Lighting Setup
Two 30 W fixtures can differ twofold in light output, and plants only care about the light that reaches the water. So the first result is "how many lumens" (net water × lumens per litre), and wattage is back-calculated from the fixture efficacy. LED output also falls with junction temperature: 25 °C is the rating point and each extra degree costs about 0.6%, so at 45 °C you have to buy roughly 14% more nominal wattage.
Size a light by lumens, not watts: a 600×400×400 planted tank holds 88 L of net water, wants about 3,074 lm at the medium tier, which at 110 lm/W is roughly 28 W of LED.
Units
Input parameters
in
in
in
Light tier
lm/W
h
°C
Result
- Net water
- 23.2 US gal
- Surface area
- 2.5 ft²
- Lumens required
- 3.1 klm
- Suggested LED wattageRange 25.4–38.1 W
- 31.8 W
- Light actually deliveredAfter the 110 lm/W rating and the junction-temperature loss
- 3.1 klm
- Illuminance at the surface
- 13,475 lx
- Hours per day
- 7 h
Tier comparison
| Tier | Lumens per litre | For this tank | Suggested hours |
|---|---|---|---|
| Low (easy plants) | 20 lm/L | 1.8 klm | 6 h |
| Medium (most planted tanks) | 35 lm/L | 3.1 klm | 7 h |
| High (demanding plants, carpets) | 55 lm/L | 4.8 klm | 8 h |
| Coral (marine) | 90 lm/L | 7.9 klm | 9 h |
Notes
- Sized in lumens (light output) rather than watts: two 30W fixtures can differ twofold in output
- LED output falls with junction temperature: 25 °C is the rating point, and each extra degree costs about 0.6%
- Wattage is back-calculated as lumens ÷ efficacy ÷ derating, so changing the fixture means changing one number
- Lumens measure light for the eye, not PAR; for corals go by the PAR figure the maker publishes
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 tankNet water uses the inner cavity × water level, and surface area uses the inner footprint — both matter, because plants care about water depth and lit area.
- Step 2Pick a light tierLow = easy plants, medium = most planted tanks, high = demanding plants and carpets, coral = marine. The tier sets lumens per litre (20 / 35 / 55 / 90).
- Step 3Enter the fixture efficacyIn lm/W: ordinary LED strips run about 80–100, COB about 120–150, tubes about 60–80. Changing the fixture means changing this one number.
- Step 4Read lumens and wattageThe wattage range is ±20% because neither efficacy nor depreciation is exact; the surface illuminance (lx) lets you sanity-check against a maker PAR figure.
Frequently asked questions
- How many watts per litre do I need?
- Do not size by watts. Size by lumens: 20 lm/L low, 35 medium, 55 high, 90 for coral. A 600×400×400 planted tank holds 88 L of net water, so the medium tier wants about 3,074 lm; at 110 lm/W that is roughly 28 W, with a 25–38 W range. Wattage moves with efficacy, lumens do not.
- How long should the light stay on?
- The tier suggests it: 6 h low, 7 h medium, 8 h high, 9 h coral. Past 10 h the tool warns about algae — algae exploit long photoperiods better than plants, so the extra hours are rarely worth the risk. On a new tank start at 6 h and add 30 minutes a week.
- Why does a 45 °C fixture need more nominal wattage?
- LED output falls with junction temperature: 25 °C is the rating point and each extra degree costs about 0.6%, so at 45 °C roughly 88% remains. Nominal wattage therefore has to be compensated by 1 ÷ 0.88 ≈ 1.14×. In the default case 27.9 W at 25 °C becomes 31.8 W at 45 °C — a quantified version of the fixture "looking dimmer" in summer.
- Are lumens the same as PAR?
- No. Lumens measure light as the eye sees it; PAR (photosynthetically active radiation, in μmol/(m²·s)) counts the 400–700 nm photons a plant can use. Red and blue light score poorly in lumens but strongly in PAR, so a red-heavy fixture can be dim in lumens and bright in PAR. Treat these figures as a floor, and go by the maker PAR figure for corals and for colouring up red plants.
- How high should the light hang?
- Surface illuminance = lumens ÷ surface area, about 13,000 lx in the default case. Once the water column passes 450 mm a strip light has lost more than half its output at the substrate, and demanding plants stall: moving the fixture closer to the water, or using lenses, beats adding wattage.
- Do I need CO₂?
- At the high tier, yes. Light, CO₂ and nutrients limit each other: raise light without enough CO₂ and the plants cannot use the extra energy while algae can. This tool covers light only — target 20–30 mg/L of CO₂ separately.