The watt formula
Watts = gallons x 0.5 x rise in F. Rise is target water temperature minus the coldest room temperature. The page then picks the next size in this list: 25, 50, 75, 100, 150, 200, 250, 300, 400, 500. If the raw watts are 150 or more, it uses two heaters that each cover half, so one failure does not leave the tank unheated.
A 10 F rise is 5.0 watts per gallon. A 5 F rise is 2.5. A 15 F rise is 7.5. Examples at a 10 F rise: 10 gallons is 50 watts, 20 gallons is 100 watts, 29 gallons is 145 watts and rounds to 150, 55 gallons is 275 watts and becomes two 150s, 75 gallons is 375 watts and becomes two 200s.
This rule is a hobby sizing guide. It runs higher than Aqueon's own chart on many large tanks. Heat loss also depends on a lid, a draft, and whether the tank is against an outside wall. Check the water with a separate thermometer after the heater has run.
What size heater for a 20 gallon aquarium
For a 10 F rise, both this rule and Aqueon's chart say 100 watts. Aqueon lists 50 watts if the rise is only 5 F. Their separate wattage band puts a 50 watt heater on tanks of about 5 to 20 gallons and a 100 watt heater on about 15 to 40, which overlaps on purpose.
Heater for a 55 gallon tank
Aqueon's temperature chart, for a room of 68 to 72 F, lists 150 watts at a 5 F or 10 F rise and 200 watts at 15 F. The 0.5 watt rule at 10 F is 275 watts (two 150 watt heaters). Use Aqueon's 150 watts when the tank is covered and the room really stays in that band. Use the higher result when the room falls further, the tank is open, or it sits on an outside wall.
Heater for a 75 gallon tank
Aqueon lists 200 watts at 5 F and at 10 F, and 300 watts at 15 F. The rule at 10 F is 375 watts, shown as two 200s. Same advice: the chart assumes a normal room and a cover. Two smaller heaters, placed at opposite ends, heat more evenly than one large unit.
Aqueon heater chart
Published guide for raising the water above a 68 to 72 F room. This table is their chart, not the output of the 0.5 watt formula. Source: Aqueon freshwater setup guide.
| Aquarium | 5 F rise | 10 F rise | 15 F rise |
|---|---|---|---|
| 5 gallon | 50 W | 50 W | 50 W |
| 10 gallon | 50 W | 50 W | 50 W |
| 20 gallon | 50 W | 100 W | 100 W |
| 30 gallon | 100 W | 100 W | 150 W |
| 55 gallon | 150 W | 150 W | 200 W |
| 75 gallon | 200 W | 200 W | 300 W |
| 100 gallon | 300 W | 300 W | 300 W |
| 125 gallon | 2 x 200 W | 2 x 200 W | 2 x 200 W |
| 150 to 210 gallon | 2 x 300 W | 2 x 300 W | 2 x 300 W |
Aqueon also publishes overlapping wattage bands: 50 W for about 5 to 20 gallons, 100 W for about 15 to 40, 150 W for about 30 to 55, 200 W for about 50 to 75, and 300 W for about 65 to 90. A 29 gallon is not its own row. It sits next to their 30 gallon row (100 watts at a 10 F rise on the chart). The rule of thumb for 29 gallons at a 10 F rise is 150 watts.
Leave a drip loop on the cord, do not bury a heater in gravel, and unplug it before a water change exposes the glass tube. A stuck heater can overheat a small tank. The thermometer is the check, not the dial alone.
Frequently asked questions
What size heater for a 20 gallon aquarium?
At a 10 F rise this page's rule is 0.5 x 10 = 5 watts per gallon, so 100 watts. Aqueon's chart for a covered tank in a 68 to 72 F room also lists 100 watts for a 20 gallon at a 10 F rise, and 50 watts if you only need a 5 F rise.
What size heater for a 55 gallon tank?
The 0.5 watt rule at a 10 F rise is 275 watts, which this page rounds to two 150 watt heaters. Aqueon's chart lists a single 150 watt heater for a 55 gallon at a 10 F rise in a normal room with a cover. Use 150 watts in that milder case, and the higher figure if the room is cold or the tank is open.
What size heater for a 75 gallon tank?
At a 10 F rise the rule is 375 watts, shown here as two 200 watt heaters. Aqueon's chart lists 200 watts for a 10 F rise and 300 watts for a 15 F rise. A lid and a room that stays near 70 F are the conditions for the lower chart.
What watt heater for a 10 gallon tank?
Both the rule and Aqueon's chart land on 50 watts for a 10 gallon at a 5, 10, or 15 F rise on the chart, and the rule is also 50 watts at a 10 F rise (10 x 5). A 15 F rise on the rule is 75 watts.