Lighting Calculator
Plants do not see watts — they see the photons that reach them. Find out how much light your fixture actually delivers.
Find the fixture's PPF
Manufacturers list it in µmol/s. If it is missing, use lumens; the calculator converts approximately based on spectrum.
Enter the area the fixture lights
If the fixture spans the tank, enter the tank footprint. A shorter fixture lights a smaller area and the ends stay dark.
Measure height and depth
The distance from water surface to fixture and the water depth are separate inputs. On pendant fixtures, height is the single most influential one.
Read the band, not the number
The result is an estimate and can be off by ±30%. What matters is whether you are in the low, medium or high band — plant selection and the CO2 decision follow from that.
Set light and CO2 together
Turning up high light without CO2 grows algae. CO2 and dosing need to be settled before the light goes up.
How much PAR does a planted aquarium need?
Measured at the substrate: below 30 µmol/m²/s is low light and works without CO2 with Anubias, Microsorum and Cryptocoryne. 30–50 is medium light, where plant selection is widest. 50–80 is high light and grows algae without CO2 and steady dosing. Above 80 only makes sense with CO2, fertiliser and water changes all in place.
Why is the watts-per-litre rule no longer used?
Watts measure electricity drawn, not light produced. Two LED fixtures at the same wattage can differ threefold in what reaches the substrate; the rule worked in the fluorescent era when fixtures were all alike. The right measure today is photon flux per unit area (PPFD, µmol/m²/s).
How much does raising the fixture cut the light?
More than people expect. Light is not only absorbed by water, it also spreads out: at 45 cm depth the water absorbs around 10% while spread-related loss can exceed 70%. Raising a fixture by 20 cm noticeably lowers substrate PAR in most setups — in exchange it distributes light more evenly and reduces dark corners.
Can PAR be derived from lumens?
Approximately. Lumens are weighted for the eye's sensitivity to green and undercount the red and blue photons plants use. White LEDs run at roughly 0.015 µmol/s per lumen, and plant-spectrum fixtures higher. Where the manufacturer publishes a PPF figure, always use that instead.
Does this replace a PAR meter?
No. The model treats the fixture as a rectangular surface and applies the closed form for that geometry; lens design, reflectors and spectrum are outside it. Real values can differ by ±30%. The purpose is to show which band you are in and what changing the mounting height does.
Does a longer photoperiod make up for weak light?
Only partly. Total daily light (DLI) rises, but photosynthesis does not scale linearly with intensity; under low light some plants stay leggy no matter how many hours you run. Longer photoperiods also give algae more opportunity. In practice 6–8 hours is the right starting point for most layouts, and intensity is set before duration.