Aquarium Heater Calculator
Enter your tank volume, target water temperature and the coldest the room gets. The calculator sizes the heater in watts - about one watt per litre in a cold room - and flags when two heaters beat one.
Heater packaging quotes vague litre ranges. This one runs the actual arithmetic on your temperature gap, explains why oversized costs nothing extra to run, and prices the redundancy that turns a failure into a drift.
How to use
- Enter litres and the species' target temperature - tropicals run 24-27°C.
- Enter the coldest the room genuinely gets at night in winter.
- Buy the wattage shown, a second for anything over 200 litres, and a glass thermometer that outranks the dial.
Frequently asked questions
What size heater for a 100 litre aquarium?
Roughly 75-100 W depending on the room: litres times the temperature gap times 0.09. A cold room with an 8°C gap wants the higher end; a warm flat can run less - but never run a heater flat out at its limit.
Is a bigger aquarium heater wasteful?
No - the thermostat governs duty, not wattage, so an oversized heater runs shorter bursts at the same cost and survives cold snaps with margin. Undersized is the expensive mistake: it runs flat out, arrives late, and dies young.
Should a big tank have two heaters?
Yes, past about 200 litres: one at each end spreads the heat instead of pooling it, and if one fails the other turns an emergency into a slow drift you will notice on the thermometer first.
How accurate is the dial on an aquarium heater?
Treat it as a suggestion with a few degrees of drift - the independent thermometer is the fact. Cheap stick-on strips drift more; a small glass thermometer settles every argument for pocket change.