Your lux meter or phone reads lux — the plant cares about PPFD. Pick your light type and read the value straight off. Free, no sign-up.
The conversion factor depends on the spectrum — which is why light type is the decisive input here. The same lux reading means a different number of photons under HPS than under LED. Meter reads foot-candles? Multiply by 10.764 to get lux.
An approximation — a proper quantum meter stays more accurate.
| Light type | Divide lux by |
|---|---|
| LED Full Spectrum | ÷ 67 |
| HPS | ÷ 82 |
| CMH / LEC | ÷ 62 |
| CFL | ÷ 75 |
| T5 | ÷ 72 |
These calculators give you reference values for your own judgement. They replace neither a measurement on site nor expert advice — all figures without guarantee.
Lux measures brightness as the human eye sees it. Our eyes are most sensitive to green and barely register red and blue — exactly the two bands a plant does the most with. So a lux meter weights light wrongly the moment you want to know what actually reaches the plant.
PPFD (photosynthetic photon flux density) counts the light particles landing on a square metre each second instead — and only those in the photosynthetically usable range from 400 to 700 nanometres. It is given in µmol/m²/s. That is the unit grow lights and lighting plans speak in.
There is no universal conversion factor from lux to PPFD — it depends on how the light is spread across the wavelengths. A sodium lamp puts out heavily in the yellow-orange, where the eye is sensitive: it produces a lot of lux per photon, so the factor is high. A full-spectrum LED spreads light more widely and delivers more usable photons at the same lux reading.
That is why the table gives a separate value for each light type. Pick the wrong one and you can be twenty percent out — the difference between decent lighting and not enough of it.
The factors are approximations for typical fixtures, and they scatter considerably within one type. Full-spectrum LEDs are quoted anywhere between roughly ÷54 and ÷69 depending on the model; the 67 used here sits at the cautious end. "CMH" is not one thing either: the 3100 K agro version lands around ÷59, a 4200 K lamp around ÷65. The factor 62 used here sits between the two.
In practice that means: the manufacturer's data beats every rule of thumb. Many lighting makers publish PPFD maps or spectral curves for their models — if you have them, use them instead of a table value. This calculator is for everyone who does not.
A lux meter is cheap and measures brightness for the human eye. Together with the right factor it gets you a usable estimate — good enough to set hanging height and to find where a fixture falls off at the edges.
A PAR or quantum meter measures photons directly and needs no conversion factor at all. It costs several times as much, but it is the only method that holds true regardless of spectrum. If you want to compare fixtures of different types, there is little way around one — that is precisely where every conversion factor fails.
A word on CO₂: higher PPFD only pays off while the plant also gets enough carbon dioxide. Without enrichment the ceiling usually sits around 900 µmol/m²/s — beyond that more light no longer raises yield, only the electricity bill.
Measure at canopy height, not on the floor and not right at the lamp. Hold the sensor level, measuring face up towards the light — tilted, you will read consistently low.
Take several points: in the centre under the lamp and at the corners of the footprint. Edge fall-off is considerable on most fixtures, and a single reading from the middle suggests an evenness that is not there. And step aside — your own shadow skews the reading.
Phone apps as lux meters are better than nothing but imprecise: the camera sensors are not calibrated, and readings vary noticeably between devices. Fine for a rough picture, not for fine-tuning.
Once you have the PPFD figure, the next step is the daily total: only together with the photoperiod does it tell you how much light the plant actually gets. That is what the PPFD to DLI calculator works out — and it names the number of hours behind every reference value.
The lux reading is divided by a factor that depends on the light type: roughly 67 for full-spectrum LED, 82 for HPS, 62 for CMH/LEC, 75 for CFL and 72 for T5. Example: 40,000 lux under an LED comes to about 597 µmol/m²/s.
Multiply foot-candles by 10.764 to get lux, then use the calculator above. One foot-candle is one lumen per square foot, one lux is one lumen per square metre, and a square metre holds 10.764 square feet. Example: 3,000 fc equal 32,292 lux, which under a full-spectrum LED comes to roughly 482 µmol/m²/s. Everything else in the conversion stays the same — the light type still decides.
Because lux weights brightness by the sensitivity of the human eye, while PPFD counts photons in the 400–700 nm range. How the two relate depends on the lamp's spectrum — and HPS is a completely different one from LED. Even within one type the values scatter: full-spectrum LEDs sit anywhere between roughly ÷54 and ÷69 depending on the model.
Accurate enough in practice to judge hanging height and coverage — but it stays an estimate. If you have spectral data or a PPFD map for your fixture, use those. If you want to compare different light types directly, you need a PAR meter; no conversion factor gets you there.
As a guide: seedlings and clones roughly 200–400 µmol/m²/s, vegetative growth around 400–600, flower about 600–900. Higher values usually only pay off with added CO₂. Even coverage across the whole footprint matters more than the peak reading in the centre.
In principle yes, but only as a rough guide. The light sensors in smartphones are not calibrated, and readings differ noticeably between devices. For fine-tuning hanging height, a lux meter or a quantum sensor is the better choice.
PPFD is a snapshot — how much light is arriving right now. DLI (daily light integral) adds that up across the whole photoperiod of a day and is given in mol/m²/d. What counts for the plant in the end is the daily total, not the instantaneous value. You can convert both ways in the PPFD to DLI calculator.
Yes. This calculator uses the same conversion factors as the lux to PPFD calculator in the CRIS app. There it sits right next to the grow journal, so measured values land on the right entry straight away.
Lux → PPFD, VPD, DLI, pH, EC and more — in the CRIS app, right next to your grow journal, fully offline and without an account.