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.
200–500 fc. Clemson Extension’s preferred range for medium-light plants. It says these do well within several feet of a west- or south-facing window, or in an east-facing exposure.
- Spider plant
- Boston fern
- Prayer plant
- Rubber plant
- Fiddle-leaf fig
Bands and example plants per NC State Extension (the 25–75 fc band) and Clemson Extension (75 fc and up). The category names are this site’s labels.
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 farther back in the room. Sensor faces the light source.
- Take it around midday 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 reads in the bright-indirect range in the morning and the direct-sun range in the afternoon gets a daily blast of direct sun — relevant for fiddle leaf figs and other sun-sensitive plants.
The categories
The foot-candle bands come from extension tables. The bottom band matches the 25 to 74 fc that NC State's Extension Gardener Handbook gives as the light needed to maintain growth in low-light plants. The four bands above it match the preferred ranges in Clemson Extension's light categories: low (75 to 200 fc), medium (200 to 500), high (500 to 1,000) and very high (1,000+). Lux values are approximate conversions (1 fc ≈ 10.764 lux). The example plants come from the same two sources:
| Lux range | Foot-candles | Category | Example plants |
|---|---|---|---|
| 250–800 | 25–75 | Very low / deep shade | Cast-iron plant, Chinese evergreen, dracaena, philodendron (NC State: remain attractive for 24 to 36 months at 25–50 fc) |
| 800–2,200 | 75–200 | Low / dappled | Parlor palm, pothos, snake plant, Chinese evergreen, cast-iron plant (Clemson low-light table) |
| 2,200–5,500 | 200–500 | Medium / bright indirect | Spider plant, Boston fern, prayer plant, rubber plant, fiddle-leaf fig (Clemson medium-light table) |
| 5,500–10,800 | 500–1,000 | Bright indirect to some direct | Jade plant, wax plant (hoya), burro's tail, aloe, ponytail palm (Clemson high-light table) |
| 10,800+ | 1,000+ | Direct sun | Hibiscus, wax begonia, geranium (Clemson's very-high-light examples) |
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 on how long the light lasts. Duration matters: Clemson Extension says it is possible to make up for a lack of light by increasing how long the plant is exposed to it, and NC State's handbook says longer day length can make up for low light intensity in some species.
The calculator above uses rounded thresholds. NC State's Plant Toolbox categories (example) are based on hours of direct sun (deep shade under 2 hrs, partial shade 2–6 hrs, full sun 6+ hrs), not lux, so the match is approximate. Combined with a midday lux reading, it gives a usable category for most indoor settings.
What the numbers mean for common spots
NC State's Extension Gardener Handbook gives these reference light levels:
- Library or classroom: about 20 fc (roughly 215 lux), below the calculator's bottom threshold.
- Office: about 50 fc (roughly 540 lux), inside the calculator's very-low band.
- Supermarket: about 100 fc (roughly 1,080 lux).
- Light shining into a window: 100 to 5,000 fc (roughly 1,080 to 54,000 lux), depending on the window and the sun.
- Outdoors: about 1,000 fc on an overcast day and 10,000 fc in full sun.
Without a meter, the same handbook describes low light as more than 8 feet from a window, dim corners, the centers of rooms, hallways, north-facing rooms, and spaces with only artificial light; medium light as 4 to 8 feet from south and west windows, and west windows that get no direct sun; and high light as within 4 feet of large windows that face south, east, or west.
Reading the room — without an app
If you genuinely can't use a meter, the classic shadow test still works:
- Hold a hand 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. Iowa State Extension describes bright indirect light the same way: "light bright enough to cast a distinctive shadow but does not have sun rays hitting the foliage directly."
When light isn't enough
Genuinely dim rooms can be helped with grow lights. NC State says light for air plants "can be supplemented with LED grow lights." Distance matters: NC State's handbook says a 40-watt fluorescent tube produces 120 fc at 1 foot but only 75 fc at 2 feet, and that fluorescent fixtures are typically set about 12 inches above the plants.
For dedicated growing, Clemson Extension notes that artificial lighting is widely used to supplement or replace natural light, and NC State's handbook notes that horticultural LED grow lights emit the wavelengths plants use and last much longer than conventional lights.