest room temperature

How cold the room gets in winter or at night — the heater must cover this gap.

Recommended heater

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Temperature rise to hold —

Results are estimates for a typical glass tank in a heated home. A poorly insulated room, a chiller, or an open-top tank will need more. Two smaller heaters are safer than one large one on big tanks.

How the heater calculator works

Tell the calculator how much water your tank holds, the temperature you want to keep, and how cold the room gets. It works out the temperature gap the heater has to bridge, sizes the wattage for that gap, and rounds up to the nearest standard heater you can actually buy. Everything updates as you type and the link captures your inputs so you can save or share it.

The formula

A heater only replaces the heat the tank loses to the room, so the wattage scales with both the water volume and the temperature rise: watts = liters × ΔT × 0.2, where ΔT is the target minus the coldest room temperature. The 0.2 factor is calibrated so a 100-liter tank held about 5 °C above a normal room lands near a 100 W heater — the familiar ‘1 watt per liter’ rule — while a cold room needing a 10 °C rise asks for roughly 2 watts per liter.

Worked example

A 200-liter tank you want to keep at 26 °C, in a room that drops to 20 °C, has a 6 °C rise to hold. That's 200 × 6 × 0.2 = 240 W, which rounds up to a 250 W heater. On a tank this size, two 150 W heaters are a safer choice than one 250 W unit: if a heater sticks on or fails off, a smaller one does less harm.

One heater or two?

Below about 200 liters a single correctly sized heater is fine. On larger tanks, splitting the wattage across two heaters gives redundancy — a failed heater can't cook or chill the whole tank — and spreads warmth more evenly. Always pick a heater with a reliable thermostat and verify the temperature with a separate thermometer.

Frequently asked questions

What size heater do I need for my aquarium?: It depends on the water volume and how far above room temperature you need to keep it — not volume alone. As a starting point this calculator uses about 0.2 watts per liter for each °C of rise, so a 100-liter tank kept 5 °C above the room needs roughly a 100 W heater. Enter your numbers above for a size rounded to a real heater.
Is the ‘watts per gallon’ rule reliable?: Only loosely. Rules like ‘5 watts per gallon’ assume an average room and temperature gap. A tank in a cold garage needs far more per gallon than one in a heated living room. Sizing from the actual temperature rise, as this tool does, is more accurate.
Should I use one heater or two?: On tanks over about 200 liters, two smaller heaters that add up to the required wattage are safer than one large heater. If one fails on, the other can't overheat the tank as fast; if one fails off, the other limits how far the temperature drops. They also spread heat more evenly.
Can a heater be too powerful?: An oversized heater cycles in short bursts and, if its thermostat sticks on, can overheat the tank quickly. Match the wattage to the tank rather than buying the biggest one. This calculator rounds up only to the next standard size, not far beyond your need.
Where should I measure room temperature?: Use the coldest the room realistically gets — winter nights or when the heating is off — because that's when the heater works hardest. Sizing for the warmest part of the day would leave the tank cold when it matters.