What size heater do I need? Aquarium heater wattage by tank size

The right heater depends on two things: how much water you have and how much colder your room gets than your tank. Find your tank below, then read across to the coldest your room gets.

By ScapeMath · Updated November 3, 2026 · Every number on this page is computed with the same formulas as our calculators.

Want the exact figure for your tank? Enter your exact water volume, coldest room temperature, target temperature and top-off, and the calculator sizes the heater for you. Open the free heater size calculator.

The short answer

For a closed tank kept at 78 °F (about 26 °C) in a 68 °F (20 °C) room: a 20 gallon needs a 100 W heater, a 29 gallon 150 W and a 55 gallon 200 W. Colder rooms, warmer water and open tops all need more. Each suggestion is the estimated heat loss plus 25% headroom, so the heater isn't running flat out, rounded up to a common heater size.

Heater size chart in gallons

Water at 78 °F, closed lid. Pick the column for the coldest your room gets at night or in winter.

Totals over 300 W are split across two heaters. The last column is the estimated loss plus 25% headroom, per gallon, before rounding to a heater size. It falls as tanks get bigger, which is why one watts-per-gallon rule can't fit every tank.

Heater size chart in litres

Water at 25 °C, closed lid.

Is "5 watts per gallon" right?

The old rule of thumb is 3–5 watts per gallon. It treats heat loss as if it grew with the water, but heat escapes through the glass, and a tank with 8 times the water has only about 4 times the glass. So the rule oversizes large tanks and can undersize small ones in a cold room.

How the estimate works

A heater only has to replace the heat the tank loses, which grows with the glass area and with the gap between water and room temperature (Q = U·A·ΔT). We anchor that to a published rating, the Fluval E100 (100 W for up to 120 L at 26 °C water in a 20 °C room), and scale it: loss = 100 W × (litres ÷ 120)^2/3 × (temperature lift ÷ 6 °C). The same formula gives 97 W for 150 L at a 5 °C lift, in line with EHEIM's 100 W rating for that job. The full method, worked examples and limits are on the heater size calculator page.

Choosing and fitting the heater

• Size for the coldest night, not the average day. Basements, cold floors and outside walls need the colder column.
• Open tops lose more. Every litre of top-off a day is about 28 W of extra loss.
• Two heaters for big tanks. Two smaller heaters with the same total fail more safely: one stuck on can't cook the tank alone, and if one dies the other keeps it warm.
• Put it in the flow, near the filter outlet or in the sump, so the warm water mixes instead of sitting in one corner.
• Check it with a thermometer. Heater dials are often a degree or two off, and a separate temperature controller adds a safety cut-off.

Common questions

What size heater do I need for a 20 gallon tank?

In a 68 °F room with the water at 78 °F, a closed 20 gallon loses about 68 W, so with 25% headroom a 100 W heater. In a 60 °F room it needs 200 W.

What wattage heater for a 100 litre aquarium?

At 25 °C water in a 20 °C room, a closed 100 L tank loses about 74 W: a 100 W heater with headroom. In a 16 °C room or basement, 200 W.

How many watts per gallon does an aquarium heater need?

It isn't a fixed number. Heat escapes through the glass, and bigger tanks have less glass per gallon, so the watts per gallon fall as tanks grow. In a 65 °F room with water at 78 °F, the estimated loss plus 25% headroom works out to 7.0 W per gallon for a 10 gallon but 3.0 W per gallon for a 125.

Is 5 watts per gallon a good rule?

Roughly, for mid-size tanks in a cool room. It oversizes big tanks (625 W for a 125 gallon, which needs about 231 W in a 68 °F room) and can undersize small ones in a cold room: a 10 gallon at 78 °F in a 60 °F room loses about 77 W, more than the rule's 50 W.

Does a warmer tank need a bigger heater?

Yes, in proportion to the gap between water and room. Keeping a 29 gallon at 82 °F instead of 78 °F in a 68 °F room raises the heat loss from 87 W to 122 W, and the suggestion from 150 W to 200 W.

Does an open-top tank need a bigger heater?

Yes. Evaporation carries heat away: every litre of daily top-off is about 28 W of continuous loss (107 W per US gallon a day). Enter your top-off in the heater calculator to include it.

References

1. Fluval E-series heater instruction manual (ratings at 26 °C water, 20 °C room)
2. EHEIM thermocontrol ratings as compiled by Aquarium Stocking (maker specs)
3. Aqueon heater wattage guidelines (PDF)
4. Engineering ToolBox: Water heat of vaporization vs temperature (2441.7 kJ/kg at 25 °C)
5. Incropera, F.P. et al. Fundamentals of Heat and Mass Transfer, Wiley (overall heat transfer, Q = U·A·ΔT)

The tables on this page are generated from the calculators' own code and checked by automated tests. Last reviewed November 3, 2026. Found an error? Tell us.

Calculators for this job

• Heater wattage

  How many watts your tank needs, from volume, room and target temperature, and optional evaporation.

• Tank volume & water weight

  How many gallons or litres your tank holds: rectangular, cylinder, bow-front, corner and hexagon, plus water weight.

• Filter size & flow rate

  What GPH or L/h filter your tank needs, how much flow is lost to media and plumbing, and reef return pump and powerhead flow.

• Water change & dilution

  What a water change does to nitrate, salinity or hardness, how much to change to hit a target, and RO/tap blends.

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