There is no single conversion factor. Lux is what your eye sees, PPFD is what the leaf counts, and the ratio between them is set by the light source. So this converter makes you name the source, and tells you when the answer is solid and when it is a guess.
185 µmol·m⁻²·s⁻¹ PPFD
Over 8 hours that is 5.3 mol·m⁻²·day⁻¹ DLI.
Sunlight is the one case where this conversion is dependable, because its spectrum barely changes. A reading taken at a window is a sunlight reading.
Factors from Apogee Instruments, a maker of PAR sensors. The white LED option is not theirs and is not in the table below: it is our midpoint of a band, and the reasoning is in when the answer is solid.
A lux meter weights every wavelength by how bright it looks to a human eye, which peaks in the green and falls away sharply at both ends. A PAR meter counts photons between 400 and 700 nm and does not care what colour they are. Red photons drive photosynthesis and barely register as brightness; the eye is most alive to exactly the green light that leaves reflect away.
So the ratio between the two numbers is a property of the lamp, not of physics. Two lights that read identically on a lux meter can hand a plant noticeably different photon counts, and a converter that offers one number without asking about the source has not removed that problem, only hidden it.
| Light source | Lux per µmol·m⁻²·s⁻¹ | 10,000 lux comes to |
|---|---|---|
| Sunlight | 54 | 185 PPFD |
| Ceramic metal halide, CMH930-Agro | 59 | 169 PPFD |
| Ceramic metal halide, CMH942 | 65 | 154 PPFD |
| Metal halide | 71 | 141 PPFD |
| Cool white fluorescent | 74 | 135 PPFD |
| High pressure sodium | 82 | 122 PPFD |
The bottom row is the clearest case. High pressure sodium pours output into the yellow and orange, which is close to where the eye is most sensitive, so it reads bright for the photons it actually delivers. Reading a sodium lamp with a lux meter and a sunlight factor overstates the light reaching the plant by about half.
For a houseplant on a windowsill this whole problem goes away. The light is sunlight, the spectrum is stable, and 54 is a dependable divisor. If your plant is at a window, take the reading and trust the conversion.
Under a grow light it is a different situation, and the honest answer is that a lux meter cannot settle it. White LED is the hardest case, because two lamps sold under the same description can carry quite different spectra: the plausible band runs from about 65 to about 75 lux per micromole, bracketed by Apogee's own figures for agricultural ceramic metal halide at one end and cool white fluorescent at the other. We set the LED option to 70, in the middle of that band, and you should read the result as give or take roughly seven per cent rather than as a measurement.
If the number matters to you, the way out is not a better converter. It is the manufacturer's PPFD chart for that fixture at that hanging height, or a PAR meter. Both measure photons instead of inferring them.
PPFD is an instant: photons arriving right now. Plants respond to the total over a day, which is why the calculator also gives DLI, the daily light integral in moles per square metre. A modest PPFD over a long day can beat a bright burst over a short one, and that is the number most published growing guidance is really about.
Neither number describes a spot on its own. The same windowsill can be a good place in April and a poor one in December, and a spot that has the light can still be wrong on temperature, humidity or airflow. If you want that answered for one specific place in your home rather than converted, the free spot checker scores a spot on all four, and the lux lookup turns a reading into the tier it lands in. What real rooms actually measure is published as open data, and running that through this converter is worth doing once: the median indoor reading in it, 2,349 lux, comes to about 44 PPFD, which over an eight-hour day is 1.25 mol·m⁻²·day⁻¹. Put your own reading in above and see where your spot sits against that.
Only if you know what the light source is. Lux weights every wavelength by how bright it looks to a human eye, and PPFD counts photons between 400 and 700 nm regardless of colour. Two lamps that read the same in lux can deliver very different photon counts, so a converter that offers a single number is hiding the assumption rather than removing it.
About 54 lux per micromole per square metre per second, so lux divided by 54 gives PPFD. That figure comes from Apogee Instruments. It is the factor to use for a plant on a windowsill, because the light coming through the glass is sunlight.
Because its spectrum is different. Apogee publishes 82 lux per micromole for high pressure sodium, 74 for cool white fluorescent, 71 for metal halide and 59 for one agricultural ceramic metal halide. A high pressure sodium lamp puts a lot of output where the eye is most sensitive, so it reads bright in lux for the photons it actually delivers.
Daily light integral is the total number of photosynthetically active photons a spot receives in a day, in moles per square metre. It is PPFD multiplied by the number of seconds of light, divided by one million. A steady 200 PPFD for eight hours is about 5.8 mol per square metre per day.
Well enough to compare one spot with another, not well enough to quote to three digits. Phone sensors are uncalibrated and sit behind cover glass. Treat the output as a band rather than a figure, which is the same advice we give for lux readings anywhere on this site.
Yes, with the same caveat. Multiply PPFD by the factor for your source to get lux. It is the same assumption running the other way, so it inherits the same uncertainty.
That is the same number upside down. One divided by 54 is 0.0185, so multiplying lux by 0.0185 and dividing lux by 54 are the same operation. Foot-candles bring a second conversion with them, since one foot-candle is 10.764 lux.