Heater Wattage
The familiar "1.2 W/L" rule describes **heat-up**, while the reason a tank never reaches its setpoint is usually **heat loss**. This tool separates them: steady state is loss area × 10 W/(m²·K) × ΔT (undersize it and the setpoint is unreachable), and heat-up assumes the whole tank is raised from room temperature within 12 hours. The larger of the two, times 1.25, rounded up to a standard rating.
Heater power is the larger of two stages: a 600×400×400 tank needs only about 65 W to hold 26 °C, but about 154 W to climb from 18 °C to 26 °C within 12 hours — so the standard 200 W rating wins.
Units
Input parameters
in
in
in
°C
°C
°C
Covered?
Result
- Net water in the cavity
- 22.8 US gal
- Heat-losing area
- 8.7 ft²
- Suggested heater ratingAbout 1.16 W/L (the usual rule of thumb is 1.2 W/L)
- 100 W
- Steady-state requirement
- 65 W
- Heat-up requirement
- 77 W
- Power-density floor
- 69 W
- From the low point to target
- 6.8 h
Standard ratings
| Rating | Power density | Holds the setpoint |
|---|---|---|
| 25 W | 0.29 W/L | Too small |
| 50 W | 0.58 W/L | Too small |
| 75 W | 0.87 W/L | Usable |
| 100 W | 1.16 W/L | Usable |
| 150 W | 1.73 W/L | Usable |
| 200 W | 2.31 W/L | Usable |
| 300 W | 3.47 W/L | Usable |
| 400 W | 4.63 W/L | Usable |
| 500 W | 5.78 W/L | Usable |
| 600 W | 6.94 W/L | Usable |
| 800 W | 9.25 W/L | Usable |
| 1000 W | 11.57 W/L | Usable |
Notes
- 限制在升温窗口：按 12 小时内把整缸水从室温拉到位计算。
- The result is the largest of steady-state, heat-up and the power-density floor, times 1.25, rounded up to a standard rating
- **Steady state** is loss area × 10 W/(m²·K) × ΔT: below it the tank never reaches the setpoint
- **Heat-up** assumes the whole tank is raised from room temperature within 12 hours (an overnight start-up is normal), matching the familiar 1.2 W/L rule
- The loss coefficient is empirical (glass conduction, evaporation and convection together) and higher than a bare-water figure, which is why theory-based sizing comes out too small
- A bigger heater warms the tank faster, but past the steady-state requirement it is mostly waste: heat loss sets the ceiling
The calculator above follows the unit system selected at the top of this page; the explanatory text below stays metric (mm / L / kg / ℃).
Need a full design?
- Want to turn the dimensions into a 3D model and a complete bill of materials
- Enter length, width and height in the aquarium designer and it generates the 3D model, dimensions, glass cut list, silicone usage and equipment selection, all exportable as CSV and PNG.
How to use it
- Step 1Measure the tank and note whether it is coveredLoss area = surface (30% if covered) + four walls. A cover cuts the required wattage sharply and reduces evaporation.
- Step 2Enter the lowest room temperatureUse the coldest days of winter, not an average — the heater has to be sized for the worst case.
- Step 3Enter the target and the post-change lowThe target is what steady state must hold; the low point only answers "how long to recover", which after a water change is usually 1–2 K.
- Step 4Read the three power figures and the rating tableSteady state, heat-up and the density floor — the largest, rounded up to a standard rating; the table marks which ratings hold the setpoint.
Frequently asked questions
- How do I choose a heater wattage?
- Take the larger of two stages. A 600×400×400 tank (86 L net) at 18 °C room aiming for 26 °C, covered: steady state is about 65 W (0.81 m² of loss area × 10 W/(m²·K) × 8 K) and heat-up about 77 W (from room temperature within 12 hours). The larger, 77 W × 1.25, is 96 W, which rounds up to 100 W. Drop the room to 12 °C and steady state climbs to 174 W, so 200 W or 300 W is needed.
- Why did a 1.2 W/L heater fail to hold temperature in winter?
- Because 1.2 W/L describes heat-up, while the binding limit is often heat loss. The same 86 L needs 65 W covered, 121 W uncovered and 174 W at 12 °C room temperature. When the heater is below the steady-state requirement it runs at 100% duty and still misses the setpoint — a cover or a warmer room beats a bigger heater.
- Is a bigger heater always better?
- No. Past the steady-state requirement it only speeds up the first warm-up (about 1.1 h for the whole tank at 200 W against 2.3 h at 100 W in the default case), and it brings two problems: a concentrated heat load makes the tank less even, and if the thermostat sticks — a common failure — the water heats too fast for the fish to react. Above 200 W the tool suggests splitting into two units so that one failure does not take the whole tank.
- Do I need two heaters?
- Above 200 W, yes, for two reasons: one unit failing still holds most of the temperature, and two units at opposite ends even out the water. Split the total (200 W into 2×100 W) and set both to the same target — a primary and a backup is not needed.
- Why is recovery after a water change so much faster than the first warm-up?
- Because a water change usually drops the temperature 1–2 K while a cold start climbs 8 K or more from room temperature. In the default case (86 L, 200 W, 22 °C to 26 °C) that is about 2.8 h; the same heater raising the whole tank from 18 °C takes far longer, which is exactly why the heat-up window drives the sizing.
- Do I still need a heater in summer?
- Once room temperature is at or above the target the heater never runs, and the tool simply reports "not needed". What you need then is cooling: a fan drops 2–4 °C by evaporation (and needs top-ups), while a chiller is stronger but costlier and noisier. Note that dissolved oxygen falls sharply above 30 °C, which is the real summer risk.