Grow Light DLI Calculator
Lux, PAR, and DLI for 20 popular houseplants — the light math most guides skip
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DLI Requirements for 20 Popular Houseplants
Data synthesized from Iowa State Extension, University of Florida IFAS, and published greenhouse research. Ranges are indoor-adapted, not commercial greenhouse.
| Plant | DLI min | DLI max | Light level | Typical window |
|---|---|---|---|---|
| Snake Plant (Sansevieria) | 1 | 4 | Low | North-facing, or 4-6 ft back from any window |
| ZZ Plant (Zamioculcas) | 1 | 4 | Low | North-facing, or interior rooms with indirect light |
| Pothos (Epipremnum) | 2 | 6 | Low-Medium | East-facing, or 2-3 ft back from south/west |
| Peace Lily (Spathiphyllum) | 2 | 6 | Low-Medium | North or East-facing |
| Philodendron | 3 | 8 | Medium | East-facing, or filtered South/West |
| Monstera Deliciosa | 4 | 10 | Medium-Bright | East or filtered South-facing |
| Fiddle Leaf Fig (Ficus lyrata) | 8 | 15 | Bright | South or bright East/West-facing, within 3 ft of window |
| Calathea / Prayer Plant | 2 | 6 | Low-Medium | North or East-facing, avoid direct sun |
| Spider Plant | 3 | 8 | Medium | East or filtered South-facing |
| Boston Fern | 2 | 6 | Low-Medium | North or East-facing |
| Rubber Plant (Ficus elastica) | 4 | 10 | Medium-Bright | East or filtered South-facing |
| Aloe / Cactus / Succulent | 12 | 25 | Very Bright | South or West-facing, direct sun |
| African Violet (flowering) | 8 | 15 | Bright | East-facing, or under grow light 12 hrs/day |
| Orchid (Phalaenopsis) | 6 | 12 | Medium-Bright | East-facing, filtered south |
| Bird of Paradise | 10 | 20 | Very Bright | South or West-facing, direct sun OK in morning |
| Alocasia | 4 | 10 | Medium-Bright | East or filtered South-facing |
| Anthurium | 6 | 12 | Medium-Bright | East or filtered South-facing |
| Hoya | 8 | 15 | Bright | East or South-facing, within 3 ft of window |
| String of Pearls / Dolphins | 10 | 20 | Very Bright | South-facing, direct morning sun |
| Chinese Evergreen (Aglaonema) | 1 | 4 | Low | North-facing, or interior rooms |
The Three Light Numbers — What They Actually Mean
Lux measures illuminance weighted to human eye sensitivity. It is what your phone's light-meter app reads. Useful as a rough proxy but not what plants actually photosynthesize.
PPFD (µmol/m²/s) measures photon flux density in the photosynthetically active range (400-700nm). This is what plants actually respond to. Measured by PAR meters ($300-1500) or approximated from Lux using the conversion factors above.
DLI (mol/m²/day) integrates PPFD over a 24-hour photoperiod. Formula: DLI = PPFD × photoperiod_hours × 3600 / 1,000,000. A low-light plant surviving at 50 PPFD for 12 hours receives 2.16 mol/m²/day DLI — exactly in its survival zone.
Honest Limits
- DLI ranges are averages across species — individual cultivars vary by 20-30%.
- Lux-to-PPFD conversion assumes full-spectrum light. Narrow-band LEDs (blurple / pink) have different conversion ratios.
- Window glass blocks 10-40% of light, more for UV-coated or tinted windows. Outdoor readings are not directly usable indoors.
- I am a developer and houseplant hobbyist, not a horticulturist. For commercial growing or plant pathology, consult a licensed extension agent.
FAQ
What is DLI and why does it matter for houseplants?
DLI (Daily Light Integral) is the total amount of photosynthetically active light a plant receives in a 24-hour period, measured in mol/m²/day. It is the single most useful light number for houseplants because it captures both intensity AND duration — a bright windowsill for 2 hours delivers roughly the same DLI as a dim corner for 8 hours. Iowa State Extension publishes DLI ranges for common plants: low-light plants do fine at 1-4 mol/m²/day, while flowering plants often need 10-20.
How do I convert Lux to PAR or DLI without expensive equipment?
For sunlight coming through a window, divide Lux by roughly 50-55 to estimate PPFD (µmol/m²/s) — this is the standard approximation used in Iowa State extension materials. For fluorescent or white LED grow lights, divide by 70-75 instead. Once you have PPFD, multiply by photoperiod hours × 3600 / 1,000,000 to get DLI. The calculator on this page does the math for you, but the conversion factors are public and in the research.
Why does my north-facing window plant look sad even though it gets light all day?
North-facing windows in the Northern Hemisphere typically deliver only 100-800 Lux during most of the day — that works out to roughly 2-15 µmol/m²/s PPFD, or about 0.1 to 1 mol/m²/day DLI over a 12-hour photoperiod. That is genuinely low-light territory where only species like Snake Plant, ZZ Plant, and Cast Iron Plant will actually thrive. Pothos can survive but won't grow fast. Flowering plants, Monstera, and calatheas will progressively decline over months. The fix is either rotating the plant weekly to a brighter spot, or supplementing with a 20-30W LED grow light on a 12-hour timer.
Do grow lights actually produce the PPFD they claim?
Sometimes. Reputable brands (Soltech, Mars Hydro, Spider Farmer) publish PPFD maps measured at specific distances. Cheaper Amazon grow lights often overstate PPFD by 30-60% because they measure at 2 inches from the diode rather than at plant height. Rule of thumb: for a 12-hour photoperiod, a 20W full-spectrum LED at 18 inches gives roughly 200-300 µmol/m²/s PPFD directly under, translating to about 8-12 mol/m²/day DLI — enough for flowering houseplants like African Violets and most herbs, overkill for low-light foliage plants.
What is the difference between a Lux meter and a PAR meter?
A Lux meter measures light intensity weighted to human eye sensitivity — it gives you illuminance in Lux. A PAR meter (or PAR quantum sensor) measures photosynthetically active radiation in the 400-700nm range, weighted to plant photosynthesis — it gives you PPFD in µmol/m²/s. PAR meters (Apogee, LI-COR) cost $300-1500; Lux meters cost $20-50 or are built into every smartphone. For sunlight and white LEDs, Lux-to-PPFD conversion is accurate enough that most hobby growers can skip the PAR meter entirely.
Is one of my houseplants getting too much light?
Signs of too much light: faded or bleached leaves (loss of chlorophyll), crispy edges that do not respond to watering, leaf curl toward or away from the light source, and for variegated plants, burn marks on the white sections first. DLI above about 25 mol/m²/day will stress most indoor-adapted plants, even full-sun species. Move the plant 1-2 feet further from the window, add a sheer curtain, or rotate it so it gets filtered light instead of direct afternoon sun. Calathea, Philodendron, and Fern species burn especially fast at high DLI.
Related Tools
- Watering Schedule Generator — factors light into watering frequency.
- Light Requirement Matcher — simpler lookup: window direction → which plants thrive.
- AI Plant Identifier Comparison — once you know the plant, match its light needs here.