Aquarium Lighting Calculator
How much light a planted tank needs depends on the plants you want to grow, not the fish. This tool converts your tank size and a target light level into a lumen range and an approximate LED wattage. It also explains PAR — the measurement that actually matters — and why the old watts-per-gallon rule no longer works in the LED era.
Find your lighting target
How this calculator works
Hobbyist guidance for LED-era planted tanks commonly lands around 10–20 lumens per litre for low light, 20–40 for medium, and 40–60+ for high light. The calculator takes the midpoint of your chosen band, applies a ±25% range, adds ~35% for tall tanks (light falls off steeply with depth), and converts lumens to watts using a typical modern LED efficacy of ~90 lumens per watt.
PAR: the number that actually matters
Plants don’t use lumens — lumens weight light by human eye sensitivity. Plant growth tracks PAR (photosynthetically active radiation), measured as PPFD in µmol/m²/s at the substrate. Rough hobby bands: 15–30 low, 30–80 medium, 80+ high. Reputable planted-tank fixtures publish PAR-at-depth charts; when a fixture offers one, trust it over any lumen math, including ours. Lumens are the useful fallback because nearly every light lists them.
Why watts-per-gallon died
The old rule — 1–2 W/gal low, 2–3 medium, 3–5 high — was written for T8/T12 fluorescent tubes. A watt of modern LED produces roughly 3–5× the usable light of a watt of 1990s fluorescent, so applying the rule to an LED fixture over-lights a tank spectacularly. If you see watts-per-gallon advice online, check the date and the technology it assumes. For fluorescent fixtures the old rule is still roughly valid; for LEDs, use lumens or PAR.
Light, CO2 and algae — the triangle
Light sets the demand for CO2 and nutrients. Push high light onto a tank that can’t supply matching CO2 and fertiliser, and algae — which thrives on imbalance — takes the surplus. This is the single most common planted-tank failure: buying a powerful light “to be safe”. If you don’t inject CO2, medium light is the sensible ceiling, 6–8 hours a day on a timer. It is far easier to grow healthy plants under modest light than to fight algae under strong light.
Frequently asked questions
How many hours should my aquarium light run?
6–8 hours for most planted tanks, on a timer for consistency. New setups often start at 6 to starve algae during the unstable first weeks. Fish-only tanks can run whatever schedule suits viewing — fish need a day/night rhythm, not intensity.
Does colour temperature (Kelvin) affect plant growth?
Far less than intensity does. Anything from ~5,000K to ~8,000K grows plants well; choose by the look you prefer. Full-spectrum fixtures with red+blue peaks score slightly better on PAR-per-watt, but no Kelvin choice rescues inadequate intensity.
Can I use a regular household LED bulb or shop light?
Yes, especially over open-top or budget setups — a 6,500K household LED grows low-medium light plants fine. You lose waterproofing, spread optimisation and PAR data, but the photons are real. Compare by lumens.
My plants melt even though the numbers say my light is right. What gives?
Light is one leg of three. Check CO2 (see the CO2 estimator), fertilisation, and whether the plant was grown emersed at the farm (melting and regrowing is normal for those). Also verify your fixture’s real output — cheap fixtures often deliver half their claimed lumens.