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Enter your values and press Calculate.
Enter your tank size in gallons (use actual water volume, not the number on the box, since substrate and decorations displace 10 to 15 percent), your target water temperature, and the coldest temperature the room realistically reaches, such as an overnight winter low, not the daytime average. The calculator returns the temperature lift, the watts-per-gallon rate for that lift, the calculated wattage, the next standard heater size to buy, and whether one heater or two is recommended.
A heater does not heat a tank so much as replace the heat the tank leaks to the room, and that leak grows with the temperature difference. That is why the same 20 gallon tank needs about 60 watts at a 5 degree lift, 100 watts at a 10 degree lift, 150 watts at a 15 degree lift, and 200 watts at a 20 degree lift. The flat 5 watts per gallon rule is only correct in the middle of that range, which is why this calculator uses lift-based bands instead. Small tanks under 10 gallons get a 25 percent uplift, because a small body of water has far more glass surface per gallon to lose heat through.
Measure actual water volume: a tank sold as 20 gallons often holds about 18 gallons once substrate, rock and wood are in. Enter the target temperature for your fish (most community tanks run 76 to 80 F) and the coldest room temperature the tank will face, such as a winter overnight low or an unheated basement. The result is rounded up to the next real heater size, because heaters are sold in steps like 25, 50, 75, 100, 150, 200, 250 and 300 watts, and sizing up slightly is safer than sizing down.
At a typical 10 degree lift in a heated room, the usual recommendations are: 5 gallons, 50 watts; 10 gallons, 50 watts; 20 gallons, 100 watts; 29 gallons, 150 watts; 40 gallons, 200 watts; 55 gallons, 300 watts; 75 gallons, 400 watts as two 200 watt units; 100 gallons, 500 watts as two 250 watt units. In a cold room with a 20 degree lift, roughly double those figures: a 20 gallon tank wants about 200 watts and a 55 gallon tank about 600 watts. Always check the heater maker's own tank rating for the unit you buy, since thermostat accuracy varies more than the maths does.
This one vs two guide starts simple: below about 55 gallons, one correctly sized heater is simpler and easier to hide. From about 75 gallons up, split the requirement into two heaters of half the wattage, one at each end of the tank. Two half-size units heat a long tank more evenly, and they protect against both failure modes: a heater stuck on cannot supply enough wattage alone to cook the tank, and a heater failed off leaves the other holding a survivable temperature until you notice. For tanks over 40 gallons, two smaller heaters are already worth considering for the same reason.
Mount the heater where water flows past it, near the filter outflow or intake, so warmed water is carried around the tank instead of pooling. Horizontal low placement works well because warm water rises. Always use a separate thermometer at the far end of the tank and never trust the heater dial alone. An external temperature controller is the best upgrade for valuable fish: it cuts power at a hard upper limit if a thermostat sticks on. Switch the heater off during water changes and let it cool before refilling, since thermal shock cracks glass heaters, and fit a drip loop so water runs off the cable instead of into the socket.
The classic errors are sizing from the daytime room temperature instead of the coldest night, using the advertised tank volume instead of actual water volume, and buying one oversized heater for a large tank. A heater sized for a 72 F living room will fall behind on a 58 F winter night, and a heavily decorated tank can hold 10 to 15 percent less water than its label claims. Oversizing has its own risk: a wildly oversized heater does not use more electricity, it just runs less often, but if its thermostat sticks on, it pushes the temperature into lethal territory far faster than a right-sized unit.
It depends on the temperature lift: about 3 watts per gallon for a 5 F lift, 5 watts per gallon for a typical 10 F lift, 7.5 watts per gallon for 15 F, and 10 watts per gallon for a cold room at 20 F below target. The old flat 5 watts per gallon rule is only correct in the middle of that range.
At a typical 10 F lift, about 100 watts, which lands exactly on a standard 100 watt heater. In a cold room that drops to 60 F overnight, the same tank wants closer to 200 watts. One heater is plenty at this volume.
About 50 watts at a 10 F lift. Small tanks swing temperature faster than big ones, so if the room is cool or draughty, step up to 75 watts and let the thermostat cycle rather than running a small heater flat out.
About 300 watts at a 10 F lift, or two 150 watt heaters at opposite ends. In a cold room, the same tank can need about 600 watts. Two smaller heaters heat a tank this size more evenly and give backup if one fails.
Under about 55 gallons, one heater is simpler. From about 75 gallons up, two half-size heaters are better: they heat evenly, a stuck-on unit cannot cook the tank alone, and a failed unit leaves the other holding temperature until you notice.
Yes, it is the single biggest factor after volume. A heater only replaces heat lost to the room, and that loss scales with the temperature difference. The same 40 gallon tank needs about 200 watts in a heated 72 F room and about 400 watts in a 58 F basement.
In good flow, next to the filter outflow or intake, fully submerged, horizontal and low if possible. Keep a separate thermometer at the opposite end of the tank, so you measure the coldest spot rather than the warm pocket near the heater.
Yes, if its thermostat sticks on. That is why oversizing is risky and why an external controller is recommended for valuable fish: it cuts power at a hard upper limit even if the heater thermostat fails.