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Aquarium Heater Size Calculator (Wattage)

Heaters fail in two modes: stuck on (cooks the tank) or stuck off (cold tank in 12 hours). Both are common. The fix is two smaller heaters running through an external controller, not one big heater trusting its internal thermostat. This calculator uses the 3–5 W/gal rule of thumb (BRS), adjusts for ambient room temperature, and recommends a redundant pair.

Heater wattage

Choose by your coldest winter night, not summer.

Recommended total wattage
300W
Redundant pair (each)
150W

Two heaters, one fails safe. Both wired through a separate controller.

W/gal used
4W/gal
ΔT 1-hour check
1830W

Watts needed to raise the tank by ambient→target in one hour.

  • ⚠ Your room is cold relative to target. Consider sizing up beyond the rule of thumb or improving insulation.

The 3–5 W/gal rule is the BRS Saltwater Guide consensus. Always wire heaters through an external controller (Inkbird, Apex, ReefKeeper) — never rely on the internal thermostat alone.

How this is calculated

Watts per gal:
  warm room (≥70 °F) : 3 W/gal
  cool room (60–70 °F): 4 W/gal
  cold room (<60 °F) : 5 W/gal
  Add +1 W/gal for tanks under 30 gal (worse surface:volume ratio).

recommended_total = tank_gal × W_per_gal
redundant_pair    = ceil(recommended_total / 2 / 25) × 25   // each heater

Sanity check (raise tank by ΔT in 1 hour):
  W = 1.16 × tank_L × ΔT_°C

The redundant pair rule: two heaters, each sized at roughly half the recommended total wattage, wired through an external controller (Inkbird ITC-308, Apex EB832, ReefKeeper, etc.). Both heaters run together when the tank is below set point. If one fails on, the other can't push the tank dangerously high because the controller cuts power. If one fails off, the other carries the tank until you notice.

Never put the heater in the display section directly. Sump or in-line is safer for fish (no burns) and easier to service. Titanium heaters are more failure-resistant than glass; cheap glass heaters are the leading cause of "I went to work and came home to a boiled tank" threads.

FAQ

Can I use one big heater instead of two smaller ones?
You can, but it's a single point of failure. The cost difference between one 300 W and two 150 W is maybe $30. The cost of replacing a tank of livestock after a single-heater failure is much more.
Do I really need an external controller?
Yes. Internal heater thermostats are not reliable. Single-fault cooked-tank stories almost always start with 'I never had problems with this heater before.' An external controller ($30 Inkbird) cuts power at set temp regardless of what the heater's internal thermostat is doing.
What about a chiller?
If your room is reliably warmer than your tank target — yes. Common in basements with metal halides, attics in summer, or in southern climates without strong AC. Sizing chillers is harder than heaters; this calculator doesn't cover them.
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Sources & references

  1. 01
    Bulk Reef Supply — 5 Minute Saltwater Aquarium Guide Ep 12: Heaters
    https://www.bulkreefsupply.com/content/post/5-minute-saltwater-aquarium-guide-ep12-heaters
  2. 02

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