PlantMeasure at leaf height with the sensor facing the window, around midday, on several days. Below about 2,500 lux most foliage plants just hold on; 10,000–20,000 lux is bright indirect light.
Take a repeatable reading
Put the sensor where the leaves are, at the top of the plant’s canopy, and point it towards the main light source, usually the window, not at the ceiling. On a phone the ambient light sensor usually sits beside the front camera near the earpiece, so hold the screen facing the window. Keep your hand, head and shadow out of the way.
Take three readings a few seconds apart and use the middle value. Note the time, date and sky conditions. Readings are only comparable when they are taken the same way, and a spot measured at noon on a sunny day cannot be compared fairly with one measured at 5 pm under cloud.
What the numbers mean
These bands are approximate midday values at leaf height. Use them to place a plant, not to prescribe its care to the last lux.
- Typical room away from windows: 50–500 lux. Too dark for any plant long term.
- Low light: 500–2,500 lux. Survival for ZZ, snake plant, cast iron plant, green pothos.
- Medium light: 2,500–10,000 lux. Steady growth for most foliage plants.
- Bright indirect: 10,000–20,000 lux. Ideal for monstera, fiddle-leaf fig and variegated plants.
- Direct sun through glass: 20,000–50,000+ lux. Suits aloe, cacti and succulents.
- Full outdoor sun: around 100,000 lux.
From an instant reading to a daily light budget
Plants respond to the total light they receive in a day, the daily light integral (DLI, in mol/m²/day), not to a single instant. A south-facing sill (in the Northern Hemisphere) might read 30,000 lux at noon but only for a few hours; a north window might read 3,000 lux almost all day. Measure several times a day to see the shape of the curve.
Lux is weighted to human vision, while photosynthesis is measured as PPFD (µmol/m²/s). For sunlight, roughly 54 lux equals 1 µmol/m²/s, so 10,000 lux of daylight is about 185 µmol/m²/s. Held for 8 hours, that gives about 5 mol/m²/day (PPFD × hours × 0.0036), a comfortable level for most foliage plants. Grow lights have different spectra, and a pink or purple LED can read deceptively low in lux, so for lamps rely on the manufacturer’s PPFD figures at a given distance.
Know your meter’s limits
Phone sensors are not calibrated horticultural instruments. They differ between models, can saturate in direct sun, and read differently depending on the angle at which light strikes them. They are very good at comparisons, telling you that one shelf is three times brighter than another, and less trustworthy for absolute values. PlantPal’s light meter is built for exactly that kind of comparison: same technique, same spot, repeated over time.
If you want more precision, a dedicated lux meter is inexpensive, and a quantum (PAR) meter measures PPFD directly, which is what serious growers use under grow lights.
Turn readings into decisions
Light falls off fast with distance from a window: a plant 2 m back often gets only 10–20% of what reaches the glass. If a plant is stretching, producing small leaves or losing variegation and its spot reads under 2,500 lux, move it closer to the light. Do this in steps over 1–2 weeks if it will now get direct sun, and watch for bleached or scorched patches.
Measure again at the change of seasons. The winter sun is lower and reaches deeper into a room, but days are short and intensity drops, so a spot that was ideal in June can be too dim in December.
