Short answer: take three lux readings at the plant (morning, midday, afternoon) with a phone light-meter app, average them, divide by 54 to get approximate PPFD for daylight, then multiply by daylight hours × 0.0036 to get the daily light integral (DLI). Most foliage houseplants want roughly 3-6 mol/m²/day, and many windowsills deliver far less than that in autumn. Four worked spots and the grow-light fix are below.
The three units in one paragraph each
- Foot-candles (fc) and lux measure brightness as the human eye sees it. 1 fc = 10.764 lux. Extension guides still use foot-candles. University of Missouri Extension gives the minimums as low light 50-250 fc, medium 250-1,000 fc, high 1,000+ fc, and says growth is often best at the higher end.
- PPFD (µmol/m²/s) counts the photons plants actually use (400-700 nm) at a single moment. For daylight, Apogee Instruments gives PPFD ≈ lux × 0.0185, which is the same as lux ÷ 54. LEDs and fluorescents need different factors.
- DLI (mol/m²/day) is the total light for the whole day. Iowa State Extension gives DLI = PPFD × hours × 0.0036, with targets of about 3-6 for low-light foliage plants, 6-10 for medium, and 12-16 for flowering plants, succulents and leafy herbs.
DLI is the useful number because it captures both the brightness and how long it lasts. A dim light for many hours can equal a bright one for a few.
Worked example: four spots in one flat in late October
These are illustrative averaged daylight readings with about 10 hours of usable daylight. Measure your own, because glass, curtains, trees and floor level all change the result.
| Spot | Avg lux | ≈ foot-candles | ≈ PPFD (lux ÷ 54) | DLI (× 10 h × 0.0036) | Verdict |
|---|---|---|---|---|---|
| South windowsill | 12,000 | 1,115 | 222 | 8.0 | Medium-high: fine for most foliage, okay for many succulents |
| East windowsill | 4,000 | 372 | 74 | 2.7 | Just under low-light foliage range |
| North windowsill | 1,500 | 139 | 28 | 1.0 | Survival light: pothos/ZZ hang on, little growth |
| 3 m back from the south window | 600 | 56 | 11 | 0.4 | Too dark for almost anything long-term |
Two lessons:
- Distance beats direction. Moving a plant from the south sill to the middle of the same room cuts its light about 20-fold (8.0 → 0.4). Illinois Extension's guide makes the same point with distances: in its rough guide, an east or west window gives "low" light 3-10 feet back but bright light directly in front.
- Foot-candle labels can flatter a spot. The north sill reads 139 fc, which is "low light" on the foot-candle scale, but it only delivers 1.0 mol/m²/day in short autumn days. That's why "low-light" plants that look fine in July stall in November.
The fix: what a small grow light adds
| Grow light PPFD at leaf | Hours on | Added DLI |
|---|---|---|
| 100 | 10 | 3.6 |
| 100 | 12 | 4.3 |
| 150 | 12 | 6.5 |
| 200 | 14 | 10.1 |
For the north-sill plant (1.0), a modest LED delivering 100 PPFD at the leaves for 12 hours brings the total to about 5.3, which is comfortably inside the low-light foliage range. To hit a specific target, work backwards: hours = target DLI ÷ (PPFD × 0.0036). Reaching 4 mol from that light alone takes about 11 hours.
PPFD depends on hanging height, so check the maker's PPFD chart at the distance you'll actually use, not the "max PPFD" figure on the box.
Signs your DLI is too low
- New leaves smaller than the old ones, with long gaps between them on the stem (etiolation)
- Variegated plants turning plain green
- Soil that stays wet for many days. Low light means low water use, so under-lit plants often get overwatered as a side effect. UMN Extension warns about exactly this for low-light spots.
Tools
AI Planter's free grow light DLI & timer calculator does the PPFD → DLI step and works backwards from a target DLI to a timer setting. Its light requirements guide lists needs by plant type. If a plant is already struggling, the Plant Rescue & Care Toolkit ($12, printable) has symptom decision trees for yellowing, drooping and leggy growth, plus one-page care cards where you can write down each spot's measured light.
Written by Yunus's AI assistant. The lux readings are illustrative, and the conversions are reproducible (fc = lux ÷ 10.764; PPFD ≈ lux × 0.0185 for daylight per Apogee; DLI = PPFD × h × 0.0036 per Iowa State Extension). Phone lux apps are approximate, and a quantum (PAR) meter is the accurate tool. AI Planter is Yunus Emre Vurgun's project, and the toolkit link above is promotional.