Heater size for a 5 gallon tank
The 5-gallon is the classic betta tank. Its heater needs are small, but bettas are tropical fish that sit near the heater — size it right and guard it.
Watts needed by room temperature
For 78 °F (25.6 °C) water in a lidded glass tank holding 4.7 gallons of real water:
| Coldest room | Lift | Heat needed | Buy | One heater | Two heaters |
|---|---|---|---|---|---|
| 73 °F (22.8 °C) | 5 °F (2.8 °C) | 7 W | 10 W | 10 W | — |
| 68 °F (20.0 °C) | 10 °F (5.6 °C) | 14 W | 25 W | 25 W | — |
| 63 °F (17.2 °C) | 15 °F (8.3 °C) | 21 W | 25 W | 25 W | — |
| 58 °F (14.4 °C) | 20 °F (11.1 °C) | 28 W | 50 W | 50 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 25 W heater covers a 10 °F lift. Two heaters are not needed at this size — a spare heater in a drawer is the better 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 2.2 °F per hour with one 25 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.
5 gallons in liters, and the real water
| Tank | Outside dimensions | Rated | Real water |
|---|---|---|---|
| 5 gallon | 16 × 8 × 10 in (40.6 × 20.3 × 25.4 cm) | 5.5 gal (21 L) | 4.7 gal (18 L) |
5 US gallons is 18.9 liters (4.2 imperial gallons). Real water is inside the glass, filled 1 inch below the rim; the heater rule above uses it.
Heating a betta tank
Bettas do best at about 76–82 °F, warmer than most living rooms, so a 5-gallon betta tank almost always needs a heater. A 25 W adjustable heater is the usual buy: it covers a room about 10 °F below the tank with some margin, without being so large that a stuck thermostat cooks a small volume.
- Bettas like to rest near warm spots and on leaves close to the surface. Mount the heater low and diagonally near the filter outflow so the heat is spread rather than pooling in one corner.
- Long-finned bettas can tear fins on sharp equipment — choose a heater with a smooth plastic guard.
- Open-top 5-gallon cubes lose heat through evaporation; the lid switch in the calculator adds 25%.
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 5 gallon tank at 15–25 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: 5 gallon. Other sizes: 3 gal, 10 gal, 20 gal, 29 gal, 40 gal, 55 gal, 75 gal · full chart.
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FAQ
How many watts per gallon does a 5 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 25 W. In a 58 °F room it rises to 50 W.
Is a 50 W heater too big for a 5-gallon tank?
It will hold temperature, but if its thermostat sticks on it heats the small volume about twice as fast as a 25 W heater would. A 25 W heater is enough for most rooms.
What temperature should a 5-gallon betta tank be?
About 78 °F (25–26 °C) is a good target for most bettas; the healthy range is roughly 76–82 °F.
How many liters is a 5 gallon tank?
5 US gallons is 18.9 liters. Filled about an inch below the rim, the tank holds about 4.7 gallons (18 L) of real water, which is what the heater has to warm.