Aquarium Heater Size Calculator
What wattage heater your tank actually needs
Aquarium heater size is not just watts per gallon — it depends on how far you are heating above the coldest the room ever gets. Enter your volume and both temperatures to get the heater wattage for your tank, rounded to a real heater size, with a one-versus-two-heater call for larger setups.
Updated July 2026 · Reviewed by the AquariumStocking team
Your tank & your room
Use the coldest realistic room temperature — a winter overnight low or an unheated basement — not the daytime average. That is the moment the heater has to cope with.
Not sure? Work out your exact gallons.
Most community tanks: 76–80 °F (24–27 °C).
Winter overnight low, not the daytime average.
Recommended heater
29 US gallons · 10.0 °F lift
Under about 55 gallons a single 150 W heater is enough and easier to hide. Pair it with an independent thermometer so you spot a failure early.
Worked example: a 20 gallon at a 10 °F lift needs 20 × 5 = 100 W, which is exactly a standard 100 W heater.
How aquarium heater wattage is calculated
A heater does not "heat" a tank so much as replace the heat the tank leaks to the room. That leak is proportional to the temperature difference, so the same volume needs very different heater wattage depending on where it sits.
- ΔT ≤ 5 °F: about 3 W per gallon — a warm room barely fights you.
- ΔT ≈ 10 °F: about 5 W per gallon — the classic rule of thumb.
- ΔT ≈ 15 °F: about 7.5 W per gallon.
- ΔT ≥ 20 °F: about 10 W per gallon — cold rooms and basements.
Values between those points are interpolated, and tanks under 10 gallons get a 25 % uplift because a small body of water has far more glass surface per gallon to lose heat through. The result is then rounded up to the next real heater size — 25, 50, 75, 100, 150, 200, 250 or 300 W.
One heater or two?
Below roughly 55 gallons, a single correctly sized heater is fine and much easier to hide behind hardscape.
At around 75 gallons and up, split the requirement into two heaters of half the wattage. Heater thermostats fail in two ways: stuck ON, which cooks the tank, and dead OFF, which chills it. Two half-size units mean a stuck heater cannot supply enough wattage to overheat the tank alone, and a dead one leaves the other holding a survivable temperature until you spot it. Long tanks also heat far more evenly with a unit at each end.
Aquarium heater size chart by tank size
Recommended heater wattage for common tank sizes at a typical 10 °F lift — a tank in a heated room held at around 78 °F. The cold-room column assumes a 20 °F lift, which is what a basement or an unheated bedroom in winter looks like. Size up, never down.
| Tank size | Litres | Typical room (10 °F lift) | Cold room (20 °F lift) | Heaters |
|---|---|---|---|---|
| 5 gallon | 19 L | 50 W | 75 W | 1 × 50 W |
| 10 gallon | 38 L | 50 W | 100 W | 1 × 50 W |
| 20 gallon | 76 L | 100 W | 200 W | 1 × 100 W |
| 29 gallon | 110 L | 150 W | 300 W | 1 × 150 W |
| 40 gallon | 151 L | 200 W | 400 W | 1 × 200 W |
| 55 gallon | 208 L | 300 W | 600 W | 1 × 300 W |
| 75 gallon | 284 L | 400 W | 750 W | 2 × 200 W |
| 100 gallon | 379 L | 500 W | 1000 W | 2 × 250 W |
| 125 gallon | 473 L | 600 W | 1200 W | 2 × 300 W |
| 150 gallon | 568 L | 750 W | 1200 W | 3 × 250 W |
Wattage guidance is an estimate. Always check the manufacturer's own tank rating for the heater you buy — build quality and thermostat accuracy vary far more than the maths does.
Heater safety essentials
- Use a separate thermometer. Never trust the dial on the heater. A cheap glass or digital thermometer at the far end of the tank is your only honest reading.
- Add an external controller. A plug-in temperature controller such as an Inkbird ITC-308 cuts power at a hard upper limit, so a stuck-on heater trips the controller instead of boiling the tank. It is the single best upgrade for any tank with expensive fish.
- Mind water changes. If the heater is not fully submerged during a drain, switch it off and let it cool before refilling — thermal shock cracks glass heaters.
- Place it in flow. Near the filter outflow or intake so warm water is distributed instead of stratifying.
- Fit a drip loop. Let the cable hang below the socket so water runs off rather than into the outlet.
Oversizing has a cost
A wildly oversized heater does not use more electricity — it just runs for less time. The real risk is failure mode: the bigger the single heater relative to the tank, the faster a stuck thermostat pushes the temperature into lethal territory.
Sizing one step up for safety margin is sensible. Putting a 300 W heater in a 10 gallon tank is not. If you want extra headroom on a big tank, add a second heater rather than one enormous one.
Frequently asked questions
Heater guides by tank size
Want the buying side rather than the maths? These guides cover what to look for at each tank size.