Equipment Guides · Jul 12, 2026 · By Fish Tank Volume Calculator Team
What Size Heater Does My Aquarium Need?
Pick the right aquarium heater wattage for your tank size and room temperature, with a simple watts-per-litre rule and a free heater calculator.
An aquarium heater is the least exciting thing you will buy for your tank and one of the most important. Tropical fish need stable, warm water, and the heater is what keeps it there through a cold night or a winter room. Buy one too small and it runs constantly without ever reaching temperature; too large and it can overshoot. The good news is that sizing a heater is mostly a matter of your tank’s volume and how cold your room gets.
This guide gives you a simple rule, a size chart for common tanks, and the reasoning behind both, so you can pick a heater with confidence rather than grabbing whatever is on the shelf.
The quick answer
A widely used starting point is about 1 watt per litre of water for a modest temperature rise of 5 to 6 C above room temperature. In a colder room you will want more, closer to 1.5 watts per litre.
- 100 litre tank: roughly a 100 to 150 watt heater
- 200 litre tank: roughly 200 to 300 watts, ideally split across two heaters
Because your room temperature and target temperature both matter, the cleanest way to get an exact figure is the aquarium heater calculator. Enter your water volume and how far above room temperature you need to warm it, and it recommends a wattage and rounds it up to the nearest standard heater size.
Heater size by tank volume
This chart uses the common watts-per-litre guide for a typical heated room. If your tank sits in a cold room, garage or conservatory, move up to the next size.
| Tank volume | Typical heater (normal room) | Cold room |
|---|---|---|
| 20 L (5 US gal) | 25 W | 50 W |
| 40 L (10 US gal) | 50 W | 75 W |
| 75 L (20 US gal) | 75 W | 100 W |
| 100 L (25 US gal) | 100 W | 150 W |
| 200 L (55 US gal) | 200 W (or 2 x 100 W) | 2 x 150 W |
| 400 L (100 US gal) | 2 x 200 W | 2 x 300 W |
Not sure of your real volume? Measure the inside of the tank and use the fish tank volume calculator to convert the dimensions to litres before you size the heater.
Why room temperature changes everything
A heater does not warm water to a fixed number, it maintains a difference between the water and the room. If your room sits at 22 C and your fish need 26 C, the heater only has to hold a 4 degree gap, which is easy. If the room drops to 15 C on a winter night, that same heater now has to hold an 11 degree gap and may not keep up.
This is why a single watts-per-litre number is only a starting point. The colder and less stable your room, the more wattage you need per litre. The heater calculator accounts for this directly by asking for your target temperature rise, so a tank in a chilly room gets a larger recommendation than the same tank in a warm one.
Why two smaller heaters beat one big one
For tanks over about 200 litres, two smaller heaters are safer than a single large one, for two reasons. First, if one heater fails on, the smaller one cannot cook the whole tank before you notice. Second, if one fails off, the other keeps the tank from crashing to room temperature. Two heaters also spread warmth more evenly across a long tank. Split the total wattage roughly in half between them.
Round up, and set it correctly
Always round the calculated wattage up to the next standard size (25, 50, 75, 100, 150, 200, 300 W). A slightly oversized heater simply switches off sooner once it hits temperature, while an undersized one runs non-stop and still falls short on cold nights.
When you install it, use a separate thermometer at the far end of the tank rather than trusting the heater’s dial, and give the heater a few hours to stabilise before you judge it. A steady reading matters far more than a precise dial setting.
Matching the heater to your fish
Most tropical community fish want 24 to 27 C. Set your target temperature from the species you keep, then set the heater’s rise as that target minus the coldest your room reaches. Bettas, for example, are strictly tropical and suffer in unheated tanks, which is one reason a proper heated setup matters so much, covered in our guide on what size tank a betta needs.
Frequently asked questions
How many watts per litre for an aquarium heater? About 1 watt per litre for a 5 to 6 C rise above room temperature, and up to 1.5 watts per litre in a cold room. Use the heater calculator for your exact tank and room.
What size heater for a 100 litre tank? Around 100 to 150 watts in a normal room. If the tank is in a cold room, lean towards 150 watts or two 100 watt heaters.
Can an aquarium heater be too powerful? A moderately oversized heater is fine, it just cycles off sooner. Avoid a very large heater on a small tank, since a stuck-on fault could overheat the water quickly.
Do I need a heater for a small tank? Yes, if you keep tropical fish. Small tanks lose and gain heat fastest, so stable warmth actually matters more, not less. Choose a compact heater rated for the volume.
Should I use one heater or two? One is fine up to about 200 litres. Above that, two smaller heaters give more even heat and a safety margin if one fails.
The bottom line
Sizing a heater comes down to two numbers: your water volume and how far above room temperature you need it. Get the volume from the fish tank volume calculator, decide your target temperature from your fish, and let the aquarium heater calculator turn both into a recommended wattage. Round up, add a thermometer, and your tank will hold a steady temperature through any season.