Heater size for a 55 gallon tank
A 55-gallon holds about 51 gallons of real water in a long, narrow 48 × 13 × 21 inch box. Split the heat across two heaters, one at each end.
Watts needed by room temperature
For 78 °F (25.6 °C) water in a lidded glass tank holding 50.8 gallons of real water:
| Coldest room | Lift | Heat needed | Buy | One heater | Two heaters |
|---|---|---|---|---|---|
| 73 °F (22.8 °C) | 5 °F (2.8 °C) | 76 W | 2 × 50 W | 100 W | 2 × 50 W |
| 68 °F (20.0 °C) | 10 °F (5.6 °C) | 152 W | 2 × 100 W | 200 W | 2 × 100 W |
| 63 °F (17.2 °C) | 15 °F (8.3 °C) | 228 W | 2 × 150 W | 250 W | 2 × 150 W |
| 58 °F (14.4 °C) | 20 °F (11.1 °C) | 305 W | 2 × 200 W | over 300 W | 2 × 200 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), split across two heaters because the tank is over 50 gallons. 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?
At this size the recommendation is two heaters: 2 × 100 W for a 10 °F lift, each at least half of the 152 W needed. Placed at opposite ends, they heat evenly; if one fails off, the other still supplies half the heat, and if one sticks on it warms the tank half as fast as a single heater would.
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 1.6 °F per hour with one 200 W heater, or 0.8 °F per hour with one 100 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.
55 gallons in liters, and the real water
| Tank | Outside dimensions | Rated | Real water |
|---|---|---|---|
| 55 gallon | 48 × 13 × 21 in (121.9 × 33.0 × 53.3 cm) | 56.7 gal (215 L) | 50.8 gal (192 L) |
55 US gallons is 208.2 liters (45.8 imperial gallons). Real water is inside the glass, filled 1 inch below the rim; the heater rule above uses it.
Dual-heater placement in a 55
The 55 is the size where two heaters become the default. Put one at each end of the 48-inch tank, each in the path of a filter intake or return, mounted low and diagonally so the full element stays under water when the level drops between top-offs. Each heater should be at least half the wattage the tank needs — the table rounds each one up.
- Failure stuck on: one 100 W heater stuck on warms the tank about half as fast as a single 200 W heater would, which buys hours to notice.
- Failure off: if one heater dies on a cold night, the other still covers most of the heat loss.
- Controller: plug both heaters into one external controller with its own probe for a second, independent cut-off.
At 21 inches tall, a 55 also benefits from good circulation so the bottom does not run cooler than the surface.
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 55 gallon tank at 165–275 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: 55 gallon. Other sizes: 3 gal, 5 gal, 10 gal, 20 gal, 29 gal, 40 gal, 75 gal · full chart.
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FAQ
How many watts per gallon does a 55 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 2 × 100 W. In a 58 °F room it rises to 2 × 200 W.
What size heater for a 55-gallon tank?
Two 100 W heaters (200 W total) to hold 78 °F in a 68 °F room. In a room about 5 °F below the tank, two 50 W heaters are enough.
Is one 200 W heater OK for a 55-gallon?
It will hold temperature, but two 100 W heaters at opposite ends heat more evenly and are safer if one sticks on or fails.
How many liters is a 55 gallon tank?
55 US gallons is 208.2 liters. Filled about an inch below the rim, the tank holds about 50.8 gallons (192 L) of real water, which is what the heater has to warm.