The 5W-per-gallon rule (and where it breaks)
The standard hobby rule of thumb is 5 watts per gallon for a 5–10°F lift over typical room temperature in an insulated home. The rule is approximate — heat loss is a surface-area phenomenon and surface-to-volume drops as tanks grow, so very large tanks don't scale linearly. Very small tanks (under ~5 gallons) also break the rule because the smallest commonly-available heaters are 25W.
The rule survives because it's right often enough — a 10-gallon tropical in a 70°F room is exactly the case 5W/gal was tuned for. The math below diverges from the rule when your room is colder, your delta is bigger, or your tank is small enough that the heater wattage floor (~25W) overshoots.
The calculator above adjusts watts-per-gallon by the actual temperature delta you specify:
- ≤ 5°F lift: ~3 W/gal
- ≤ 10°F lift: ~5 W/gal
- ≤ 15°F lift: ~7.5 W/gal
- > 15°F lift: ~10 W/gal — and consider room insulation
10-gallon tank heater size
A 10-gallon tropical (bettas, neon tetras, white clouds) targeting 76–78°F sits in the sweet spot the 5W/gal rule was tuned for. The smallest commonly-stocked heater is 25W, but for a 10g community tank 50W is the realistic floor — it gives margin for cooler nights without straining the thermostat.
| Room → target lift | Recommended | Common scenario |
|---|---|---|
| 5°F lift (mild room) | 50W | 73°F room → 78°F betta |
| 10°F lift (typical) | 50W | 68°F room → 78°F community |
| 15°F lift (cool room) | 75W | 63°F basement → 78°F |
| 20°F lift (cold room) | 100W | 58°F unheated room → 78°F |
20-gallon tank heater size
The 20-gallon long is the most-recommended beginner community size and the wattage choices line up cleanly with manufacturer SKUs. For a typical room a single 100W is the answer; if the tank lives in a colder room or you're keeping warmer-water species (discus territory), step up rather than add a second small heater — the two-heater redundancy logic doesn't kick in until ~75g.
| Room → target lift | Recommended | Common scenario |
|---|---|---|
| 5°F lift | 75W | 73°F room → 78°F |
| 10°F lift (typical) | 100W | 68°F room → 78°F community |
| 15°F lift | 150W | 63°F basement → 78°F |
| 20°F lift (cold room or discus) | 200W | 62°F room → 82°F discus |
55-gallon tank heater size
A 55g sits right at the boundary — single-heater is still defensible but the math starts to favor splitting. A 300W single heater in a typical room works; a pair of 150W heaters at the same total draw gives the redundancy that matters more as tanks get bigger and re-heating after a cold spell takes longer.
| Room → target lift | Recommended (single) | Or split (×2) |
|---|---|---|
| 5°F lift | 200W | 2 × 100W |
| 10°F lift (typical) | 300W | 2 × 150W |
| 15°F lift | 500W | 2 × 250W |
| 20°F lift (cold room) | 600W | 2 × 300W |
75-gallon tank heater size
Past 75g, default to two heaters. The redundancy math (see the next section) is the reason — same total wattage, but a single failure can't cook the tank or chill it past the danger floor. Mount them on opposite ends of the tank for even temperature and run them off separate outlets if your wiring allows.
| Room → target lift | Total wattage | Per heater (×2) |
|---|---|---|
| 5°F lift | 250W | 2 × 150W |
| 10°F lift (typical) | 400W | 2 × 200W |
| 15°F lift | 600W | 2 × 300W |
| 20°F lift (cold room) | 800W | 2 × 400W |
Why two heaters on big tanks
For tanks over 75 gallons, conservative practice is two heaters at half the recommended total wattage each. Two failure modes motivate this:
- Stuck-on heater. A failed thermostat that keeps the heater always-on cooks the tank. Two smaller heaters limit how high a single failure can drive the temperature.
- Stuck-off heater. A failed heater during a cold-night power-cycle drops the tank fast. A second still-working heater keeps the tank above the danger floor.
Target temperatures by common species
- Goldfish: 65–72°F (cool-water; usually no heater in temperate rooms)
- Most community tropicals (tetras, corydoras, guppies): 76–78°F
- Bettas: 78–80°F
- Discus: 82–86°F (high delta — requires more wattage and stable room temp)
A worked example: 75-gallon planted tank, basement at 62°F
Say you've just set up a 75-gallon planted community in a basement that sits around 62°F year-round. Your tropical fish need 78°F. That's a 16°F lift — past the 15°F band on the 75-gallon table above, so size to the next band up.
The conservative-safe answer is 800W total, split between two 400W heatersmounted at opposite ends of the tank. Not a single 800W. Past 75 gallons, two heaters at half the total wattage each is the redundancy floor: if one fails stuck-on (broken thermostat that keeps it heating non-stop), it can only push the water so far before the other heater's thermostat shuts off. If one fails stuck-off, the other still keeps the fish above the danger floor while you replace the bad unit1.
If your room drops further during a winter cold snap or a power outage runs the room cold for a night, the calculator above is sizing for steady state— for holding the temperature once you're at target — not for re-heating a tank that dropped 15°F overnight. Re-heating from a deep drop takes noticeably more wattage than holding a steady temperature. Plan for one bigger jump per year and read the trap section below.
What this calculator gets wrong if you ignore three things
The wattage tables above are tuned for typical setups: lid on, the heater fully submerged in clear water, steady-state heating. Three real-world variables move the answer enough to matter:
- Heater placement matters more than wattage on paper. A heater buried in substrate, jammed behind driftwood, or pinned against the back glass can materially reduce its effective output — the heating element creates a localized hot spot instead of warming the bulk water. Mount it vertically (or at the 45° angle some manufacturers recommend), with at least an inch of water flow around it, near the filter outlet so the warm water actually circulates. Hot-spotting also shortens heater life.
- Open-top tanks lose heat faster than covered tanks. The biggest single source of heat loss in a freshwater aquarium is evaporation off the water surface. If you run a rimless tank with no lid (popular on planted-shrimp setups), the wattage tables above are likely under-sizing. Either add a glass canopy or acrylic lid, use the next band up on the table, or accept that the heater will run nearly non-stop through the cooler months.
- Steady-state sizing ignores re-heating after a power outage. A 200W heater on a 30-gallon tank can hold 78°F all day. It will not pull a cold tank back up to 78°F quickly after an 8-hour overnight outage — re-heating from a deep drop needs noticeably more wattage than holding a steady temperature. The conservative beginner move: size for steady-state and pair the heater with an external temperature controller — an Inkbird ITC-308 or similar runs about $30 and gives you a hard upper bound that overrides a stuck-on thermostat2. For tanks 55 gallons and up, that's the cheapest insurance you can buy on a tank full of fish.
Common questions
+What size heater for a 10-gallon tank?
Use a 50W adjustable heater for most 10-gallon tropical tanks. If the room is cool and the tank needs a 15°F lift, step up to 75W; if the room is cold and the tank needs about a 20°F lift, use 100W. For a normal 68-73°F room targeting 76-78°F, 50W is the standard answer.
+How many watts per gallon for an aquarium heater?
Use about 3 watts per gallon for a 5°F temperature lift, 5 watts per gallon for a 10°F lift, 7.5 watts per gallon for a 15°F lift, and close to 10 watts per gallon for a colder room or larger lift. Round up to the next common heater size rather than down.
+Should I get one big heater or two smaller ones?
Under 55 gallons, one heater is fine. At 75 gallons and up, default to two heaters at half the recommended total wattage each. Same total heat, two failure modes covered: a stuck-on single heater can cook the tank; a stuck-off heater on a cold night can chill it past the danger floor. Two smaller heaters limit how far either failure can push the water.
+Where should I mount the aquarium heater?
Vertically (or at 45° if the manufacturer's manual allows that angle), fully submerged, with at least an inch of water flow around it. Position it near the filter outlet so the heated water actually circulates. Don't bury the heater in substrate and don't jam it behind decor — both cause hot spots that shorten the heater's life and leave cold zones at the other end of the tank.
+Do planted tanks need a bigger heater?
No. Planted-tank lighting adds at most 1-2°F to room ambient — it doesn't replace a heater. The wattage tables above stand for planted tanks too. The opposite trap is real on high-light setups in cool rooms, where day/night cycles produce temperature swings the heater struggles to smooth out.
+Should I use an external temperature controller (Inkbird) on the heater?
For tanks 55 gallons and up, yes. An Inkbird-style controller (~$30) runs the heater AND lets you set a hard upper bound that overrides a stuck-on thermostat. On tanks where a heater failure means hundreds of dollars of livestock, it's the cheapest insurance you can buy. Smaller tanks can skip the controller until you scale up.
+How long does a typical aquarium heater last?
Most submersible heaters get 2-3 years before the thermostat drifts or fails outright. The reliability tier above that — Eheim Jager, Fluval E-series, Hydor ETH — typically runs 4-5 years. Replace any heater that runs constantly when the tank is already at target temperature (stuck-on thermostat) or that isn't running while the tank is dropping (relay failed off). Don't roll the dice on a 6-year-old heater with a tank full of fish.
Related tools
- Tank volume calculator — get accurate water volume first.
- Water parameters explained — temperature is one of four parameters that matter; this covers all of them.
- Cycling a new aquarium — temperature stability matters more during cycling than after.
- 1. Eheim Jager aquarium heater technical guide — manufacturer-published wattage sizing tables, placement recommendations, and the two-heater redundancy guidance for larger tanks. The wattage bands on this page line up with Eheim's published SKU lineup (25W / 50W / 75W / 100W / 150W / 200W / 250W / 300W). ↩
- 2. Inkbird ITC-308 dual-relay temperature controller — manufacturer documentation on how the controller's hard upper-bound cutoff overrides a stuck-on heater thermostat. The relay-failsafe pattern is the same on any comparable controller (Ranco, Willhi, BN-LINK). ↩