What Size Heater for a 20 Gallon Aquarium?
Estimate aquarium heater wattage for a 20 gallon tank based on room temperature and target water temperature.
Last updated: May 2026
Who this guide is for
This guide targets common 20 gallon aquariums where heater sizing depends on temperature rise, not just gallons.
What to compare
A modest temperature rise may need less wattage than a cool room. Placement near flow and a reliable thermometer help confirm performance.
Buying guidance
Use the calculator with your room temperature, then compare adjustable heaters near the recommended watt range.
Use the calculator first
Start with the related calculator, then use this guide to interpret the result. Sizes, costs, capacities, and product ratings should always be checked against current labels, reviews, and your actual room or aquarium setup.