Aquarium heater size calculator

Find the heater wattage your tank needs based on its size and how cold the room gets, plus which heater (or pair of heaters) to buy.

Inputs

Use the lowest your room gets, e.g. at night in winter.

Recommended heater wattage

275W

Range 138–275 W

To raise 55 gal by 10 °F. Buy: 2 × 150 W heaters.

Temperature rise
10 °F
Minimum
138 W
Recommended
275 W
Heater setup
2 × 150 W heaters
Watts needed as the room gets colder
010020030040050060060657075Room temperature (°F)Heater watts (W)Your room: 138–275 W

To heat 55 gal from 68 °F to 78 °F, use about 275 W (minimum 138 W), for example 2 × 150 W heaters. The chart shows how the wattage band grows as the room gets colder.

How it works

Heat loss grows with the size of the tank and with the gap between water and room temperature. The widely used rule is watts per gallon per 10 °F of rise:

rise (°F)      = target water temp − coldest room temp
recommended W  = 5.0 × gallons × rise ÷ 10
minimum W      = 2.5 × gallons × rise ÷ 10
heater setup   = next standard size ≥ recommended W
(split across 2 heaters above 150 W)
  • Recommended is Aquarium Co-op’s rule: 5 W per gallon for a 10 °F rise, with a lid. It’s what we’d size to.
  • Minimum is half that, roughly the low end of Aqueon’s guidelines (for example, 150 W for a 55 gallon tank). A heater this small works in a lidded tank in a steady room, but it will run constantly and can fall behind on cold nights.
  • Room temperature should be the coldest your room gets, not the average. Heaters only have to win on the coldest night.
  • In Celsius, a 10 °F rise is about 5.6 °C, and 5 W per gallon is about 1.3 W per liter.

Worked example

A 29 gallon tank, kept at 78 °F in a room that drops to 68 °F:

  • Rise: 78 − 68 = 10 °F
  • Recommended: 5 × 29 × 10 ÷ 10 = 145 W
  • Minimum: 2.5 × 29 × 10 ÷ 10 = 73 W
  • Buy: one 150 W heater. Aqueon’s guide puts a 29 gallon tank in its 100 W band (15–40 gal), which falls inside our range.

In metric: a 200 L tank at 25 °C in an 18 °C room needs about 165–335 W, so two 200 W heaters.

Tips from the fish room

  • Place heaters in the flow. Put the heater near the filter outlet or a powerhead so warm water spreads evenly instead of pooling at one end.
  • Don’t trust the dial. Heater dials often read a degree or two off. Set it using a separate thermometer.
  • Unplug before water changes. A heater that sits out of the water while it’s on can crack, even with a shut-off feature.
  • Cold rooms need a lid more than more watts. Covering the tank often solves a struggling heater for the price of a glass lid.

FAQ

What size heater do I need for a 55 gallon tank?

About 140–275 W to hold 78 °F in a 68 °F room. We'd use two 150 W heaters. Aqueon's guide lists a single 150 W (30–55 gal) or 200 W (50–75 gal) heater for this size, which sits inside our range.

How many watts per gallon does an aquarium heater need?

About 2.5 to 5 watts per gallon for every 10 °F the water has to sit above the room. A lid and a warmer room move you toward the low end; a cold room, an open top or a fan moves you toward the high end.

Can a heater be too big?

A thermostat switches the heater off at the set temperature, so a slightly oversized heater is fine and warms the tank faster. A heater that's far too big is riskier if its thermostat sticks on, which is why two smaller heaters are safer than one big one.

Do I need a heater if my room is warm?

If the room never drops below your target temperature, you may not. Most homes cool off at night and in winter, so a small heater set a few degrees below your target is cheap insurance against sudden drops.

Does a lid really matter?

Yes. Most heat leaves an aquarium through evaporation at the surface. An open-top tank can need noticeably more wattage than the same tank with a lid.

Sources

  1. How to Choose the Right Aquarium Heater, Aquarium Co-op. 5 watts per gallon for a 10 °F rise above room temperature, with a lid.
  2. Aqueon Heater Wattage Guidelines, Aqueon. 50 W for 5–20 gal, 100 W for 15–40, 150 W for 30–55, 200 W for 50–75, 300 W for 65–90.

Formula and sources last checked September 30, 2026. How we test formulas.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.