Heater size for a 20 gallon tank
A 20 long and a 20 high hold similar water but different shapes. Both need about 50–75 W for a typical room — here is which and how to fit it.
Watts needed by room temperature
For 78 °F (25.6 °C) water in a lidded glass tank holding 17.6 gallons of real water:
| Coldest room | Lift | Heat needed | Buy | One heater | Two heaters |
|---|---|---|---|---|---|
| 73 °F (22.8 °C) | 5 °F (2.8 °C) | 26 W | 50 W | 50 W | 2 × 25 W |
| 68 °F (20.0 °C) | 10 °F (5.6 °C) | 53 W | 75 W | 75 W | 2 × 50 W |
| 63 °F (17.2 °C) | 15 °F (8.3 °C) | 79 W | 100 W | 100 W | 2 × 50 W |
| 58 °F (14.4 °C) | 20 °F (11.1 °C) | 106 W | 150 W | 150 W | 2 × 75 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 75 W heater covers a 10 °F lift. A two-heater setup would be 2 × 50 W — optional at this size; a spare heater is the simpler backup.
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 1.7 °F per hour with one 75 W heater. 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.
20 gallons in liters, and the real water
| Tank | Outside dimensions | Rated | Real water |
|---|---|---|---|
| 20 gallon long | 30 × 12 × 12 in (76.2 × 30.5 × 30.5 cm) | 18.7 gal (71 L) | 16.1 gal (61 L) |
| 20 gallon high | 24 × 12 × 16 in (61.0 × 30.5 × 40.6 cm) | 19.9 gal (76 L) | 17.6 gal (67 L) |
20 US gallons is 75.7 liters (16.7 imperial gallons). Real water is inside the glass, filled 1 inch below the rim; the heater rule above uses the larger of these.
20 long vs 20 high
The two standard 20-gallon tanks hold roughly the same water, so the wattage is close — the table uses the larger real-water volume of the two so it covers either. The shapes matter for fitting the heater:
- 20 long (30 × 12 × 12 in): only 12 inches tall. Check the heater's length; it must be fully under water at your lowest water level. Mounting it diagonally or horizontally along the back glass works well.
- 20 high (24 × 12 × 16 in): taller water can be a degree or so cooler at the bottom with weak circulation. Put the heater near the filter flow so warm water is carried down.
The 20 high shares the 15-gallon's 24 × 12 inch footprint and the 20 long shares the 29-gallon's 30 × 12 inches, so a 20 long is often upgraded to a 29 on the same stand. Plan on a bigger heater when you do: for a 10 °F lift the 29 needs about 100 W where the 20 long needs 50 W.
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 20 gallon tank at 60–100 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: 20 gallon long · 20 gallon high. Other sizes: 3 gal, 5 gal, 10 gal, 29 gal, 40 gal, 55 gal, 75 gal · full chart.
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FAQ
How many watts per gallon does a 20 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 75 W. In a 58 °F room it rises to 150 W.
What size heater for a 20 gallon long?
About 50 W in a 68 °F room for 78 °F water, 75 W if the room drops to about 63 °F.
Does a 20 high need a bigger heater than a 20 long?
Slightly, because it holds a little more water inside the glass. The difference is about one heater size at most in a cold room.
How many liters is a 20 gallon tank?
20 US gallons is 75.7 liters. Filled about an inch below the rim, the larger of the standard shapes holds about 17.6 gallons (67 L) of real water, which is what the heater has to warm.