Aquarium Heater Size Calculator: Watts for Your Tank and Room
Work out what size aquarium heater you need from your tank's gallons and how warm the room is, using heater makers' own watts-per-liter and volume tables.
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Most tanks need roughly 1 to 1.5 watts of heater per liter of water, which works out to about 4 to 6 watts per gallon: the lower figure in a warm room, the higher in a room that is only slightly heated. That rule comes from heater maker sera's own guide. The calculator below applies it to your tank and also shows which size EHEIM rates for that volume, using the wattage table EHEIM prints for its thermocontrol heaters.
Heater size calculator
Suggested heater
80 W
1.0 W per litre x 76 L
EHEIM thermocontrol size
75 W
rated for about 60 to 100 L
Lift above the room
8°F
the heater has to hold this gap
Watts per litre: sera's guide (1 W per litre in a warm room, 1.5 W per litre in a room that is only slightly heated). EHEIM row: the thermocontrol volume bands on eheim.com. Use the actual water volume; rock, sand and the gap under the rim take some of the tank's nominal gallons.
How the calculator works
- Volume. Enter the water volume in US gallons. The calculator converts it to liters (one US gallon is 3.785 liters).
- Watts per liter. sera's guide says to calculate about 1.5 watts per liter in rooms that are only slightly heated, and that 1 watt per liter is sufficient in a warm room. The calculator multiplies your liters by 1 or 1.5 and rounds up to the next 5 watts.
- EHEIM size. EHEIM's product data table rates each thermocontrol heater for a band of tank volumes. The calculator picks the band your tank falls in.
- Lift. It shows how far above the coldest room temperature you are asking the heater to hold the water. A lift above about 10°F is a sign to treat the room as cool and take the larger answer.
The numbers behind it
| Wattage | Rated volume (liters) | About (US gallons) |
|---|---|---|
| 25 W | 20 to 25 | 5 to 7 |
| 50 W | 25 to 60 | 7 to 16 |
| 75 W | 60 to 100 | 16 to 26 |
| 100 W | 100 to 150 | 26 to 40 |
| 125 W | 150 to 200 | 40 to 53 |
| 150 W | 200 to 300 | 53 to 79 |
| 200 W | 300 to 400 | 79 to 106 |
| 250 W | 400 to 600 | 106 to 159 |
| 300 W | 600 to 1,000 | 159 to 264 |
Gallon figures are our conversion of EHEIM's liters, rounded.
Worked examples
| Tank | Room | sera rule | EHEIM size | What we would buy |
|---|---|---|---|---|
| 5 gal betta tank (19 L) | Warm | 20 W | below the 25 W band | A 25 W preset such as Fluval's P25 (rated up to 6 gal) |
| 10 gal (38 L) | Warm | 40 W | 50 W | A 50 W heater |
| 20 gal long (76 L) | Warm | 80 W | 75 W | 75 to 100 W |
| 20 gal long (76 L) | Cool basement | 115 W | 75 W | 100 to 150 W, or two heaters |
| 55 gal (208 L) | Warm | 210 W | 150 W | 200 W, such as Fluval's E200 (rated up to 65 gal) |
| 75 gal (284 L) | Cool | 430 W | 150 W | Two 200 W heaters at opposite ends |
The two methods disagree on bigger tanks, because EHEIM's bands assume a modest lift above a normal room. When they differ, lean toward the larger answer in a cool room, and toward the maker's rating in a warm one.
Things the calculator cannot know
- Your real water volume. Rock, sand and the gap under the rim take up space. Aqueon's 20 gallon long measures 30.25 by 12.5 by 12.75 inches outside, which works out to about 19 gallons with an inch of space under the rim, before you subtract the glass and the gravel. Our tank volume calculator works it out from the dimensions.
- Lids and drafts. An open top, a fan blowing across the surface or a tank by a cold window loses heat faster.
- Your heater's own rating. Each maker sizes its heaters differently. Fluval rates its P series and E series by tank size, and adds on its P series page that if the room is more than 8°C (12°F) below the 76 to 78°F target, you should use a higher-wattage heater. Read the box.
Why a stronger heater is not the danger it seems
sera notes that a stronger heater is acceptable, since the energy needed for a given amount of warmth stays the same: a thermostat switches the heater off once the water reaches the set point. The danger with an oversized heater is a thermostat that sticks on, which can heat a small tank quickly. That is the case for a separate controller, described in our best aquarium heaters guide, rather than for an undersized heater that runs flat out all winter.
Next steps
- Best aquarium heaters: Fluval E200, EHEIM thermocontrol e, titanium heaters and a safety controller.
- Best betta heaters for 2.5 to 10 gallon tanks.
- Betta fish water temperature, if you are heating a betta tank.
Frequently asked questions
How many watts per gallon do I need for an aquarium heater?
About 4 watts per gallon in a warm room and about 6 in a room that is only slightly heated. That comes from heater maker sera's guide of 1 watt per liter in a warm room and 1.5 watts per liter in a cool one.
What size heater for a 10 gallon tank?
About 50 watts. sera's rule gives around 40 W in a warm room, and EHEIM rates its 50 W thermocontrol for about 25 to 60 liters (7 to 16 gallons). Go up a size in a cold room.
What size heater for a 55 gallon tank?
About 200 watts. sera's rule gives about 210 W in a warm room, and Fluval rates its 200 W E200 for tanks up to 65 gallons. In a cool room, consider two heaters.
Is it bad to have a heater that is too big?
Not in normal use, because the thermostat turns it off at the set point; sera says a stronger heater is acceptable. The risk is a stuck thermostat, so pair a big heater with a separate controller.
Does room temperature affect heater size?
Yes, a lot. The bigger the gap between room and water temperature, the more wattage you need. Fluval says to use a higher-wattage heater when the room is more than 12°F below your target.
Sources
Cover image: Illustration: Aquarium Gear Review (AI-generated)
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