Aquarium lighting calculator
How much light actually reaches your substrate, how long to run it, and whether it suits the plants you have.
Fixture type
High light · 2.1 mol/m²/day
An estimate from wattage, depth and mounting height — not a measurement. Real fixtures still vary by about a third either way on optics, spectrum and age. A manufacturer PAR chart or a borrowed meter beats any calculator, this one included.
Run it 5–6 hours a day.
That is a lot of light with no injected carbon behind it. Plants run out of CO2 by mid-morning, and everything past that point feeds algae instead — shorten the day, or add CO2.
Which plants are in there?
Planning the tank itself? The stocking calculator carries your plants and dimensions straight over.
Lighting is the one piece of aquarium equipment still sold on a number that means nothing. Watts told you something useful when every tank ran fluorescent tubes of roughly equal efficiency; an LED bar makes two to three times the usable light per watt, so the old watts-per-litre tables now under-rate modern fixtures and over-rate old ones by the same stroke. What plants actually respond to is PAR — the photons in the 400–700 nm range that photosynthesis can use — measured where the plants are, at the substrate. This calculator estimates that from the three things that decide it: how efficiently your fixture makes photons, how much of your tank's footprint it has to cover, and how deep the water is that they cross. Depth is the one people forget, and it is the one that dominates: the same light over a 50 cm deep tank delivers roughly half what it does over a 30 cm one, which is why tall tanks disappoint and shallow ones grow carpets on modest fixtures. The estimate is honest about its limits — optics, mounting height and a fixture's age each move it — so it gives a range, and it tells you when a manufacturer's PAR chart or a borrowed meter would answer better. Then it does the part a spec sheet cannot: it compares the light you have against the demand of the plants you actually keep, names the ones that will melt, and suggests a photoperiod that fits.
Common questions
- How many watts per litre does a planted tank need?
- None — that rule did not survive LEDs. A 30 W LED bar and a 30 W T8 tube put out completely different amounts of usable light, and depth changes the answer more than wattage does: the same fixture over a 50 cm deep tank delivers roughly half what it does over a 30 cm one. What plants respond to is PAR at the substrate, which is what this tool estimates.
- My plants look fine but I have algae — more light or less?
- Almost always less, or a shorter day. Algae is rarely a light shortage; it is light arriving faster than the plants can use it, because carbon or nutrients ran out first. Cut the photoperiod by an hour or two before changing anything else — it costs nothing, and it shows within a fortnight.
- Should I hang the light higher or sit it on the tank?
- It is a trade, not a mistake either way. On the rim gives you the most PAR at the substrate; 30 cm up costs roughly half of it. What height buys is evenness — a suspended fixture lights the corners nearly as well as the centre, which is what a carpet needs. Raise it if you have light to spare and a hotspot under the middle; keep it low if the tool says you are already short.