How many lux is low light for plants?

A common rule of thumb puts low light at roughly 50 to 250 foot-candles, about 540 to 2,700 lux. Under about 50 foot-candles is too dark for most houseplants.

Are phone light meter apps accurate?

They're approximate. Phone sensors aren't calibrated light meters, but they're fine for telling a low-light spot from a bright one. Take readings at midday and compare spots in the same session.

Daily light integral is the total amount of photosynthetically active light a spot receives in a day, in moles per square metre per day. It's calculated from PPFD (light intensity) and the number of hours the light is on.

How many lux is low light for plants?

On the scale this converter uses, low light is about 540 to 2,700 lux (50 to 250 foot-candles). Below about 540 lux is too dark for most houseplants to grow.

Can I use my phone as a light meter for plants?

Yes, for comparing spots. Phone sensors are not precise instruments and vary between models, but readings taken with the same phone at the same time of day are good enough to tell low, medium and bright spots apart.

What DLI do houseplants need?

It varies by plant. As a rough guide, shade-tolerant foliage plants manage on low single-digit DLI values, many foliage plants grow well somewhere around 4 to 10, and succulents want considerably more. Watch how the plant responds and adjust.

Plant Light Meter Converter (Lux, Foot-Candles, DLI)

Quick answer

Enter a reading from a light meter or phone app, in lux or foot-candles. The converter shows both units and a light category: too dark, low, medium, bright indirect or direct sun. If you use a grow light, enter its PPFD and hours per day to get the daily light integral (DLI).

The conversions

1 foot-candle = 10.764 lux. This is exact, because a foot-candle is one lumen per square foot and a lux is one lumen per square metre.
DLI (mol/m²/day) = PPFD (µmol/m²/s) × hours × 3,600 ÷ 1,000,000 = PPFD × hours × 0.0036

The light categories

These ranges are commonly used rules of thumb for houseplants, measured at midday:

Category	Foot-candles	Lux (approx.)
Too dark for most plants	under 50	under 540
Low light	50–250	540–2,700
Medium light	250–1,000	2,700–10,760
Bright indirect	1,000–5,000	10,760–53,800
Direct sun	5,000+	53,800+

Worked examples

A bookshelf reading of 2,000 lux is about 186 fc: low light.
A spot near an east window reading 10,000 lux is about 929 fc: medium light, close to bright indirect.
A grow light delivering 150 µmol/m²/s for 12 hours gives a DLI of 150 × 12 × 0.0036 = 6.5 mol/m²/day.

Taking a good reading

Use a free light meter app, or a dedicated lux meter if you have one.
Measure at midday on a typical day. Do it on a sunny day and a cloudy day if you can.
Hold the sensor where the plant's leaves are, facing the window or light.
Take several readings and use the average.
Measure again in winter. Light drops a lot in many homes.

Lux vs PPFD

Lux measures light as the human eye sees it. Plants use light differently, mostly the red and blue parts of the spectrum. For sunlight, lux is a reasonable guide. For grow lights, especially pink or purple LEDs, lux can be misleading. That's why grow light makers publish PPFD, which counts photons in the range plants use. To compare grow lights, use their PPFD at a stated distance and calculate DLI.

How much DLI do houseplants need?

Figures in horticultural sources vary by plant and source. Very roughly, shade-tolerant foliage plants manage on low DLI values, while succulents and fruiting plants want much more. Rather than chasing an exact number, compare your DLI with how the plant responds. Leggy, stretched growth and small new leaves suggest too little light. Bleached or scorched patches suggest too much.

Using the result

Once you know your light level, use the low-light plant finder to pick plants that suit it. Then adjust your watering with the watering schedule builder, because lower light means slower drying.

Lux to foot-candles: quick reference

These are the converter's exact results for common lux readings:

Lux reading	Foot-candles	Category
200	19	Too dark for most plants
500	46	Too dark for most plants
538	50	Too dark for most plants
1000	93	Low light
2000	186	Low light
2691	250	Medium light
5000	465	Medium light
10000	929	Medium light
10764	1000	Bright indirect light
20000	1858	Bright indirect light
50000	4645	Bright indirect light
53820	5000	Direct sun
80000	7432	Direct sun

Foot-candles to lux

Some older meters and books use foot-candles. Here's the other direction:

Foot-candles	Lux	Category
25	269	Too dark for most plants
50	538	Low light
100	1076	Low light
250	2691	Medium light
500	5382	Medium light
1000	10764	Bright indirect light
2500	26910	Bright indirect light
5000	53820	Direct sun
8000	86112	Direct sun

Notice the category changes exactly at the boundaries. 50 foot-candles is the first value classed as "Low light", 250 as "Medium", 1,000 as "Bright indirect" and 5,000 as "Direct sun". Anything below each boundary falls into the band beneath it.

DLI table for grow lights

The DLI section multiplies PPFD by hours by 0.0036. These are the results for common settings:

PPFD	8 h	10 h	12 h	14 h	16 h
50	1.4	1.8	2.2	2.5	2.9
100	2.9	3.6	4.3	5	5.8
150	4.3	5.4	6.5	7.6	8.6
200	5.8	7.2	8.6	10.1	11.5
300	8.6	10.8	13	15.1	17.3
400	11.5	14.4	17.3	20.2	23

Read across a row to see how adding hours raises the daily total. A 100 µmol/m²/s light run for 16 hours (5.8) delivers more than a 150 µmol/m²/s light run for 10 hours (5.4). Our guide to grow lights for houseplants explains how to use this when setting up a light.

Worked example: choosing a spot for a new plant

You want to put a peace lily somewhere in a living room with one west-facing window. You take three readings at midday with a phone app:

Spot	Reading	Converter result
On the windowsill	40,000 lux	3,716 fc, Bright indirect light
Two metres back, on a shelf	6,000 lux	557 fc, Medium light
Far corner	700 lux	65 fc, Low light

The windowsill gets afternoon sun and its reading will jump past 53,800 lux on a clear day. That's too much for a peace lily. The shelf, in medium light, is ideal for growth and flowering. The corner would keep it alive but probably without flowers. Our peace lily care guide explains why.

Worked example: is a grow light enough?

A shelf in a windowless office has a grow light. The maker says it delivers 120 µmol/m²/s at 30 cm. The office timer runs it for 11 hours a day.

DLI = 120 × 11 × 0.0036 = 4.75, shown as 4.8 mol/m²/day (rounded to one decimal place).

That's a reasonable figure for low-light foliage plants such as ZZ plant, pothos and snake plant. A succulent would want much more. See plants for an office with no windows.

Why phone readings vary

Phone light meters use the front camera or an ambient light sensor that's designed for screen brightness, not science. Expect them to be off by a noticeable margin, and to differ between phones. They're still useful because you're mostly comparing spots in your own home with the same phone. Tips:

Keep the phone at the same angle, facing the light source.
Remove thick cases that shade the sensor.
Avoid your own shadow.
Take three readings and use the middle one.
Note the weather. A cloudy day can read a fraction of a sunny one.

How light changes across a room

Light falls off quickly as you move away from a window. A spot one metre from the glass may get a fraction of the light right at the window, and the far side of a room much less again. Your eyes adjust so well that you rarely notice. This is why plants that seem "close enough" to a window often stretch towards it.

Window direction (northern hemisphere)	Typical light
South	Brightest, direct sun for much of the day
West	Bright, hot afternoon sun
East	Gentle morning sun, bright indirect later
North	No direct sun, steady low to medium light

In the southern hemisphere, swap north and south.

Light levels for plants in our guides

Plant	Converter category to aim for	Guide
Snake plant, ZZ plant	Low light is fine, medium or brighter for growth	Very low light plants
Pothos	Medium	Pothos care guide
Monstera	Medium to bright indirect	Monstera care guide
Spider plant	Medium to bright indirect	Spider plant care
Succulents, aloe	Bright indirect to direct sun	Use a sunny sill or strong grow light

Seasonal changes: measure twice a year

A spot that reads "Medium light" in June can drop to "Low light" in December in many homes, because the sun is lower, days are shorter and skies are greyer. If a plant that was happy all summer starts to stretch or stop growing in winter, light is the most likely reason. Take a reading in midwinter and compare.

What to do with a lower winter reading:

Move light-hungry plants closer to the window, but keep them off cold glass at night.
Clean the windows. Dirty glass can block a surprising amount of light.
Wipe dust off leaves.
Add a grow light for the darkest months.
Water less. Plants in lower light use water more slowly. The watering schedule builder has a winter setting for this reason.

Daylight vs the DLI calculator

The DLI box is for grow lights, where PPFD is constant while the light is on. Daylight changes all day, rising to a peak around midday and falling again, and it changes with clouds. You can't get an accurate daylight DLI by multiplying a single midday lux reading by the hours of daylight. The result would be far too high. For window light, use the light category instead, and use DLI for planning artificial light.

Converting lux to PPFD (and why the tool doesn't)

You'll see conversion factors online for turning lux into PPFD. They depend on the light source's spectrum, so one factor for sunlight, another for warm white LEDs, another for cool white, and none that work reliably for purple grow lights. Because a single factor would be wrong for many readers, the converter keeps the two measurements separate: lux and foot-candles for everyday light levels, PPFD for grow lights. If your grow light's maker publishes PPFD at a given distance, use that figure directly.

What the categories mean for plants

Category	What to expect
Too dark for most plants	Slow decline even for tough plants. Add a grow light or move the plant.
Low light	ZZ plant, snake plant, cast iron plant, pothos and similar survive and grow slowly.
Medium light	Most foliage houseplants grow steadily. Peace lilies can flower.
Bright indirect light	Fast growth for foliage plants. Good for monstera, rubber plant, fiddle leaf fig.
Direct sun	Succulents, cacti and sun lovers. Can scorch shade plants.

For a list of plants for each light level, try the low-light plant finder. It uses the same low, medium and bright bands.

How the converter handles your inputs

The converter rounds lux and foot-candles to whole numbers and DLI to one decimal place. If the reading is zero or missing, it shows a dash instead of a result. The light category uses the foot-candle value, so a lux reading is converted first and then compared with the 50, 250, 1,000 and 5,000 foot-candle boundaries. The DLI result only appears when both PPFD and hours are positive numbers.