Reef heater size calculator
3 to 5 watts per gallon of total water. Use 3 if your room stays within 5 °F of tank temperature, 4 for a 6 to 10 °F gap and 5 for more. Above 40 gallons, split it across two heaters, and run every heater through a controller.
Include the rear chambers or sump in the volume, not just the display. Sizes round up to the next common heater size.
Reef heater size chart
Assumes 78 °F in the tank. "2 ×" means two heaters, each at least half the target.
| Tank | 3 W/gal (warm room) | 4 W/gal (typical) | 5 W/gal (cold room) | What to buy |
| 5 gal | 15 W | 20 W | 25 W | 25 W |
| 10 gal | 30 W | 40 W | 50 W | 50 W |
| 20 gal | 60 W | 80 W | 100 W | 75 W to 100 W |
| 29 gal | 87 W | 116 W | 145 W | 100 W to 150 W |
| 30 gal | 90 W | 120 W | 150 W | 100 W to 150 W |
| 40 gal | 120 W | 160 W | 200 W | 125 W to 200 W |
| 55 gal | 165 W | 220 W | 275 W | 2 × 100 W to 2 × 150 W |
| 75 gal | 225 W | 300 W | 375 W | 2 × 125 W to 2 × 200 W |
| 90 gal | 270 W | 360 W | 450 W | 2 × 150 W to 2 × 250 W |
| 125 gal | 375 W | 500 W | 625 W | 2 × 200 W to 2 × 400 W |
| 150 gal | 450 W | 600 W | 750 W | 2 × 250 W to 2 × 400 W |
Freshwater keepers can use the same math. Many freshwater tanks run cooler, which shrinks the temperature gap and the wattage.
What size heater for a 5 gallon reef tank?
20 W in a typical home (4 W/gal), 15 W in a warm room and 25 W in a cold one. One heater: 25 W.
Verified options at 25+ W: EHEIM thermocontrol 25 W (9.57")
What size heater for a 10 gallon reef tank?
40 W in a typical home (4 W/gal), 30 W in a warm room and 50 W in a cold one. One heater: 50 W.
Verified options at 40+ W: EHEIM thermocontrol 50 W (9.57") · Fluval M series 50 W (11")
What size heater for a 20 gallon reef tank?
80 W in a typical home (4 W/gal), 60 W in a warm room and 100 W in a cold one. One heater: 75 W to 100 W.
Verified options at 80+ W: Bulk Reef Supply Titanium heater element 100 W (10.25", needs a controller) · EHEIM thermocontrol 100 W (12.56") · Fluval E series 100 W (11.75") · Fluval M series 100 W (11")
What size heater for a 29 gallon reef tank?
116 W in a typical home (4 W/gal), 87 W in a warm room and 145 W in a cold one. One heater: 100 W to 150 W.
Verified options at 116+ W: EHEIM thermocontrol 125 W (12.56") · EHEIM thermocontrol e 150 W (12.87") · Fluval M series 150 W (13")
What size heater for a 30 gallon reef tank?
120 W in a typical home (4 W/gal), 90 W in a warm room and 150 W in a cold one. One heater: 100 W to 150 W.
Verified options at 120+ W: EHEIM thermocontrol 125 W (12.56") · EHEIM thermocontrol e 150 W (12.87") · Fluval M series 150 W (13")
What size heater for a 40 gallon reef tank?
160 W in a typical home (4 W/gal), 120 W in a warm room and 200 W in a cold one. One heater: 125 W to 200 W.
Verified options at 160+ W: Bulk Reef Supply Titanium heater element 200 W (10.25", needs a controller) · EHEIM thermocontrol 200 W (15.98") · EHEIM thermocontrol e 200 W (15.19") · Fluval E series 200 W (14") · Fluval M series 200 W (13")
What size heater for a 55 gallon reef tank?
220 W in a typical home (4 W/gal), 165 W in a warm room and 275 W in a cold one. Split it across two heaters: 2 × 100 W to 2 × 150 W.
Verified options at 110+ W: EHEIM thermocontrol 125 W (12.56") · EHEIM thermocontrol e 150 W (12.87") · Fluval M series 150 W (13")
What size heater for a 75 gallon reef tank?
300 W in a typical home (4 W/gal), 225 W in a warm room and 375 W in a cold one. Split it across two heaters: 2 × 125 W to 2 × 200 W.
Verified options at 150+ W: Bulk Reef Supply Titanium heater element 200 W (10.25", needs a controller) · EHEIM thermocontrol 150 W (13.58") · EHEIM thermocontrol e 150 W (12.87") · Fluval E series 200 W (14") · Fluval M series 150 W (13")
What size heater for a 90 gallon reef tank?
360 W in a typical home (4 W/gal), 270 W in a warm room and 450 W in a cold one. Split it across two heaters: 2 × 150 W to 2 × 250 W.
Verified options at 180+ W: Bulk Reef Supply Titanium heater element 200 W (10.25", needs a controller) · EHEIM thermocontrol 200 W (15.98") · EHEIM thermocontrol e 200 W (15.19") · Fluval E series 200 W (14") · Fluval M series 200 W (13")
What size heater for a 125 gallon reef tank?
500 W in a typical home (4 W/gal), 375 W in a warm room and 625 W in a cold one. Split it across two heaters: 2 × 200 W to 2 × 400 W.
Verified options at 250+ W: Bulk Reef Supply Titanium heater element 300 W (10.25", needs a controller) · EHEIM thermocontrol 250 W (17.8") · EHEIM thermocontrol e 250 W (16.97") · Fluval E series 300 W (14") · Fluval M series 300 W (15")
What size heater for a 150 gallon reef tank?
600 W in a typical home (4 W/gal), 450 W in a warm room and 750 W in a cold one. Split it across two heaters: 2 × 250 W to 2 × 400 W.
Verified options at 300+ W: Bulk Reef Supply Titanium heater element 300 W (10.25", needs a controller) · EHEIM thermocontrol 300 W (19.92") · EHEIM thermocontrol e 300 W (19.13") · Fluval E series 300 W (14") · Fluval M series 300 W (15")
Bigger is not safer
A 300 W heater on a 20 gallon tank is 15 W per gallon. If its thermostat sticks, it overheats the tank fast. Size it right, and plug it into a controller set 1 to 2 °F above the heater, such as the Inkbird ITC-306T (heating only, 10 A) or the Innovative Marine Helio controller (rated 1000 W, high and low alarms). The Bulk Reef Supply titanium element has no thermostat, so it only works through a controller.
Watts are half the answer. Will it fit?
All-in-one rear chambers are where most heater mistakes happen. Manufacturers don't publish the water depth in these chambers, so we assume the water sits 1.5" below the chamber top. Measure yours before you buy.
| Tank | Chamber W × D × H | Assumed water depth | Target |
| Fluval Evo 13.5 | 3.5 × 2.75 × 11.25 in (community) | ~9.75" | 41-68 W |
| Coralife BioCube 32 (LED) | 3.5 × 3.125 × 17 in (community) | ~15.5" | 90-150 W |
| Coralife BioCube 16 (LED) | 4.125 × 2.1875 × 12.75 in (community) | ~11.25" | 48-80 W |
| Fluval Flex 9 (34L) | 3.875 × 2.125 × 11.375 in (community) | ~9.88" | 27-45 W |
| Fluval Flex 15 (57L) | 3.875 × 2.9375 × 12.5625 in (community) | ~11.06" | 45-75 W |
| Marineland Portrait 5 (AIO) | 9.5 × 2 × 12 in (community) | ~10.5" | 15-25 W |
| Waterbox AIO 35.2 | 4 × 4.7 × 17.7 in (mfr) | ~16.2" | 99-165 W |
| Waterbox AIO 50.3 | 6.1 × 4.7 × 17.7 in (mfr) | ~16.2" | 147-246 W |
| Waterbox AIO 65.4 | 5.9 × 4.7 × 17.7 in (mfr) | ~16.2" | 197-329 W |
| Waterbox Cube 10 (AIO) | 4.9 × 3.1 × 13.8 in (mfr) | ~12.3" | 33-55 W |
| Waterbox AIO 15 Peninsula (Cube 15) | 4.9 × 3.1 × 13.5 in (mfr) | ~12" | 46-76 W |
| Waterbox Cube 20 (AIO) | 7.9 × 3.1 × 15.7 in (mfr) | ~14.2" | 60-100 W |
| Waterbox Peninsula 25 (AIO) | 6.8 × 3.1 × 15.7 in (mfr) | ~14.2" | 71-119 W |
FAQ
How many watts per gallon for a reef tank?
3 to 5, depending on how much colder your room gets than the tank. Use 3 if the room stays within 5 °F of tank temperature, 4 for a 6 to 10 °F gap, and 5 for more than 10 °F.
Can a heater be too big?
It heats fine, but if its thermostat sticks on, a big heater overheats the tank much faster. We flag anything over 6 W per gallon as oversized and recommend a controller on every tank.
Do I count sump or rear-chamber water?
Yes. Size on total system volume. On an all-in-one tank the rear chambers hold real water, and on a sump system the sump does too.
One heater or two?
One up to about 40 gallons. Above that, two heaters at half the target each: if one sticks on it heats the tank more slowly, and if one dies the other keeps the tank from crashing overnight.
A rule of thumb only. Heat loss also depends on lids, fans, lights and room airflow. Heater specs come from manufacturer pages and manuals; see the reef heater hub for sources.