CALCULATOR · HEATING
Aquarium heater wattage calculator
The short answer
Heat a reef at 3–5 watts per gallon of actual water. A 75-gallon tank holds about 64 gallons of water, so it needs 190–320 W — best split across two elements at 60–75% each so one stuck element can’t cook the tank, always on an external controller.
The ReefRig Heater Method
3–5 W per gallon of actual water volume (~85% of nominal after rock and sand), split across two elements at 60–75% each, always on an external controller.
A 75-gallon reef holds ~64 gallons of water and needs 190–320 W of heat, split across two elements on a controller.
Heater wattage by tank size
Total watts are sized against effective water volume (~85% of nominal after rock and sand), then a stony tank splits that across two elements at 60–75% each. A soft-coral or fish-only tank can run a single element of the total wattage.
| Nominal | Water vol | Total heat | Two elements (each) |
|---|---|---|---|
| 10 gal | 9 gal | 27–45 W | 16–34 W |
| 20 gal | 17 gal | 51–85 W | 31–64 W |
| 30 gal | 26 gal | 78–130 W | 47–98 W |
| 40 gal | 34 gal | 102–170 W | 61–128 W |
| 55 gal | 47 gal | 141–235 W | 85–176 W |
| 75 gal | 64 gal | 192–320 W | 115–240 W |
| 90 gal | 77 gal | 231–385 W | 139–289 W |
| 120 gal | 102 gal | 306–510 W | 184–383 W |
| 150 gal | 128 gal | 384–640 W | 230–480 W |
| 180 gal | 153 gal | 459–765 W | 275–574 W |
| 210 gal | 179 gal | 537–895 W | 322–671 W |
| 250 gal | 213 gal | 639–1,065 W | 383–799 W |
| 300 gal | 255 gal | 765–1,275 W | 459–956 W |
Every stony tank (LPS/SPS/mixed) should run two elements on an external controller — mechanical thermostats fail closed, and two smaller heaters mean one stuck element can’t overheat the system.
LIVE CALCULATOR
Size your heater
Pick your tank size and corals. Total wattage is set by water volume; stony tanks (LPS/SPS/mixed) split it across two elements for failover.
64 gal needs 192–320 W of heat, split across two elements of 115–240 W each — one stuck-on element alone cannot cook the tank. Run both through an external controller; mechanical thermostats fail closed.