Heater size for a 40 gallon tank
Whether it is a 40 breeder (36 × 18 × 16 in) or a 40 long (48 × 12 × 16 in), plan on about 150 W for a typical room — and consider splitting it.
Watts needed by room temperature
For 78 °F (25.6 °C) water in a lidded glass tank holding 39.7 gallons of real water:
| Coldest room | Lift | Heat needed | Buy | One heater | Two heaters |
|---|---|---|---|---|---|
| 73 °F (22.8 °C) | 5 °F (2.8 °C) | 60 W | 75 W | 75 W | 2 × 50 W |
| 68 °F (20.0 °C) | 10 °F (5.6 °C) | 119 W | 150 W | 150 W | 2 × 75 W |
| 63 °F (17.2 °C) | 15 °F (8.3 °C) | 179 W | 200 W | 200 W | 2 × 100 W |
| 58 °F (14.4 °C) | 20 °F (11.1 °C) | 238 W | 250 W | 250 W | 2 × 150 W |
"Buy" is the calculator's recommendation: the heat needed rounded up to a common heater size (10, 25, 50, 75, 100, 150, 200, 250, 300 W). For an open top add about 25%; for acrylic take off about 15% — or use the calculator above with your own temperatures.
One heater or two?
One 150 W heater covers a 10 °F lift. A two-heater setup would be 2 × 75 W — worth considering for long tanks, valuable fish or as a backup if one heater fails.
If a heater's thermostat sticks on, the worst case (every watt going into the water, none lost to the room) is a rise of about 1.5 °F per hour with one 150 W heater, or 0.8 °F per hour with one 75 W heater of a pair. Real tanks lose heat, so the rise is slower, but it shows why an oversized heater is riskier in a small tank. An external controller cuts power if the water passes your set point.
40 gallons in liters, and the real water
| Tank | Outside dimensions | Rated | Real water |
|---|---|---|---|
| 40 gallon breeder | 36 × 18 × 16 in (91.4 × 45.7 × 40.6 cm) | 44.9 gal (170 L) | 39.7 gal (150 L) |
| 40 gallon long | 48 × 12 × 16 in (121.9 × 30.5 × 40.6 cm) | 39.9 gal (151 L) | 34.9 gal (132 L) |
40 US gallons is 151.4 liters (33.3 imperial gallons). Real water is inside the glass, filled 1 inch below the rim; the heater rule above uses the larger of these.
Breeder or long: where the heat goes
- 40 breeder: 18 inches front to back and only 16 tall, with about 40 gallons of real water. A single 150 W heater along the back glass, near the filter return, spreads heat well in this compact shape.
- 40 long: a 48-inch run with less water. One heater at one end can leave the far end cooler if the filter flow is weak. Two heaters of half the wattage, one at each end, even it out — the "two heaters" column in the table.
At this size a stuck-on heater is still slow to cause harm (see below), so a second heater is about even heat and backup more than safety. If you keep valuable fish, an external temperature controller is a cheap insurance policy.
How the numbers are worked out
watts = real gallons × lift (°F) × 0.3, about 0.14 W per liter per °C, for a lidded glass tank. For a 10 °F lift that is 3 W per gallon — the low end of the familiar 3–5 W per gallon rule of thumb, which is commonly quoted for heating up to about 10 °F above the room with a lid. That rule would put a 40 gallon tank at 120–200 W. Use the coldest overnight temperature, not the daytime average, and go one size up for drafty rooms. Read the heater size guide for placement and safety.
More about this size: 40 gallon breeder · 40 gallon long. Other sizes: 3 gal, 5 gal, 10 gal, 20 gal, 29 gal, 55 gal, 75 gal · full chart.
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FAQ
How many watts per gallon does a 40 gallon tank need?
About 3.0 W per gallon of real water to hold 78 °F in a 68 °F room (0.3 W per gallon per °F of lift), which rounds up to 150 W. In a 58 °F room it rises to 250 W.
What size heater for a 40 breeder?
About 150 W for 78 °F water in a 68 °F room; 200 W if the room drops to about 63 °F.
Should a 40 long have two heaters?
It helps. Two 75 W heaters at opposite ends heat the 48-inch length more evenly than one 150 W heater at one end, and leave you with heat if one fails.
How many liters is a 40 gallon tank?
40 US gallons is 151.4 liters. Filled about an inch below the rim, the larger of the standard shapes holds about 39.7 gallons (150 L) of real water, which is what the heater has to warm.