Lux → PPFD for Houseplants (When It Works, When It Doesn’t)

If you’ve ever measured 6,000 lux on your windowsill and thought “so…is that enough PPFD?”, you’ve hit the core problem: lux is for human eyes, PPFD is for leaves. The trick is knowing when a conversion is good enough—and when to skip it and measure PPFD directly. Why this piece now? Most guides list one fixed conversion, but skip spectrum, window aspect, and DLI (total daily light), which decide whether your plants actually grow.

Lux→PPFD can be a useful estimate under white LEDs or daylight if you assume a reasonable factor (often ~0.015–0.020 µmol·m⁻²·s⁻¹ per lux). It breaks down with pink/blurple spectra, tinted glass, or colored rooms. When in doubt, measure PPFD at the leaf and set targets by DLI. 

Plain terms: PPFD = “how many plant-useful photons hit the leaf each second.” DLI = “how many arrive across the whole day.” Iowa State explains why PPFD is the best intensity unit for indoor plants and how to compute DLI. 

Myth vs Fact: Lux→PPFD in real homes 

Myth: “There’s a universal conversion factor.”

Fact: The factor depends on spectrum (SPD), room reflections, and glazing. White LEDs and daylight cluster in a similar ballpark; “blurple” or narrow-band fixtures do not. Reputable instrument makers show that factors vary and give examples to illustrate the spread. 

Myth: “Phone lux apps give me PPFD.”

Fact: Apps read lux (eye-weighted) unless they explicitly model PAR; some estimate PPFD after you choose a light type. Use them for evenness and relative changes; confirm plant-level PPFD or compute DLI from PPFD to guide care. 

Myth: “If lux is high, I’m covered.”

Fact: 12,000 lux from a warm LED can yield less PPFD than 8,000 lux of cool daylight hitting the same leaf. The DLI you deliver (PPFD × hours × 0.0036) is what predicts growth. 

“In my flat, I’d rather run a steady 90 PPFD for 12 h than chase peaks. The leaves tell you when it’s right.”

A pragmatic plan that works indoors 

1) Pick the right metric today. If you have a PAR/PPFD reading, use it. If not, use lux as a ballpark only under white LEDs or daylight; avoid conversions for magenta fixtures.

2) Set safe starting bands. For foliage aroids and many houseplants, steady 60–100 PPFD for 10–12 h is a gentle, growth-positive baseline in small apartments; raise +10 PPFD every 1–2 weeks only if new leaves stay happy. For genus-specific ranges and presets, see our Aroid Light & PPFD Guide. 

3) Think in DLI, not just peaks. Compute DLI = PPFD × hours × 0.0036. If winter growth stalls, extend daylength before cranking intensity to avoid hot, crispy margins near radiators or dry air. 

“When London winter drops my window to ~5 DLI, I add two hours on the timer instead of pushing intensity—new leaves stay smooth.”

Numbers you can use now (with one “white-LED” assumption) 

Context first: The table below uses a commonly seen white-LED/daylight example factor to illustrate the math. Real-world results vary by spectrum and room, so treat this as a starting estimate—then adjust by leaf response. For factors and why they differ, see Apogee. 

Midday lux (leaf)	Approx. PPFD (µmol·m⁻²·s⁻¹)	DLI @ 12 h (mol·m⁻²·day⁻¹)
1,000	18.5	0.80
3,000	55.5	2.40
5,000	92.5	4.00
8,000	148	6.39
12,000	222	9.59

How we computed it: PPFD ≈ lux × 0.0185 (example white-LED/daylight factor), then DLI = PPFD × hours × 0.0036. Iowa State’s guide covers the DLI formula; Apogee discusses conversion factors. Badges: [Academic], [Manufacturer spec]. 

A safe “starter rule” for mixed rooms 

Box:

• Midday 5k–8k lux at the leaf under white LED or bright east window ≈ ~90–150 PPFD.
• Run 10–12 h for ~4–6.5 DLI; extend to 14 h in deep winter before raising intensity.
• Keep leaf surface ≤ 28 °C (82 °F) and RH ~55–65 % to tolerate the top of the band.
• Adjust in +10 PPFD steps every 1–2 weeks.

For species that care about pattern stability, see Anthurium clarinervium indoor care to translate lux/PPFD into mix, moisture, and distance settings. 

Third-party experiences

Common pattern: Growers who track DLI (not just lux) report steadier foliage without crispy margins.

Frequent pitfall: Trusting a single conversion factor indoors leads to over-lighting in dry rooms near radiators or west windows.

What works for many: Gentle 80–100 PPFD with 12 h photoperiod + moderate RH beats chasing greenhouse numbers for foliage plants.

Anonymised statement (paraphrased): “My cabinet read 10k lux but leaves still shrank—switching to PPFD readings and extending hours fixed it.”

Anonymised statement (paraphrased): “The same lux under two bulbs gave different growth; the cool-white bar outgrew the warm one.”

Transparency: Experiences above were compiled from public (international) forums, Reddit and public Facebook groups; these are not this site’s own hands-on trials.

Science in one paragraph (accessible):

Lux and foot-candles track perceived brightness, not plant-useful photons. PPFD measures the photons that actually hit the leaf, and DLI sums them across the day: DLI = PPFD × hours × 0.0036. That’s why PPFD (and DLI) guide indoor targets better than lux alone. 

Authority quote: “PPFD is…one of the most useful units of measure used to determine the light intensity needs of an indoor plant.” — Iowa State University Extension. 

Q&A mini-guide (real questions, concise) 

How do I use a lux app without lying to myself?

Measure at leaf height midday, assume a white-LED/daylight factor only, convert to PPFD, then compute DLI. If growth stalls, extend daylength before raising intensity. 

What PPFD should I aim for with foliage aroids on a shelf?

Start 60–100 PPFD for 10–12 h; increase in +10 PPFD steps if new leaves match size/quality and stay unscorched. See our Aroid Light & PPFD Guide. 

Is 12,000 lux too much by a south window?

Not necessarily—diffuse and watch leaf temperature. Aim to keep peaks comfortable (≤ ~130 PPFD) and extend photoperiod for DLI instead of cranking intensity into dry air.