How to Measure Light for Houseplants, Beyond the Window

The short answer: measure the light where the leaves are, not where the window is. Use a light meter or a phone light-meter app that reads foot-candles or lux. Take readings at several times of day and compare them with published bands. University of Maryland Extension uses 25–100 foot-candles for low light, 100–500 for medium-bright, 500–1,000 for high, and over 1,000 for direct sun indoors (Lighting for Indoor Plants). Window direction is a useful first guess, but distance from the glass, obstructions and the season can change the answer a lot. Measure again in winter.

Why your eyes misjudge plant light

Your eyes keep adjusting to the light around you. In dim conditions your pupils dilate, so a dark room looks brighter even though the light in it hasn't changed. Virginia Tech's extension guide says this makes it "nearly impossible" to judge correctly how much light a plant is getting (Virginia Tech, SPES-720). A spot that feels bright to you may be in the low-light range on a meter.

Meters have limits too. Foot-candles and lux are "photometric" units, based on the light the human eye detects, which is mostly green. A single reading is also only a snapshot, because natural light changes all day (Purdue Extension, HO-238-W). Most extension guidance on windowsill houseplants, including the UMD, Illinois and Penn State pages cited here, still uses foot-candles, so they're a practical place to start.

What low, medium, bright and direct light mean in numbers

Sources don't agree on exact cutoffs. Penn State Extension notes that county extensions, horticulturists and engineers "vary in their recommendations", and gives 100 foot-candles as a common baseline for low light (Penn State Extension). Here are two extension schemes side by side:

Light level	UMD Extension (foot-candles)	Approx. lux (UMD bands)	Illinois Extension (foot-candles)
Low	25–100	270–1,080	about 75
Medium	100–500 ("medium-bright")	1,080–5,400	about 150
High / bright	500–1,000	5,400–10,800	about 300
Direct sun indoors	over 1,000	over 10,800	about 1,500

Sources: UMD Extension and Illinois Extension. Lux values are converted from the UMD bands at 1 foot-candle = 10.764 lux (Iowa State Extension) and rounded. For comparison, direct outdoor sun peaks at about 10,000 foot-candles (UMD Extension). Use the bands to compare spots in your home, not as exact thresholds.

Window direction is a starting point, not an answer

UMD Extension pairs north-facing windows with low-light plants, east and west windows with medium-bright light, and south (and some west) windows with high light (UMD Extension). Illinois Extension adds distance, which shows how quickly one window gives you several light zones (Illinois Extension: Lighting):

Light level	North window	East or west window	South window
Low (about 75 fc)	a few feet away	3–10 ft away	15–20 ft away
Medium (about 150 fc)	directly in front	a few feet away	3–10 ft away
High (about 300 fc)	not listed	directly in front	up to 5 ft away
Direct (about 1,500 fc)	not listed	not listed	directly in front

Both schemes come from US sources and assume a Northern Hemisphere home. Treat the distances as a rough guide, then measure.

Distance: light falls off fast

Light intensity drops quickly as a plant moves farther from its light source (UMD Extension). In the Illinois table, the same south window gives direct light right in front of it and low light 15–20 feet back. So moving a plant a few feet toward or away from the glass can matter as much as which wall the window is on.

Obstructions: trees, buildings, screens and curtains

Curtains, insect screens, weather and the season all change how much light reaches a plant (UMD Extension). Trees and shrubs outside can block sunlight with their leaves in summer and let in more light in winter once the leaves have fallen (Illinois Extension). In a Penn State Extension article, a Master Gardener describes nearby tall buildings and thick translucent window glass cutting the light in her home (Penn State Extension). Two homes with the same window direction can measure very differently.

Three ways to read the light

The shadow test

This is the quickest check and needs no equipment. Hold your hand between the light and the spot where the plant sits, and look at the shadow. N.C. Cooperative Extension explains that when the sun shines directly on a plant it casts a crisp shadow, bright indirect light gives a fuzzy, soft shadow, and in low light the shadow is faint (N.C. Cooperative Extension: Light Considerations for Indoor Plants). The RHS likewise describes bright shade as bright light that "does not cast a sharply defined shadow" (RHS: How to grow houseplants). The test doesn't give you a number, so use it to compare spots and confirm with a meter.

A phone light-meter app

Iowa State Extension suggests measuring with a light meter or a smartphone app (Iowa State Extension). Apps read the phone's built-in light sensor. Virginia Tech notes that the precision you need depends on the job: a phone app can give homeowners useful information, while professional growers may invest in dedicated sensors (Virginia Tech). To get the most out of one:

Measure at foliage level, where the leaves are (Iowa State Extension).
Hold the phone level and steady. Purdue gives the same advice for dedicated sensors: keep them level and at plant height (Purdue Extension, HO-238-B-W).
Take readings at different times of day and on more than one day, because one reading is only a snapshot (Purdue Extension).
Note whether the app reports lux or foot-candles. Divide lux by 10.764 to get foot-candles (Iowa State Extension).

Treat app readings as a way to compare one spot with another, not as lab measurements.

A handheld light meter

A light meter or a photographic exposure meter measures light intensity in foot-candles (Illinois Extension). Even so, Penn State's article says the meter is "a handy tool, but knowing precise light levels is only a guideline" (Penn State Extension). If you're setting up grow lights, a quantum (PAR) sensor is the better tool, because it measures PPFD, the light plants use for photosynthesis, in µmol·m⁻²·s⁻¹ (Virginia Tech).

Light through the year: measure again in winter

A spot that works in July may be too dark in January. Iowa State Extension says light levels are "particularly low in the winter" because of short days, the sun's low angle and overcast weather (Iowa State Extension). The RHS says houseplants benefit from full sunlight in winter, when daylight is weak, and warns that strong summer sun can scorch some plants through glass (RHS).

Autumn and winter: measure again, and move plants closer to the brightest windows if readings drop. UMD Extension suggests moving plants closer to a light source to raise light levels (UMD Extension: Low Light Impacts).
Spring and summer: watch plants you moved into direct sun for winter. Pull them back before they scorch. See how to spot pothos sunburn.

How long a plant needs light

Duration matters as well as brightness. UMD Extension advises giving plants no more than 16 hours of light a day in total, because most plants need a dark period (UMD Extension). Illinois Extension says 14–16 hours is sufficient and that plants should not get more than 16 hours (Illinois Extension). Some flowering plants respond to day length. Poinsettia, kalanchoe and Christmas cactus form buds only when days are short, with 11 hours of daylight or less (UMD Extension). If you add grow lights, see why lights shouldn't run 24 hours a day.

Reading the plant: signs of too little and too much light

Your plant gives its own readout, though more slowly than a meter.

Too little light: spindly, "leggy" stems stretching toward the light, faded leaf colour, poor flowering and poor growth (UMD Extension); few or no flowers (UMD Extension). See leggy pothos and small pothos leaves.
Too much direct light: leaves turn pale, then brown, and die (UMD Extension). See pothos sunburn and bleached fiddle-leaf fig leaves.
After a move: plants take time to adjust to lower indoor light. Plants brought inside after a summer outdoors may drop leaves or turn yellow, as weeping figs often do (UMD Extension). Give a plant time after a move before you decide the new spot has failed.

Plant-specific light guides: pothos, Monstera, orchids and cacti.

When the window isn't enough

The RHS says no plant will tolerate complete darkness, such as a windowless room (RHS: Easy care houseplants). Iowa State Extension says many houseplants benefit from supplemental light, either in winter or all year (Iowa State Extension). When you shop for grow lights, compare PPFD figures rather than foot-candles or lux, because Virginia Tech advises against foot-candles, lux and watts for measuring plant light (Virginia Tech). For how those numbers work, see PPFD and DLI explained. Next steps: grow light vs sunlight, how to choose a grow light and grow lights for indoor plants. To work out how long to run a light, try our grow-light DLI calculator.

Frequently asked questions

What does bright indirect light mean?

It means a bright spot that the sun's rays don't touch directly. N.C. Cooperative Extension says bright indirect light gives a fuzzy, soft shadow, while direct sun on the leaves casts a crisp one, and the RHS describes bright shade as bright light that does not cast a sharply defined shadow. For numbers, UMD Extension calls 500–1,000 foot-candles high light and anything over 1,000 foot-candles direct sun indoors.

How many lux is low light for houseplants?

University of Maryland Extension puts low light at 25–100 foot-candles, which is roughly 270–1,080 lux at 10.764 lux per foot-candle. Other sources set the cutoffs differently. Penn State Extension gives 100 foot-candles as a common baseline for low light.

Are phone light meter apps accurate enough for houseplants?

For comparing spots in your home, usually yes. Iowa State Extension suggests a light meter or smartphone app, and Virginia Tech notes that a phone app can give homeowners useful information, while professional growers use dedicated sensors. Measure at leaf level, hold the phone steady, and take several readings across the day, because one reading is only a snapshot.

Is a north-facing window enough for houseplants?

For low-light plants, often yes. UMD Extension pairs north windows with low-light plants. Illinois Extension rates the spot directly in front of a north window as medium light and a few feet back as low. Measure, because nearby trees and buildings can change the reading.

Why does my plant struggle in winter in the same spot?

Indoor light drops in winter. Iowa State Extension points to short days, the sun's low angle and overcast weather. Measure again in autumn and move plants closer to your brightest windows if readings fall.

Can I use foot-candles to set up a grow light?

It's better not to. Foot-candles and lux are based on human vision, and Virginia Tech recommends against using them for plant lighting. For grow lights, compare PPFD at leaf height and the number of hours the light is on.