Lux and PPFD: How to Convert Between Light Units for Your Plants

If you’re growing plants indoors, you’ve probably seen lights advertised in lumens or lux—but then heard that plants don’t care about lux, they care about PPFD. So what gives? Why are there two different units, and how do you convert between them?

In short: lux measures light as humans see it, while PPFD measures light as plants use it. The conversion depends on the type of light source you’re using—and in many cases, it’s better to skip the conversion and measure PPFD directly.

Here’s what you need to know.

What is Lux? (Human Vision)

Lux is a unit of illuminance—the amount of visible light falling on a surface, weighted to match how the human eye perceives brightness. It takes into account that our eyes are most sensitive to green-yellow light (around 555 nm) and less sensitive to blue and red .

1 lux = 1 lumen per square meter

In the US, you may also see foot-candles (1 foot-candle = 1 lumen per square foot ≈ 10.76 lux)

Lux is great for describing how bright a room feels to a person. But plants don’t see light the same way we do.

What is PPFD? (Plant Vision)

PPFD stands for Photosynthetic Photon Flux Density. It measures the number of photons in the 400–700 nm wavelength range—the exact range plants use for photosynthesis—that hit one square meter each second .

It’s expressed in µmol/m²/s (micromoles per square meter per second).

For comparison:

Full summer sunlight at noon delivers about 2,000 µmol/m²/s PPFD, which equals roughly 108,000 lux

A typical grow light might provide 150–500 µmol/m²/s depending on the setup

The key difference: PPFD counts every photon in the PAR range equally, regardless of color, because each photon has the potential to drive photosynthesis.

Why You Can’t Just Use One Conversion Factor

Here’s the catch: lux and PPFD measure different things, and the relationship between them changes depending on the light spectrum.

If you have 10,000 lux from sunlight and 10,000 lux from an HPS lamp, the PPFD will be different because:

Sunlight has a broad spectrum including plenty of blue and red

HPS lamps emit strongly in the yellow-orange range where human eyes are sensitive, but with relatively less blue

This means a lux meter can give you a rough estimate, but it will be most inaccurate for light sources that are heavy in red or blue—which many grow lights are .

Common Conversion Factors (Lux → PPFD)

To convert lux to PPFD, multiply your lux reading by the conversion factor for your light type :

Examples:

30,000 lux of sunlight ≈ 30,000 × 0.0185 = 555 µmol/m²/s

30,000 lux from full-spectrum LED ≈ 30,000 × 0.0185 = 555 µmol/m²/s

30,000 lux from HPS ≈ 30,000 × 0.0122 = 366 µmol/m²/s

Going the Other Way: PPFD → Lux

If you know the PPFD and want lux, use these factors :

Example: 500 µmol/m²/s from an HPS lamp ≈ 500 × 83 = 41,500 lux

A Quick Rule of Thumb for White LEDs

For white LEDs (common in home grow setups), a widely cited rule of thumb is: 1,000 lux ≈ 15 µmol/m²/s PPFD .

This is based on the spectral characteristics of typical white LEDs and is accurate to within about 7% for most practical purposes. It’s a handy shortcut if you don’t have a table handy.

Foot-Candles? No Problem

If you’re working with foot-candles instead of lux, the same principle applies. Multiply foot-candles by the conversion factor for your light source :

Example: 10,000 foot-candles of sunlight ≈ 10,000 × 0.200 = 2,000 µmol/m²/s

The Bottom Line: Should You Convert?

For rough estimates, yes. If all you have is a lux meter, these conversion factors will get you in the ballpark.

But if you’re serious about plant growth, measure PPFD directly. A quantum sensor (PAR meter) gives you the actual photon count in the 400–700 nm range, without relying on spectral assumptions. Light sources vary widely—even between different brands of “full-spectrum” LEDs—and the conversion factors are only approximate .

The table below shows the practical difference: the same 30,000 lux yields very different PPFD values depending on the source :

So use the conversion when you need to, but invest in a PAR meter or a smartphone app that measures PPFD directly (like Photone) for better accuracy . Your plants will thank you.