# Aquarium heater wattage calculator

> Enter your tank size in US gallons, the target minus room temperature in °F, and the room temperature where the tank sits. A 29 US-gallon tank at 78 °F target with a 72 °F room needs about 150 W — the worked example below uses the same inputs so the numbers shown here always match the calculator.

Use the interactive calculator on the page: https://petguidecalc.com/aquarium/heater-wattage/. Enter your tank size in US gallons, the temperature rise needed (target water °F minus room °F), and the room temperature where the tank sits. The recommendation rounds to the nearest common 25 / 50 / 75 / 100 / 150 / 200 / 300 W aquarium heater step.

## How many watts does my aquarium heater need?

A common rule of thumb is 5 W per US gallon, then +50% if the temperature rise is over 10 °F, plus +1 W per gallon if the room is below 65 °F. Round the result to the nearest common heater step (25 / 50 / 75 / 100 / 150 / 200 / 300 W), and pick a second smaller heater for long tanks, big deltas, or cold rooms.

## Common tank sizes at a glance

| Tank | US gal | Room °F | Target °F | Heater | Second heater? |
|---|---|---|---|---|---|
| 24 × 12 × 16 in (20 US gal high) | 20 | 72 | 78 | 100 W | No |
| 24 × 12 × 16 in (20 US gal high) | 20 | 65 | 78 | 150 W | No |
| 48 × 13 × 21 in (55 US gal) | 55 | 72 | 78 | 300 W | No |
| 48 × 13 × 21 in (55 US gal) | 55 | 65 | 78 | 300 W | No |
| 48 × 18 × 21 in (75 US gal) | 75 | 72 | 78 | 300 W | Yes |
| 48 × 18 × 21 in (75 US gal) | 75 | 65 | 78 | 300 W | Yes |
| 72 × 18 × 27 in (150 US gal) | 150 | 72 | 78 | 300 W | Yes |
| 72 × 18 × 27 in (150 US gal) | 150 | 65 | 78 | 300 W | Yes |

Wattage is rounded to the nearest common heater step (25 / 50 / 75 / 100 / 150 / 200 / 300 W). "Second heater" flags setups where one heater is unlikely to keep the tank stable.

## How the heater wattage is calculated

Start at 5 W per US gallon. For a 29 gallon tank that is 145 W before any adjustments.

If the temperature rise is greater than 10 °F the demand is multiplied by 1.5× to keep up with the steady-state heat loss through the glass.

If the room is colder than 65 °F, add 1 W per gallon (cold rooms typically need more heater capacity, and a second smaller heater gives better heat distribution and redundancy if one fails).

Round the raw demand to the nearest common aquarium heater step (25 / 50 / 75 / 100 / 150 / 200 / 300 W) so the recommendation is always a real product size, not an arbitrary number.

Worked example: a 29 gallon tank with a 6 °F rise and a 72 °F room has a raw demand of 145 W, rounds to 150 W, and fits a single heater.

## Aquarium heater FAQ

**Q: How many watts per gallon do I need for an aquarium heater?**

The hobby rule of thumb is 5 W per US gallon. Add a 50% boost if the temperature rise is greater than 10 °F, and +1 W per gallon if the room is below 65 °F. Round to the nearest common heater step before buying.

**Q: Do I need one heater or two?**

A single heater is enough for small to medium tropical tanks in a normal room. Two smaller heaters give better heat distribution on long tanks (over about 55 US gallons), big temperature rises (> 15 °F), or very cold rooms (< 60 °F), and a backup if one fails.

**Q: What if my room is below 65 °F?**

Cold rooms run the heater almost continuously and benefit from extra wattage. Pick a heater one step larger than the formula suggests and consider running two smaller heaters in series so neither cycles too aggressively.

**Q: Should I oversize the heater?**

Modest oversizing (one step up) is fine in cold rooms. Going two or three steps over can short-cycle the thermostat and shorten heater life; instead, add a second heater for capacity.

**Q: Does tank shape change the heater size?**

Wattage scales with water volume, so a tall narrow tank and a short wide tank of the same US gallonage need the same heater. Long tanks benefit from two smaller heaters placed at opposite ends so heat distributes evenly.

## References

Wattage steps and the 5 W/gal rule of thumb come from mainstream aquarium hobby sources; the calculator rounds to those same steps so the recommendation maps to a real product.

- [Plane heating with a transparent heater film in a fish tank](https://doi.org/10.1016/j.heliyon.2024.e24066) — Panama G, Jin J, Kim DJ, Lee SS, Heliyon, 2024.
- [How to choose an aquarium heater](https://www.aquariumcoop.com/) — Aquarium Co-Op, Aquarium Co-Op. Hobby reference for the 5 W per US gallon rule of thumb, the 25–300 W common heater steps, and the case for two smaller heaters on long tanks.
- [Choosing the right aquarium heater size](https://www.liveaquaria.com/) — LiveAquaria, LiveAquaria. Retail reference confirming the same wattage steps and the recommendation to oversize modestly in cold rooms rather than dramatically oversize in normal rooms.
- [Aquarium heater sizing basics](https://www.thesprucepets.com/aquarium-supplies-5082682) — The Spruce Pets, The Spruce Pets. Pet-care reference confirming the 5 W/gal baseline, the +50% boost above a 10 °F temperature rise, and the second-heater guidance for cold rooms and long tanks.

_Educational resource, not veterinary advice. PetGuideCalc — https://petguidecalc.com_