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Aquarium Heater Size Calculator (and What It Costs to Run)
Pick a heater by the water your tank actually holds and by how much warmer than the room you want it. This calculator does both steps. Then it shows what that heater would cost to run each month at your electricity rate.
Aquarium heater size calculator
Water in the tank–
Temperature rise needed–
Estimated heating need–
Heater size to look for
–
What that heater costs to run (30 days)
| Heater switched on | kWh / month | Cost / month |
|---|
A thermostat switches the heater off once the water is warm, so it rarely runs all day. A smart plug with an energy meter shows your real figure.
How the calculator works
Water volume. It multiplies length by width by the depth of the water itself: the tank height minus the gap at the top and the substrate at the bottom. One US gallon is 231 cubic inches, or about 3.785 liters. This is why a “20-gallon” tank often holds closer to 16 or 17 gallons once it is set up.
Heating need. A rule of thumb you will see in many fishkeeping guides is about 3 to 5 watts per gallon when the water sits around 10 °F above the room. The calculator scales that range by your actual temperature rise. Holding a tank 20 °F above the room takes roughly twice the power of holding it 10 °F above. Tanks under 10 gallons lose heat faster for their size, so for them it uses 5 to 6 watts per gallon, in line with hygger’s chart for a 5-gallon tank. It then rounds the middle of the range up to a common heater size, or down if rounding up would go past the top of the range.
Running cost. Watts × hours on × 30 days ÷ 1,000 gives kWh per month. Multiply by your price per kWh. A heater only draws power while it is switched on, so the table shows three cases. Our guide to fish tank electricity costs explains what pushes a heater toward the high end and how to measure your own tank.
Check the result against the heater maker’s chart
The calculator gives a starting point. The chart for the exact heater you buy matters more, because it reflects how that heater is built. Makers do not all agree. hygger, for example, publishes a chart in its aquarium heating guide that lists 50 W for a 10-gallon tank kept 9 °F above the room and 75 W for an 18 °F rise. The same article also quotes a simpler rule of 5 watts per gallon. If the chart for your heater and this calculator disagree, go with the chart, and keep a separate thermometer in the tank.
Things that make a tank need more heat
- A cold room, a basement, or heating that switches off at night.
- An open top. Evaporation cools the water, especially in dry winter air.
- Lots of surface agitation or air bubbles, which speed up evaporation.
- A small tank. Small volumes lose heat faster for their size.
- Glass against an outside wall, a window, or a draft.
A lid, a spot away from windows and doors, and a sheet of foam behind the tank all cut the work the heater has to do.
Related guides
- Betta fish heater guide: sizes for 3 to 10-gallon betta tanks and what to look for in a small heater.
- How much electricity does a fish tank use?: the full monthly bill, from the light and filter to the heater.
- Betta fish tank setup guide: the whole setup, step by step.
Use this calculator on your site
Link to this page from a forum post, care sheet or blog whenever it helps a reader size a heater. If you quote a result, please link back to timesripple.com/aquarium-heater-calculator so readers can run their own numbers.
Sources: watts-per-gallon range as commonly quoted in fishkeeping guides; heater chart from hygger’s Aquarium Heating Basics Part Two; 2026 electricity price forecast from the US Energy Information Administration’s Short-Term Energy Outlook; evaporation and aeration effects as described in Aquarium Science 14.13. Checked October 2026. This calculator gives an estimate, not a guarantee.
