How do I measure light for my plants?

Pick a clear day and one hour, set your phone down at the exact spot at pot height with the sensor facing up, and read the light tier, not the precise number. Compare spots by the pattern.

Can you use your phone as a light meter for plants?

Yes, for the decision you are making, which tier a spot falls into, and how your spots rank against each other. Not for a figure you would quote to three digits. Lay the phone flat and screen up at the spot, at the height the pot will sit, on a clear day.

Is a phone accurate enough to measure plant light?

Not to the number. In two of our own calibration rounds, one identical reading sat on top of real values of 1,800 and 3,500 lux. That is why the honest output is a tier labelled as an estimate. For sorting spots and comparing them (the decision that actually saves plants) that is plenty.

How can I tell if a room gets enough light for plants?

You can’t, and that is the trap in the question. A room is not one light level: in the room we measured, the window read 20,000 lux and a corner 18. Ask it about the spot instead. The exact place the pot will stand.

Do LED lights count as light for plants?

Plants use LED light perfectly well; the problem is quantity, not type. Ordinary room and ceiling LEDs sit far below what a plant needs to grow. They read as close to dark on a meter even when the room looks bright to you. Purpose-built grow lights are a different intensity class altogether.

What time of day should I measure?

Late morning to early afternoon on a clear day, and use the same hour for every spot so you’re comparing fairly. Re-check when the seasons turn, since a spot’s light changes through the year.

How to Measure Light for Plants With Your Phone

You can’t eyeball plant light. Human eyes adapt so smoothly that a spot with a fraction of a window’s light still reads as “bright enough”, which is why so many plants decline in spots their owner was sure about. The good news: the instrument you need is already in your pocket, and the whole thing takes about ten seconds a spot.

Worked example: Wax plant (Hoya) 2 m back from an east window, air 22 °C and 50% humidity, medium airflow. Your spot will score differently. Run it for your own spot →

The method

The published sources measure in two different units, on several different scales, and one of them supplies the reason a reading has to be taken where the pot is.

Published by	What it measures	In its own words
extension.psu.edu	Both units	“The light meter I purchased reads both foot candles and the metric measurement of lux.”
extension.psu.edu	100 footcandles	“a common baseline for low light is 100 footcandles”, with one plant’s own spot measured “as low as 25-foot candles in mid-morning”.
extension.msstate.edu	1,000 fc, then 100 a few feet in	Not a floor but a falloff, from the same source: “The intensity of the light can drop from 1,000 to 100 foot-candles in the distance of only a few feet.”
extension.arizona.edu	Four and eight feet	“High Light: areas within four feet of large south-east or west facing windows”; “Low Light: areas more than eight feet from windows as in the center of a room, a hallway or an inside wall.”
open.edu	200–300 lux	“Daylight penetrating windows into a building can provide illuminance levels of around 200–300 lux in typical naturally lit offices for most of the day.”
open.edu	100 000 lux	“Sunlight is extremely bright and its illuminance can reach 100 000 lux on the ground.” Overcast is still “at least 5000 lux”.

What the published sources measure and in what unit, copied exactly as each writes it. Two units, several scales, and one falloff figure that explains why a reading has to be taken at the spot. 6 figures transcribed from 4 sources, read end to end on 30 August 2026. Units are copied exactly as each source writes them; nothing here is converted, and nothing here is our own measurement. All of them, from every page, are one free download.

A hand holding a phone with a blank screen beside a large variegated schefflera in a bright, glazed office interior — The instrument is already in your pocket, and where you hold it is the whole question. A reading at the leaf and a reading at the window are two different numbers.

Pick a clear day, and one hour. Measure every spot at roughly the same time (late morning to early afternoon is ideal) so you’re comparing like with like.
Set the phone down at the exact spot. Not near it: on the sill, on that shelf, in that corner, at the height the pot will actually sit, lying flat and screen up.
Read the tier, not the third digit. Phone light sensors are rough, and that’s fine. You’re sorting the spot into a tier (dim, workable, bright, direct) not measuring to the lux. Compare the pattern across your spots.

Why rough numbers are enough

For deciding where a plant can live, the tier a spot falls into matters far more than the exact reading. Two phones will disagree on the absolute number and agree completely on the pattern. Every step from the window costs most of the light. That pattern is the whole insight. Six spots in one measured room ran from 20,000 lux at the glass to 18 in the corner: you don’t need precision to act on a gap like that.

The plant you are holding	What the light is doing	Verdict
Wax plant (Hoya)	A little dim, but workable	Worth a try, 67/100
Cast iron plant	Just the right amount of light	Great fit, 99/100
Swiss cheese plant	Just the right amount of light	Great fit, 100/100
Aloe vera	Too dim here	Not the best fit right now, 42/100

One spot, one reading, four plants. The reading did not change; the answer did. This is why the tier is the useful output: it is the plant that decides what a number means, and the bands are wide enough that a rough reading still lands in the right one. Only one thing moves down the table. The spot itself does not: one east-facing spot, one metre in, on the same morning, with 22 °C, 50% humidity and medium airflow held the same on every row. The middle column is only what the light is doing. The verdict is the whole spot, so a plant can have the light it wants and still lose points on warmth, humidity or airflow.

Where you held the phone	Estimated light	What the light is doing	Verdict
0.3 m, against the glass	6,500 lux	Just the right amount of light	Great fit, 98/100
1 m back	5,000 lux	A little dim, but workable	Worth a try, 67/100
2 m back	3,150 lux	A little dim, but workable	Worth a try, 64/100
3 m back	1,984 lux	Too dim here	Not the best fit right now, 47/100
4 m back	1,250 lux	Too dim here	Not the best fit right now, 46/100

The same hoya, the same room, the phone held at five different places. This is why the rule is to measure where the pot will actually stand, not where it is convenient to hold the phone. The spot is what moves here. What does not: the same plant, the same east-facing room, the same morning, with 22 °C, 50% humidity and medium airflow on every row.

If your meter reports foot-candles rather than lux, multiply by 10.764 to compare, or just multiply by ten. The unit is the easy part. Agreeing on what the numbers mean is where it falls apart.

Can you use your phone as a light meter?

A handheld incident light meter lying on a windowsill, its display showing a shutter speed and an aperture rather than an illuminance value — A photographer’s meter reads the same light and answers a different question: this one is showing 1/125 at f/2.8, not foot-candles. The unit is half the problem.

Yes. For the decision you are actually making. Not for a figure you would quote.

That distinction is hardware: no iPhone app can read the light sensor.

Here is the honest limit, from our own calibration rounds: one identical phone reading, taken twice, sat on top of real values of 1,800 and 3,500 lux: nearly a factor of two apart. A phone reads light through whatever is in front of it, so its answer shifts with the spectrum and the angle of the scene; consumer ambient-light sensors carry a wide spread of their own before any software touches the number.

Those calibration values are ours, and so is the pile behind them: 204 readings at 104 real spots, published as aggregates rather than asserted.

That is why a tier, labelled an estimate, is the honest output. And three confident digits are not. A number that looks precise and isn’t is worse than a band, because it makes you sure about the one thing you should stay humble about.

Where a phone is dependable

Where a phone is dependable: ranking. Read two spots the same way, minutes apart, and the order they come out in holds even when the absolute figures wander. Ranking is exactly what placing a plant needs.

Whether a gap that size actually changes where the plant goes is a separate question, and we put it through our own engine to find out: at 1,800 lux and at 3,500 lux the same monstera scores 65 and 93, while 900 and 10,764 land five points apart, at 61 and 66. The error lands somewhere, but only inside one narrow band.

No phone handy? Use the shadow

The shadow test gets you the tier with nothing at all. The shape of the shadow a spot casts tells you most of what you need to know.

Shadow your hand casts	What the spot is
Sharp, dark, hard-edged	Direct sun. Too much for most houseplants, right for succulents and cacti
Soft-edged but clearly there	Bright indirect. The band nearly every care tag is written for
Faint, or no shadow at all	Low light. Tolerators only, and nothing will grow quickly

The shadow has a second use. A soft-edged but clear one is also how you check whether a spot is genuinely bright indirect, where a sharp dark shadow means direct sun and a faint one means too dim.

The ten-second version

Clear day, late morning.
Phone flat at the spot, screen up, at the height the pot will sit.
Read the tier the number lands in; ignore the digits.
Repeat at the next candidate spot and trust the order: whichever reads higher really is brighter, even when neither number is exact.

Then give the tier a job. Describe the spot to the free spot checker (direction, distance from the glass, the plant you mean to put there) and it turns that description into a verdict for that plant, in the browser, nothing to install.

By GrowSpot · Published July 23, 2026 · Last updated August 30, 2026

Plant photo credits