Aquarium heater size calculator
Enter your tank's volume, the water temperature your fish need and the coldest your room gets. You get a wattage range, a heater size to buy, and a note when you don't need a heater at all.
Result
100 W heater
- Wattage range
- 90–130 W
- Per litre
- 1.47–2.2 W
- Lift
- 11 °C
What will it cost to run? Enter 100 W in the electricity cost calculator.
How the calculator works
A tank loses heat in step with the gap between the water and the room (Newton's law of cooling). So the watts you need rise with the litres you heat and with the lift: how far the water must sit above the coldest room.
- The range. The aquarium maker JBL gives two worked figures: about 1 W per litre to warm water 5 °C above a 20 °C room, and 200 W for a 100-litre tank warmed from a 15 °C room to 30 °C. That is 0.2 and about 0.13 W per litre for each degree of lift, and the range runs between them: litres × lift × 0.13 to litres × lift × 0.2.
- No lid. Open water loses extra heat by evaporation. JBL puts an open lid at about 1–2 °C of cooling, so the calculator adds 2 °C to the lift.
- The heater to buy. The largest common size (25, 50, 75, 100, 150, 200, 250, 300 W) that stays inside the range, so the tank copes with a night a little colder than you entered. If none fits, the next size up.
- Big tanks. From 200 litres, or when one heater can't cover the range, the wattage is split across two or more heaters. JBL says professionals use two heaters on larger tanks, sized so they only reach the target together.
Choosing the temperature
Set the target to the middle of your fishes' range. Each species' care page lists it, and the stocking checker shows the range all your fish share. Some fish, like those on the cold-water fish list, may need no heater at all.
Before you buy
- Measure your room. The presets are starting points, not your home. Read a thermometer on the coldest nights; a ground-floor room or a cold wave can be colder.
- Use a separate thermometer and check the heater's setting against it, as setting up your first freshwater aquarium explains.
- Work out the running cost. A heater draws power only while it is heating. Enter its watts in the aquarium electricity cost calculator.
- Get ready for a North Indian winter. Heater safety, insulation, power cuts and which fish cope unheated are in how to keep an aquarium warm in winter in India.
- Plan for summer too. In much of India the same tank needs cooling in the hot months: see how to cool an aquarium in summer and the summer heat planner.
Common questions
What size heater do I need for my aquarium?
It depends on how many litres you heat and how far above the coldest room the water must be. The aquarium maker JBL puts it at about 1 W per litre to warm water 5 °C above a 20 °C room, and 200 W for a 100-litre tank warmed from a 15 °C room to 30 °C. That is roughly 0.13–0.2 W per litre for each degree, the range this calculator uses.
Do I need an aquarium heater in India?
It depends where you live. Delhi's Safdarjung fell to 4.3 °C (40 °F) on 16 January 2026, so tropical tanks in the North need a heater in winter. A sensor study of Chennai homes, reported by Down To Earth, found rooms rarely below 31 °C (88 °F) from October 2025 to April 2026, so a heater there may never switch on. Keep a thermometer in the tank through the coldest weeks before you decide.
Is a bigger heater safer?
No. JBL advises against buying a heater that is too strong just to be safe: in a small volume it may not switch off quickly enough. For large tanks, JBL says professionals use two heaters that only reach the target temperature together.
Does a lid change the heater size?
Yes. Open water loses heat by evaporation, and JBL puts an open lid at about 1–2 °C of cooling. The calculator adds 2 °C to the lift for a tank without a lid.
Should the heater stay on in summer?
A heater's thermostat switches it on only when the water falls below its setting, so in a hot room it does nothing. You can unplug it for the hot months, but plug it back in before winter.
Sources
- JBL press release: Warming, heating, cooking (27 Nov 2012; heater sizing, oversizing and two heaters, in German)
- Wikipedia: Newton's law of cooling (heat loss in proportion to the temperature gap)
- JBL: Summer heat, but not in the aquarium please (open lid, about 1–2 °C)
- Gupta, Gregg and Antony (Oxford Brookes University, RESIDE): Wintertime patterns of residential electricity use and indoor temperature in the composite climate of India (citing Singh 2016, Roorkee and Chandigarh flats)
- Down To Earth: New study sheds light on indoor nighttime heat retention in Indian homes (13 May 2026)
- PIB: BEE notifies new energy performance standards for air conditioners (6 Jan 2020)
- Business Today: Cold wave in North India, Safdarjung 4.3 °C (17 Jan 2026)
More tools
Keep your tank's tests, water changes and costs together in AquaLog.