Aquarium heater size calculator

How it works

The heater has to replace the heat the tank loses to the room. That loss grows with the size of the tank and with the gap between the water and the air, so the calculator multiplies the two:

watts = gallons × temperature rise (°F) × k × small-tank factor

• Temperature rise is your target water temperature minus the lowest room temperature you expect.
• k is 0.25 W per gallon per °F for a tank with a lid, and 0.375 (50% more) for an open top.
• Small-tank factor is (20 ÷ gallons)^⅓ for tanks under 20 gallons, and 1 for bigger tanks. A 10 gallon tank gets ×1.26 and a 5 gallon tank ×1.59.

In metric: watts = litres × rise (°C) × 0.119 with a lid, or × 0.178 for an open top, times the same small-tank factor. We then round up to the next common heater size (25, 50, 75, 100, 150, 200, 250, 300 W).

Worked example: 10 gallons, lid

A 10 gallon tank with a lid, in a room that drops to 65°F, with water kept at 78°F:

1. Rise: 78 − 65 = 13°F.
2. Small-tank factor: (20 ÷ 10)^⅓ = 1.26.
3. Watts: 10 × 13 × 0.25 × 1.26 ≈ 41 W.
4. Next common size up: 50 W.

Worked example: 200 litres, open top

A 200 litre (53 gallon) tank with no lid, in a room that drops to 18°C, with water kept at 25°C:

1. Rise: 25 − 18 = 7°C (12.6°F).
2. Watts: 200 × 7 × 0.178 ≈ 250 W.
3. It's over 50 gallons (189 L), so split it: 250 ÷ 2 = 125 W each, rounded up to two 150 W heaters. A lid would cut the need to about 166 W.

Aquarium heater size chart

Heater size for a tank with a lid, by how much warmer than the coldest room temperature you keep the water. Tanks over 50 gallons are shown split between heaters. For an open top, use the calculator above.

Example: a 68°F room with 78°F water is a 10°F rise. For a 10–20°F rise, the formula gives 2.5–5 watts per gallon, close to the common 3–5 watts per gallon rule of thumb.

Why we use these numbers

The usual rule of thumb. Fishkeepers often size heaters at about 3–5 watts per US gallon. It is a rule of thumb, not a measured value, and it doesn't say how cold the room is. Our 0.25 W per gallon per °F gives 2.5–3.75 W per gallon for a 10–15°F rise, the gap between a 65–68°F room and 78–80°F water, and 5 W per gallon for a 20°F rise. So for an ordinary room, the calculator lands in the same range.

Manufacturer charts. Heater makers publish size charts but don't say what room temperature they assume. Read at the largest tank each heater is rated for, the two charts listed in the sources work out to 2.5–3.3 W per gallon (Aqueon) and 1 W per litre, about 3.8 W per gallon (Hydor). Both fall within the 2.5–5 W per gallon our formula gives for a 10–20°F rise. Charts differ between makers and models, so if you already own a heater, check its own rating too.

Small tanks. A small tank has more glass and water surface for each gallon, so it cools faster per gallon. Surface area grows with volume to the power of ⅔, which is where the (20 ÷ gallons)^⅓ factor comes from. This factor is our estimate from that geometry, not a figure from a maker's chart. From 20 to 90 gallons the charts keep watts per gallon about level, so we do too.

Open tops. Water that evaporates takes heat with it, about 2.45 MJ for every kilogram. A tank that loses 1 gallon a week to evaporation is losing about 15 W of heat around the clock. The standard evaporation formula for pools puts an open 20 gallon tank (24 × 12 inch surface) at 78°F, in still air at 68°F and 50% humidity, at roughly 40 W. Through the glass, the same tank loses about 50 W. Adding 50% for an open top sits between those two estimates. Real evaporation depends on humidity and drafts, so treat it as a planning figure.

Choosing and using your heater

• Size for the coldest night, not the average day. A heater that's just big enough in the afternoon will fall behind when the heating goes off.
• Put it where water moves, such as near the filter outlet, so warm water spreads through the tank instead of pooling at one end.
• Use two heaters in big tanks. Over 50 gallons, split the wattage between two heaters at opposite ends. One failing won't chill the tank, and one sticking on heats it more slowly.
• Don't oversize by much. One size up is fine. A heater several sizes too big can cook a small tank fast if its thermostat sticks.
• Trust a thermometer, not the dial. Check the water with a separate thermometer for a few days after setting the heater, and adjust.
• Keep the heat in. A lid is the cheapest way to cut heating. In a cold room, keep the tank off outside walls and away from windows and doors.
• Unplug before water changes. Turn the heater off before you lower the water level or lift it out, and follow the maker's instructions on letting it cool.

Not sure what temperature to aim for? Each fish profile lists its range, and the stocking calculator shows the range all your fish share.

Related calculators and guides

• Aquarium volume calculator: find your tank size in gallons or litres from its dimensions.
• Stocking calculator: see the temperature range all your fish share.
• Filter flow calculator: check your filter moves enough water to spread the heat.
• Aquarium weight calculator: check your stand and floor.
• Unit converter: switch between °F and °C, gallons and litres.
• Tank size guides: what fits in each common tank size.
• Fishkeeping guides: setup, cycling and maintenance.

Sources

1. Aqueon, Heater wattage guidelines (chart, PDF): 50 W for 5–20 gal, 100 W for 15–40 gal, 150 W for 30–55 gal, 200 W for 50–75 gal, 300 W for 65–90 gal.
2. Hydor, Theo heater: 25 W for 10–25 L up to 300 W for 200–300 L.
3. Aqueon, Submersible glass heaters: adjustable from 68 to 88°F.
4. Engineering ToolBox, Evaporation from a water surface: evaporation formula and heat of evaporation of water.
5. Wikipedia, Aquarium heater: using two or more heaters in larger tanks as a backup.

Brands are named only as sources for their published sizing charts. We don't recommend any heater brand.