Aquarium Heater Size Guide: How Many Watts Per Gallon?
Get the right heater wattage for your tank. Too small won't heat properly; too large can overheat.
Quick Rule
3-5 watts per gallon is the standard recommendation. Use the higher end (5W/gal) if your room is cold or you need a large temperature increase.
Why Heater Size Matters
An undersized heater will struggle to maintain temperature, especially in cold rooms. An oversized heater can overheat your tank if the thermostat fails.
Undersized Heater
- ✗ Runs constantly (higher electric bill)
- ✗ Can't maintain target temp in cold weather
- ✗ Wears out faster
- ✗ Temperature fluctuations stress fish
Oversized Heater
- ⚠ Risk of overheating if thermostat fails
- ⚠ Less precise temperature control
- ⚠ Higher initial cost
- ✓ Will definitely reach target temp
Heater Size Chart
Use this table to find the right heater wattage based on your tank size and room temperature:
| Tank Size | Room is 68-72°F | Room is 60-68°F | Room is Under 60°F |
|---|---|---|---|
| 5 gallon | 25W | 50W | 75W |
| 10 gallon | 50W | 75W | 100W |
| 20 gallon | 75W | 100W | 150W |
| 29 gallon | 100W | 150W | 200W |
| 40 gallon | 150W | 200W | 250W |
| 55 gallon | 200W | 250W | 300W |
| 75 gallon | 250W | 300W | 2x 200W |
| 100+ gallon | 300W or 2x heaters | 2x 200W | 2x 300W |
Pro tip: For tanks 75+ gallons, use two heaters instead of one large heater. This provides redundancy and more even heating.
The Watts Per Gallon Rule
The standard formula is:
Heater Wattage = Tank Gallons x Watts Per Gallon
3 Watts/Gallon (Warm Room)
Room temperature is 70-75°F. Only need to raise water temp 5-10 degrees.
4 Watts/Gallon (Average)
Room temperature is 65-70°F. Typical home heating situation.
5 Watts/Gallon (Cold Room)
Room is under 65°F or you need to raise temp by 15+ degrees. Basements, garages.
Types of Aquarium Heaters
Submersible Heaters
Most common type. Fully underwater, usually horizontal or angled. Good heat distribution.
Best for Most TanksHang-On Heaters
Partially submerged, hangs on tank rim. Older design, less efficient.
Budget OptionInline Heaters
Installs in canister filter tubing. Hidden from view. Requires external filter.
Advanced SetupSubstrate/Under-Tank Heaters
Cables or mats under gravel. Good for planted tanks. Gentle, even heating.
Planted TanksBest Aquarium Heaters by Tank Size
Best for 5-10 Gallon: Fluval E50
50W electronic heater with LCD display. Shows actual water temperature. VueTech technology detects heater exposure and auto-shutoff.
Check Price on Amazon →Best for 20-40 Gallon: Fluval E200
200W version of the E-series. Precise to 0.5 degrees. Color-coded LCD (green=correct, red=off). Slim profile.
Check Price on Amazon →Best Budget: Aqueon Pro
Shatterproof design, lifetime warranty. Available 50W-300W. No digital display but reliable thermostat.
Check Price on Amazon →Best Inline: Hydor ETH
External thermal heater for canister filters. Completely hidden from tank. 200W or 300W options.
Check Price on Amazon →As an Amazon Associate, we earn from qualifying purchases.
Heater Placement Tips
Near water flow
Place heater near filter outlet for even heat distribution.
Horizontal or angled
Horizontal placement gives more even heating than vertical.
Use a thermometer
Never trust the heater dial. Always verify with a separate thermometer.
Don't bury in substrate
Heaters need water flow around them. Don't cover with gravel.
Don't run dry
Always unplug heater during water changes. Running dry = cracked heater.
Calculate Your Heater Size
Use our free calculator to find the exact wattage you need based on your tank size and room temperature.
Open Heater CalculatorFrequently Asked Questions
Can I use two smaller heaters instead of one large one?
Yes! This is actually recommended for larger tanks (75+ gallons). It provides backup heating and more even temperature distribution.
How do I know if my heater is working?
Most heaters have an indicator light that glows when heating. Always use a separate thermometer to verify actual water temperature.
How long does an aquarium heater last?
Quality heaters typically last 3-5 years. Replace immediately if you notice temperature fluctuations or physical damage.
Should I turn off my heater in summer?
Only if room temperature consistently stays above your target tank temp. Most heaters won't heat if water is already warm enough.