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Aquarium Heater Size Calculator
A heater has to replace the heat your tank loses through its glass, its surface and the water that evaporates. Enter your tank, the water temperature you want and how cold the room gets at night. The calculator works out that heat loss, picks a heater size with room to spare, and checks what would happen if the heater stuck on, which is the failure that kills fish.
Heater size calculator
Heater you need
- Heat loss on the coldest night
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- Likely range estimate
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- Heater power with headroom
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- Energy that night
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- Through the glass
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- Through the top
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- Through the base
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- Water to top off
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Heaters fail. Keep a separate thermometer where you can see it, unplug a heater before you take it out of the water or do a big water change, and let it sit in the tank for 15–30 minutes before you plug it in. A temperature controller that cuts the power is the best protection against a heater that sticks on.
Equipment for this tank
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Next steps
- Stocking: check your fish share a temperature
- Tank volume: your real water after substrate and decor
- Tank sizes chart: dimensions of 25 standard tanks
How the calculator works
Most heater charts say "3–5 watts per gallon". That ignores the things that really decide the size: how much colder the room is than the water, whether the tank has a lid, and its shape. A tall tank and a long tank of the same gallons lose different amounts of heat. So the calculator adds up where the heat actually goes:
- Glass. Warm glass loses heat to the room by air rising past it (natural convection) and by radiation. The glass itself barely slows this down; acrylic, which insulates better, does.
- Top. An open water surface loses heat the same two ways, plus by evaporation, which is often the biggest single loss. A glass lid stops most evaporation.
- Base. A tank standing on a cabinet loses only a little heat downwards.
The evaporation rate uses the method of Shah (ASHRAE Transactions, 2014), the larger of a natural-convection equation and an air-movement equation. It was checked against 537 measurements from 29 studies, from laboratory dishes of 0.022 m² to swimming pools, with an average error of about 16% (Shah 2023). The convection and radiation terms are the standard textbook ones for still air (h = 1.42 × (ΔT ÷ height)0.25 for the sides, 1.32 × (ΔT ÷ L)0.25 for the top; emissivity 0.92 for glass, 0.95 for water). Three parts are our own estimates, because we could find no measurements for home aquariums: a lid halves the heat lost through the top and cuts evaporation to a tenth, the base loses 2 W per m² per °C, and a fan means air moving at 0.5 m/s. That is why the result shows a range.
Dividing by 0.6 leaves 40% headroom, so the heater runs at most about 60% of the time on the coldest night and can still warm the tank after a water change. Why not just buy a much bigger heater? Because if its thermostat sticks on, it keeps heating. The water settles where the heater's watts equal the heat loss, which you can work out: room temperature + heater watts ÷ heat loss per degree. The calculator shows that number. When one big heater could cook the fish, it picks a smaller one that still copes, or two smaller heaters. The 30 °C limit comes from our stocking data: of 84 common species, 69 are listed as safe only up to 28 °C, and just one beyond 30 °C.
Example 1: 20 gallon long, open top
A 20 gallon long (30 × 12 × 12 in, 3/16 in glass) at 78 °F (25.6 °C) in a room that drops to 68 °F (20 °C) at night: a difference of 5.6 °C.
- Glass: the sides below the waterline are 0.596 m². Convection 3.00 + radiation 5.41 = 8.41 W/m²·°C; with the glass, U = 8.09. 8.09 × 0.596 × 5.6 = 26.8 W.
- Surface: the top is 0.232 m². (3.53 + 5.59) × 0.232 × 5.6 = 11.8 W.
- Evaporation: Shah's natural-convection equation gives 0.174 kg per m² per hour, so 0.97 L a day, which carries away 27.4 W.
- Base: 2 × 0.232 × 5.6 = 2.6 W. Total 68.6 W, or 12.3 W per °C.
- Heater: 68.6 ÷ 0.6 = 114 W, so a 125 W heater. But if a 125 W stuck on, the water would settle at 20 + 125 ÷ 12.3 = 30.1 °C (86 °F). One size down, a 100 W still copes (on about 69% of the coldest night), and stuck on it settles at 20 + 100 ÷ 12.3 = 28.1 °C (83 °F).
Answer: one 100 W heater, with a thermometer in view. A glass lid would nearly halve the heat loss (to 38 W), and a 50 W would then do.
Example 2: 55 gallon with a glass lid, cool room
A 55 gallon (48 × 13 × 21 in, 5/16 in glass) with a glass lid at 80 °F (26.7 °C), room down to 65 °F (18.3 °C).
- Glass: 101.6 W, by far the biggest loss once the lid is on.
- Top: 15.6 W through the lid, plus 6.1 W of evaporation through the gaps (0.2 L a day). Base: 6.7 W.
- Total: 130 W, so 130 ÷ 0.6 = 217 W: over 200 W, so two heaters of 125 W.
- Safety: one 125 W stuck on: 18.3 + 125 ÷ 15.6 = 26.3 °C (79 °F). Safe.
Answer: 2 × 125 W, one at each end.
Example 3: 75 gallon, open, cold room with a fan
A 75 gallon (48 × 18 × 21 in) at 78 °F with no lid, in a room that falls to 62 °F (16.7 °C), with a fan moving air over the water.
- Glass: 115 W. Surface: 45 W. Base: 10 W.
- Evaporation: with moving air, Shah's air-movement equation wins: 4.9 L a day, carrying away 137 W, nearly half the total.
- Total: 308 W, so 513 W with headroom: 2 × 300 W. One stuck on: 25.3 °C. That night uses 7.4 kWh.
A glass lid on the same tank cuts the loss to about 160 W, enough for 2 × 150 W, and saves more than 4 L of top-off water a day.
Aquarium heater size chart
Heater sizes from the calculator for standard tanks kept at 78 °F (25.6 °C), with still room air and 50% humidity. Each cell shows open top / glass lid. "2 ×" means two heaters, one at each end.
| Tank | Room 72 °F 22 °C | Room 68 °F 20 °C | Room 62 °F 17 °C |
|---|---|---|---|
| 10 gallon | 50 W / 25 W | 75 W / 2 × 25 W | 2 × 75 W / 2 × 50 W |
| 20 gallon long | 100 W / 50 W | 100 W / 50 W | 150 W / 2 × 75 W |
| 29 gallon | 100 W / 50 W | 125 W / 75 W | 2 × 125 W / 2 × 75 W |
| 40 gallon breeder | 150 W / 75 W | 200 W / 100 W | 2 × 150 W / 150 W |
| 55 gallon | 200 W / 100 W | 2 × 125 W / 150 W | 2 × 200 W / 2 × 125 W |
| 75 gallon | 200 W / 100 W | 2 × 150 W / 150 W | 2 × 300 W / 2 × 150 W |
| 90 gallon | 200 W / 125 W | 2 × 150 W / 150 W | 2 × 300 W / 2 × 150 W |
| 125 gallon | 2 × 150 W / 150 W | 2 × 300 W / 2 × 125 W | 3 × 300 W / 2 × 200 W |
For comparison, makers' charts are simpler. Aqueon's guide assumes a 68–72 °F room and, for example, lists a 150 W heater for a 55 gallon at a 5–10 °F rise. EHEIM rates its heaters by tank size alone (the 150 W for 53–79 gallons). Both are fine in a warm room; the chart above shows when they are not.
FAQ
What size heater do I need for my fish tank?
It depends mostly on how much colder your room gets than the water, and whether the tank has a lid. As a rough guide, a lidded 29–75 gallon tank in a 68 °F room needs about 2–3 W of heater per gallon, while a small or open tank in a cold room can need 5–10 W per gallon. Enter your tank above for the exact figure.
Is 5 watts per gallon right?
It's a safe upper rule for small tanks in cool rooms, but it oversizes large lidded tanks, and an oversized heater is more dangerous if it sticks on. Big tanks lose less heat per gallon, because they have less glass and surface for each gallon of water.
Should I use one heater or two?
Two smaller heaters, one at each end, spread the heat and protect you both ways. If one fails off, the other keeps the tank from getting cold quickly; if one sticks on, it is too small to cook the fish. The calculator suggests two above 200 W, or whenever one big heater would be risky.
Does a lid really save heat?
Yes, usually more than any other change. On an open tank, evaporation is often the biggest single heat loss. A glass lid stops most of it, so the heater does less work and you top off less water.
Can a heater be too big?
Its thermostat switches it off at the set temperature, so a big heater is fine while it works. The danger is a stuck thermostat: the water then climbs until the heat loss matches the heater's full power. The calculator shows that temperature for your setup.
Do goldfish or coldwater fish need a heater?
Usually not in a heated home. Goldfish, white cloud minnows and other coldwater fish are fine at normal room temperature. A heater only helps to stop big swings in a very cold room.
What size heater do I need for a 40 gallon tank?
For a 40 gallon breeder kept at 78 °F, the chart above gives 100 W with a glass lid in a 68 °F room, or 200 W with an open top. In a 62 °F room, use 150 W with a lid, or two 150 W heaters on an open tank. Enter your own room temperature in the calculator for an exact figure.