Every houseplant care guide uses vague light categories — "bright indirect," "low light," "partial shade." Your phone has a lux meter built in (via free apps), and it gives you an actual number. This converter turns the number into the category and tells you which plants fit your spot.
Lux → light category converter
Use any free lux meter app on your phone. Hold the phone at the plant's location, sensor facing the light source, at midday. Enter the number below.
East-facing window, behind a sheer curtain on south/west. The sweet spot for most tropical houseplants — enough light to grow well, not enough to scorch leaves.
- Monstera deliciosa
- Calathea / Goeppertia
- Prayer plant
- Anthurium
- Rubber plant
- Aluminum plant
How to take a useful reading
A few minutes of effort produces a number you can actually plan around:
- Install a free lux meter app. "Light Meter" (iOS) and "Lux Light Meter Free" (Android) are both decent. Either uses your phone's ambient light sensor.
- Take the reading at the plant's actual location — not on the windowsill if the plant sits 3 feet back. Sensor faces the light source.
- Take it at solar noon ± 2 hours on a clear day. Morning and evening readings vary too much to compare.
- Take readings across the day if the spot gets direct sun at certain hours. A spot that's 2,500 lux at 10am and 25,000 lux at 2pm is technically a "bright indirect" location that gets a daily blast of "direct sun" — relevant for fiddle leaf figs and other sun-sensitive plants.
The categories
Calibrated to standard horticultural ranges referenced in extension sources. NC State Extension uses this framework for their Plant Toolbox cultural conditions:
| Lux range | Foot-candles | Category | Notable plants |
|---|---|---|---|
| 250–800 | 25–75 | Very low / deep shade | Parlor palm, ZZ, snake plant |
| 800–2,200 | 75–200 | Low / dappled | Pothos, philodendron, peace lily, spider plant, Boston fern |
| 2,200–5,500 | 200–500 | Medium / bright indirect | Monstera, calathea, prayer plant, anthurium, rubber plant |
| 5,500–10,800 | 500–1,000 | Bright indirect to some direct | Fiddle leaf fig, pilea, air plants |
| 10,800+ | 1,000+ | Direct sun | Jade plant, cacti, succulents, citrus, herbs |
Why "bright indirect" doesn't have a clear number
Most plant care guides won't commit to a specific lux value because "bright indirect" depends on the species and the duration. Three hours at 8,000 lux is functionally different from twelve hours at 3,000 lux even though the totals are similar.
The calculator above uses the rounded thresholds that align with NC State's cultural condition categories (deep shade under 2 hrs direct sun, partial shade 2–6 hrs, full sun 6+ hrs). Combined with a midday lux reading, it gives a reliable category for most indoor settings.
What the numbers mean for common spots
Approximate ranges for typical indoor locations:
- Windowless interior room with overhead fluorescent only: 100–300 lux. Below the calculator's bottom threshold. Only parlor palm, ZZ, and snake plant survive.
- North-facing window, plant on sill: 800–2,500 lux. Low to medium. Most foliage plants do well here.
- East-facing window, plant on sill: 2,000–6,000 lux. Medium to bright indirect. The sweet spot for most tropicals.
- South-facing window, sheer curtain, plant 2 ft back: 3,000–8,000 lux. Bright indirect — ideal for monstera, calathea, anthurium.
- South-facing window, no curtain, plant on sill: 15,000–50,000 lux. Direct sun. Succulents only.
Reading the room — without an app
If you genuinely can't use a meter, the classic shadow test still works:
- Hold a hand 12" above a piece of white paper in your plant's spot.
- No shadow = very low light
- Soft, blurry shadow = low light
- Defined but soft shadow = medium / bright indirect
- Sharp, hard-edged shadow = direct sun
It's coarser than the calculator, but it doesn't require an app, and it correlates well enough with the lux categories for most decisions.
When light isn't enough
Genuinely dim rooms can be helped with grow lights. NC State recommends LED grow lights to supplement natural light for air plants and other high-light species. A basic LED grow bulb in a regular fixture puts out 1,000–3,000 lux at 12" away — enough to bump a "very low light" spot into the "low light" range.
For dedicated growing, full-spectrum LED panels can produce 10,000+ lux at the plant level, replacing window light entirely.