How to Measure Light Levels for Houseplants

Plant care labels say "bright indirect light" or "low light" with no numbers attached. But every window delivers a measurable quantity of light, and every plant has a measurable threshold below which it stops growing. Closing that gap, from vague labels to actual numbers, is the single most useful thing you can do to stop putting plants in the wrong spot.

The Two Main Units: Lux and Foot-Candles

Lux is the SI unit of illuminance, measuring lumens per square metre. Foot-candles are the imperial equivalent: one foot-candle equals approximately 10.76 lux. Most plant care guides written for the UK and Europe use lux; US guides often use foot-candles. For practical purposes, divide foot-candles by 10 to get approximate lux.

Here is how the numbers map to real window conditions:

• Under 200 lux: Very low light. A north-facing window in winter in the UK, or an inner hallway. Reading in this light is uncomfortable without a lamp.
• 200 to 1,000 lux: Low to medium light. A north-facing window in summer, or 1 to 2 metres from a south-facing window on an overcast day.
• 1,000 to 5,000 lux: Bright indirect light. Within 60 cm of a south or east-facing window without direct rays hitting the plant.
• 5,000 lux and above: Bright direct light. On the windowsill of a south-facing window in summer, with unobstructed sun reaching the plant.

The Shadow Test (Free, Instant)

Before reaching for any app or meter, try this: hold your hand 30 cm above a plain sheet of white paper at midday. The shadow your hand casts tells you the light level at that spot.

• Hard, sharp shadow with clearly defined edges: Direct sunlight, 5,000 lux or above.
• Soft shadow with blurred edges: Bright indirect light, roughly 1,000 to 5,000 lux.
• Very faint shadow, barely visible: Low light, under 500 lux.
• No visible shadow at all: Very low light, under 100 lux.

This is the fastest field test available and is accurate enough for most placement decisions. It takes ten seconds and requires nothing you do not already have.

Using Your Phone as a Light Meter

Modern smartphones estimate lux using the ambient light sensor built into the camera assembly. Apps like Lux Light Meter (available on both iOS and Android) read this sensor and display a numerical lux value. Accuracy is typically plus or minus 20 to 30 percent compared to a calibrated meter. That margin is good enough for plant placement decisions, where the relevant thresholds are wide ranges rather than precise cut-offs.

A few tips to get reliable readings:

• Take readings at the plant's leaf level, in the spot where you plan to place it, not standing back looking at the window.
• Measure at the brightest part of the day, usually between 11am and 2pm, to understand the peak available light.
• Take a second reading on an overcast day, or in the depths of winter, to understand the seasonal range. A spot that reads 3,000 lux on a July afternoon may read 300 lux on a December morning.

Dedicated Light Meters

For anyone who wants more precision, a dedicated horticultural light meter costs between £15 and £40 on Amazon and gives accurate lux readings without relying on a phone sensor's variability. Some meters also measure PAR (photosynthetically active radiation), expressed in micromol per square metre per second. PAR is more directly relevant to plant growth than lux because it measures only the wavelengths plants use for photosynthesis, but it is harder to interpret without reference tables. For home use, lux is simpler and the reference values are more widely documented.

To use a dedicated meter: set it at the plant's canopy level, point the sensor upward toward the light source, and take readings at 9am, noon, and 3pm on a representative day. Averaging these three values gives a reasonable picture of daily light exposure at that spot.

Reference Table by Plant Type

These ranges represent the comfortable operating window for each group, not the absolute minimum for survival. Plants can survive below the lower bound for a time, but growth will slow or stop.

Plant group	Examples	Comfortable lux range
Low-light survivors	Pothos, ZZ plant, cast iron plant, dracaena	300 to 600 lux (minimum 50 to 300 lux)
Medium light	Peace lily, spider plant, Chinese evergreen	300 to 800 lux
Bright indirect	Monstera, philodendron, bird's nest fern, begonia	800 to 3,000 lux
High light	Succulents, cacti, citrus, most herbs	3,000 to 8,000 lux
Direct sun requirement	Some cacti, fruiting plants	8,000 lux and above

The figures above are for survival and reasonable growth. Optimal growth often requires the higher end of each range. A monstera sitting at 800 lux is surviving; at 2,000 lux it is growing steadily.

Seasonal Variation Matters

A window that delivers 4,000 lux in July may deliver 400 lux in December in the UK. The sun sits much lower in the sky in winter, the days are shorter, and overcast skies are the norm rather than the exception. Many plant problems attributed to overwatering or the wrong soil type are actually caused by plants being placed where they thrive in summer and then staying in that same spot as light collapses in winter.

If you can only measure once, measure in winter. A spot that works in winter works year-round. A spot that only works in summer will leave your plants in trouble for four to five months of the year. If you have light-hungry plants in a south-facing window in summer, check whether they need supplemental light or a closer position to the glass once October arrives.

FAQ

Can I trust the light level reading on plant apps?

Plant apps that use the camera sensor are roughly accurate within 20 to 30 percent for relative comparisons. Use them to compare spots in your home, for instance to confirm that a corner gets half the light of a particular window, rather than treating the number as a precise calibrated measurement. For plant placement decisions, this level of accuracy is more than sufficient.

What is the minimum lux for any plant?

Around 50 lux is the lowest threshold for survival in the most tolerant species, including cast iron plants and certain dracaenas. Below 50 lux, no plant survives long-term without supplemental artificial light. Even the most shade-adapted houseplants need some measurable light to photosynthesise.

My south window has a building outside blocking direct sun. How does that affect readings?

Significantly. A large nearby building can reduce a south-facing window from 20,000 lux or more on a clear day to 2,000 to 5,000 lux of reflected sky light. The obstruction effectively converts direct sun into bright indirect light. Always measure with the obstruction in place rather than assuming the window direction tells you the full story. The direction of your window sets the ceiling; obstructions lower that ceiling considerably.

Does the colour temperature of light matter for plants?

For photosynthesis, yes. Blue light in the 5,000 to 7,000 Kelvin range drives vegetative growth; red light in the 2,700 to 3,200 Kelvin range drives flowering. Natural daylight contains both. Standard warm-white LED bulbs are biased toward the red end of the spectrum and are somewhat less efficient for plant growth than full-spectrum lights, but the practical difference matters more for dedicated plant rooms or propagation setups than for keeping the average houseplant alive on a windowsill.

I measured 800 lux at my plant but it is still not growing. What am I missing?

Check duration. 800 lux for 4 hours delivers far less total light energy than 800 lux for 10 hours. Total daily light integral (DLI), which is peak lux multiplied by hours of exposure, matters more than peak lux alone. Also verify that soil moisture, temperature, and nutrients are not independently limiting growth. A well-lit but pot-bound plant, or a well-lit plant with waterlogged roots, will not grow regardless of the light reading.