Aquarium Heater Size Chart: Wattage by Tank Size
Heater wattage lookup for 5 to 125 gallon aquariums at 5, 10, 15, and 20 degree F temperature rises, plus single vs dual heater setups and the watts-per-gallon rule.
Heater wattage is set by two numbers: how many gallons you are heating and how far above room temperature the tank must sit. A 55 gallon in a warm living room needs half the wattage of the same tank in a cool basement. The chart below covers every standard tank size at four temperature rises, so you can size a heater for your actual room instead of guessing from the box label.
Quick answer: in a typical heated home, plan on about 5 watts per gallon: a 10 gallon needs 50 W, a 20 gallon needs 100 W, a 29 gallon needs 150 W, a 55 gallon needs 300 W (better as 2 x 150 W), and a 75 gallon needs about 400 W (2 x 200 W). Cold rooms need more, warm rooms less. Run your exact numbers through the heater size calculator.
Heater wattage chart by tank size and temperature rise
Temperature rise is your target tank temperature minus the coldest temperature the room reaches, usually overnight in winter. Values are total watts; round up to the nearest heater size you can buy, and split the total across two heaters on tanks over 40 gallons.
| Tank size | +5°F rise | +10°F rise | +15°F rise | +20°F rise |
|---|---|---|---|---|
| 5 gallon | 25 W | 25 W | 50 W | 50 W |
| 10 gallon | 25 W | 50 W | 75 W | 100 W |
| 15 gallon | 50 W | 75 W | 125 W | 150 W |
| 20 gallon | 50 W | 100 W | 150 W | 200 W |
| 25 gallon | 75 W | 125 W | 200 W | 250 W |
| 29 gallon | 75 W | 150 W | 225 W | 300 W |
| 40 gallon breeder | 100 W | 200 W | 300 W | 400 W |
| 55 gallon | 150 W | 275 W | 425 W | 550 W |
| 65 gallon | 175 W | 325 W | 500 W | 650 W |
| 75 gallon | 200 W | 375 W | 575 W | 750 W |
| 90 gallon | 225 W | 450 W | 675 W | 900 W |
| 125 gallon | 325 W | 625 W | 950 W | 1250 W |
Example: your house drops to 68°F on winter nights and you keep a 29 gallon community tank at 78°F. That is a 10 degree rise, so the chart says 150 watts. The same tank in a 62°F basement is a 16 degree rise, which lands between the 15 and 20 degree columns, so 250 to 300 watts is the safe pick. Rises above 20 degrees on large tanks get expensive to run; insulating the tank, adding a tight lid, or warming the room is usually cheaper than more wattage.
The watts-per-gallon rule
All the numbers above come from one simple formula: watts = gallons x temperature rise (°F) x 0.5. Expressed as the familiar watts-per-gallon rule, it scales like this:
| Temperature rise | Watts per gallon | Typical situation |
|---|---|---|
| +5°F | 2.5 W/gal | Warm room, tank near room temperature |
| +10°F | 5 W/gal | Normal heated home, tropical tank at 76 to 80°F |
| +15°F | 7.5 W/gal | Cool room, or a warm tank such as discus at 84°F+ |
| +20°F | 10 W/gal | Cold basement or garage in winter |
The old advice of 3 to 5 watts per gallon is just the middle rows of this table. It works fine for most homes and breaks down exactly where you would expect: warm apartments where it oversizes, and cold basements where it undersizes badly.
Recommended heater setup by tank size
For a standard tropical tank in a heated home, roughly a 10 degree rise, these are sensible off-the-shelf setups. Two heaters at opposite ends heat more evenly and fail safer than one big unit.
| Tank size | Simple setup | Safer setup (40 gal and up) |
|---|---|---|
| 5 gallon | 25 W | n/a |
| 10 gallon | 50 W | n/a |
| 15 gallon | 75 W | n/a |
| 20 gallon | 100 W | n/a |
| 29 gallon | 150 W | n/a |
| 40 gallon breeder | 200 W | 2 x 100 W |
| 55 gallon | 300 W | 2 x 150 W |
| 75 gallon | 2 x 200 W | 2 x 200 W |
| 90 gallon | 2 x 250 W | 2 x 250 W |
| 125 gallon | 2 x 300 W | 2 x 300 W, plus a lid in cold rooms |
Well-Reviewed Heater Picks
Fluval M 200-Watt Submersible Heater
$35.99 on Amazon
Slim 200 W heater for tanks in the 40 to 65 gallon band.
hygger 200W Heater with External Controller
$37.99 on Amazon
Auto shutoff out of water, over-temp alarm, 20 to 40 gallons.
Placement, verification, and safety
Mount the heater near flow, such as the filter outlet or a powerhead, so warm water circulates instead of pooling in one corner. Put a separate digital thermometer at the far end of the tank; built-in heater dials are often off by several degrees, and the thermometer is what catches a stuck heater before it hurts livestock. On tanks of 55 gallons and up, an external thermostat controller that cuts power at a ceiling temperature is cheap insurance against the single most damaging heater failure, sticking on.
Unplug any heater and let it cool for 10 to 20 minutes before water changes that expose it to air. A hot glass heater tube hitting air, or cold refill water, is the classic way they crack.
Plan the rest of the build
Confirm the temperature your species actually want on the aquarium temperature chart, and if you are not sure of your gallons, get them from the volume calculator first. The heater size calculator runs this chart's math on your exact room and target temperatures, and our best aquarium heaters roundup compares specific models. Every lookup table lives at the reference charts hub.
Frequently Asked Questions
What size heater do I need for a 20 gallon tank?
In a typical heated home where the tank sits about 10 degrees F above room temperature, a 20 gallon tank needs a 100 watt heater. If the room runs cold, such as a basement that drops to the low 60s, step up to 150 watts so the heater is not running flat out all day. If the room stays warm and the rise is only about 5 degrees, a 50 watt heater is enough.
How many watts per gallon should an aquarium heater be?
Size by temperature rise, not by a single flat rule. Use roughly 2.5 watts per gallon for a 5 degree F rise above room temperature, 5 watts per gallon for a 10 degree rise, 7.5 watts per gallon for a 15 degree rise, and 10 watts per gallon for a 20 degree rise. The common 3 to 5 watts per gallon guideline is simply the middle of that range, matching the 8 to 10 degree rise found in most heated homes.
Is it better to use one heater or two?
Above about 40 gallons, two smaller heaters that add up to the target wattage beat one large unit. Placed at opposite ends of the tank they heat more evenly, and they fail safer: if one sticks on it is too weak to cook the tank quickly, and if one dies the other keeps the water from crashing until you notice. For example, run two 150 watt heaters in a 55 gallon instead of a single 300.
Can an aquarium heater be too powerful?
A moderately oversized heater is safe while its thermostat works, because it simply switches off sooner. The danger is a big heater stuck in the on position in a small tank, which can overheat the water in hours. Stay within one size of the chart recommendation, verify temperature with a separate thermometer, and consider an inline thermostat controller as a second layer of protection on larger heaters.
Why does my heater run constantly and never reach temperature?
The heater is undersized for the real temperature rise. Wattage needs are set by the gap between your coldest room temperature and the target tank temperature, and by heat loss through glass and the open top. Check the chart against your actual coldest overnight room temperature, add a lid to cut evaporative heat loss, move the tank off exterior walls and out of drafts, and step up a wattage size if it still cannot hold the set point.
Do goldfish or coldwater tanks need a heater?
Usually not in a heated home. Goldfish prefer roughly 65 to 72 degrees F and white cloud minnows tolerate into the low 60s, so normal room temperature suits them. A heater only makes sense for coldwater fish if the room drops into the 50s or swings widely day to night, in which case a small heater set as a floor, for example at 65, protects them from rapid drops without making the water tropical.
Planning or running a tank?
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