Light Planner

Tools
How much light is hitting the canopy?
Canopy PPFD700
1001,600 µmol/m²/s

Measured at the top of the canopy with a quantum sensor (a phone lux app is a rough stand-in at best).

What stage is she in?
Hours of light per day12 h
6 h24 h
Daily Light Integral
30.2
mol/m²/day (DLI)
In the zone
700
PPFD µmol/m²/s
42
Est. flower yield g/m²
Where you sit for flower
What to do about it
Is more light worth it? flower
Push PPFD up to1,000
from your current PPFD1,600
+€0
0 g
Extra bud
€0
Extra value
€0
Extra power
How this works (and what it can't tell you)

The yield numbers come from a real trial: Rodriguez-Morrison et al. (2021) grew 384 cannabis plants at canopy light levels from 120 to 1,800 µmol/m²/s. The headline finding surprises people — yield rose in a straight line the whole way, with no plateau. Twice the light, near enough twice the bud, right up to the highest level they tested.

So why do the stage bands above stop calling it "ideal" past ~1,000? Because the ceiling isn't the plant — it's everything around the plant. Push PPFD up and she needs more water, more nutrients, more airflow and ideally added CO₂ to use it. Run her hot and hungry under a big light and you'll bottleneck somewhere else and blame the light. The bands are about what most home tents can actually feed, not the plant's limit.

More light does not mean stronger bud. The same trial found THC and CBD percentages didn't move with light at all. You get more grams at the same strength — a bigger pile, not a stronger one.

The DLI (Daily Light Integral) is just your PPFD added up over a day's lighting hours. It's the honest way to compare setups on different photoperiods: 1,000 PPFD for 12 h and 750 PPFD for 16 h deliver almost the same daily light.

Honesty notes: the flower band is research-backed; the seedling and veg bands are conventional grower ranges, not from this paper. The yield figure is a flower-cycle estimate from the trial's regression (Y = 0.0122·PPFD + 33.8, R²=0.59) — a fair guide, not a promise; your genetics, training and feed move it. The fixture-estimate mode is exactly that — an estimate; a €15 quantum sensor pays for itself. Source: Rodriguez-Morrison, Llewellyn & Zheng (2021), Frontiers in Plant Science, doi.org/10.3389/fpls.2021.725078 (open access).

Light is the input most new growers get backwards — too little in flower, too much on seedlings. The light planner turns your PPFD, measured or estimated from your fixture, plus the stage and your hours on, into DLI: the total daily light your plant actually gets. It tells you whether that is short, in the zone, or into the territory where extra light just costs you electricity for no more bud. If you are weighing up a bigger light, it sets the extra grams against the extra power, so the call is made on numbers rather than the wattage printed on the box.

Questions

What are PPFD and DLI?

PPFD is how much usable light lands on your canopy right now; DLI is the total over the whole day — PPFD across your hours of light. Plants respond to the daily total, so DLI is the number that matters. The planner works it out from your PPFD and hours so you do not have to do the maths.

What light level should I aim for?

The planner judges by PPFD at the canopy for each stage: roughly 150 to 350 for seedlings and clones, 300 to 600 through veg, and 600 to 1000 in flower. On the usual 18/12 hours that is a DLI of about 10 to 23, then 19 to 39, then 26 to 43. For seedlings and veg it flags light that is more than needed; in flower, above 1000 is the high-yield range, which pays back only if your CO2, airflow and feed can match it.

I have no PPFD meter — can I still use it?

Yes. Give it your fixture's PPF in µmol/s from the spec sheet — not the wattage — along with your tent width and depth and how much of the light reaches the canopy, and it estimates the PPFD for you. An estimate is rougher than a reading — a cheap meter or a phone sensor sharpens it — but it is enough to tell whether you are miles off or roughly in the zone, which is usually the question.

Will a bigger light grow more?

Only up to a point, and after that it just raises the bill. Past the plant's light ceiling the extra photons do not become extra grams — they become heat and electricity. The planner weighs the likely extra yield against the extra power, so you can see whether the upgrade pays before you buy it.