Most light guides answer in adjectives: bright indirect, medium, dappled shade. The numbers behind those words are below, in lux and in foot-candles. What a tier chart cannot tell you is how often a real room reaches them, and for that we have readings: the middle of the indoor spots people measured with a light meter came in at 566 lux, under the floor most houseplants start at.
The four tiers, and who lives in each
Most horticultural sources answer this in foot-candles and most building sources answer it in lux, which is the first reason the numbers look incompatible.
| Published by | The figure | In its own words |
|---|---|---|
| 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”. |
| 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 | 150 / 300 / 500 lux | Recommended task levels: circulation areas 150, classroom desk 300, office desk 500. |
| extension.illinois.edu | 75 / 150 / 300 / 1500 fc | Its four bands, verbatim: “Low Light (75 foot candles)”, “Medium (average) Light (150 foot candles)”, “High or Bright Light (300 foot candles)”, “Direct (1500 foot candles)”. |
| extension.msstate.edu | 50–500 / 500–1,000 / 1,000+ fc | “High represents light conditions that are 1,000 or more foot-candles in intensity”, “medium represents 500 to 1,000 foot-candles”, “low represents 50 to 500 foot-candles” — and the same page also heads sections at 25–75, 75–150 and 150–1,000. |
| extension.psu.edu | Both units | “The light meter I purchased reads both foot candles and the metric measurement of lux.” |
Nearly every houseplant you can buy is sold into one of four light tiers. The boundaries below are the classic interior-growing brackets, written in both units. Our own care data files each species into the same four tiers, which is where the shares further down this page come from.
| Tier | Foot-candles | Lux, roughly | What it means indoors |
|---|---|---|---|
| Low | 25–75 | 270–800 | Survival light for tough species; nothing grows fast here. It is also where most indoor spots sit: from a metre back from the window, the middle reading in every distance band falls in this tier. |
| Medium | 75–150 | 800–1,600 | The big foliage group is comfortable from here up. Indoors it takes a spot close to the window: only the readings taken right at the glass had a middle reading above 800. |
| High | 150–1,000 | 1,600–10,800 | Where bright-light plants actually want to live: near a window without midday sun on the leaves. 32% of the indoor readings people logged reach it. |
| Very high | 1,000+ | 10,800+ | Sun on the sill for part of the day. Succulents, citrus and most herbs only really run here, and only 4% of indoor readings got that far. |
The tiers overlap in practice, because a plant does not read the bracket it was filed under, and the lux column is rounded on purpose: a boundary like 800 lux is a convention that different references draw at slightly different places, so a reading of 750 is not meaningfully different from one of 850.
A plant listed in a lower tier is almost always happy in the tier above it, within reason. The reverse is where trouble starts: a high-tier plant in a medium-tier spot declines slowly enough that the light rarely gets the blame.
Where those conventions meet real rooms is the part we can show rather than assert: 159 readings taken at 137 real spots, of which 30 of the 111 indoor ones fall below 270 lux, under the floor of the lowest tier on this page.
How bright real rooms are, by distance from the window
The usual way to place those tiers in a room is by description. Readings let us place them by measurement instead. These are the indoor readings where the person noted how far the spot was from the window:
| How far the spot was from the window | Readings | Middle half | Median |
|---|---|---|---|
| At the glass (0–0.5 m) | 10 | 516–2,832 lux | 1,618 lux |
| A metre or so back (1–1.5 m) | 32 | 233–2,892 lux | 526 lux |
| Two to three metres (2–3 m) | 16 | 94–613 lux | 485 lux |
| Further than three metres (3.5–5 m) | 14 | 295–599 lux | 441 lux |
Only the readings taken right at the glass have a middle above 800 lux, the floor of the medium tier. A metre or so back, the middle reading is already 526, and it barely moves after that: 485 at two to three metres, 441 further in. The spread inside each band is the bigger story: a metre back, the middle half runs from 233 to 2,892 lux, so a distance on its own cannot tell you what your spot gets.
We also split the same readings by which way the window faces. At this sample size that split did not give a pattern we would trust, so we are not printing it: distance shows up in these readings, and direction does not yet.
What is already published, and in what units
Before answering it ourselves we read what is already published. Twenty-four sources end to end on 29 August 2026, from which we transcribed 569 numeric light figures, then re-fetched every page to check each number and its unit; six were thrown out.
Two things came out of that. Only 51 of the 569 are in lux — the unit your phone gives you. 258 are in foot-candles, 106 in PPFD or DLI, 80 are hours of exposure. And on the five plants where two of those sources both publish a foot-candle range, the two ranges share only a quarter to a half of their combined span: a golden pothos is 25–75 fc in one and 50–100 fc in the other.
They are not contradicting each other about physics. They are drawing a convention in slightly different places, for a plant, in a unit you do not have. So this page answers the other question instead — the one our own data can answer exactly.
What your reading actually unlocks
The tiers above are the convention. This is the part the convention leaves out: how much of the planted world a given reading opens up, and how often a real room reaches it.
| If your reading is at least | It meets the floor for | Indoor readings that reach it |
|---|---|---|
| 270 lux | 5% | 73% |
| 800 lux | 44% | 44% |
| 1,600 lux | 79% | 32% |
| 10,800 lux | 100% | 4% |
Two things fall out of that table. Going from 800 lux to 1,600 nearly doubles what will grow for you, and the share of indoor spots that can supply it falls from 44% to 32%, so that step is worth making wherever a spot allows it. The last tier is the opposite trade: it opens the remaining 21% of the library, and only 4% of the indoor spots people measured ever get there. That is why the herbs and the succulents keep failing on windowsills that look bright.
The sources, read end to end on 29 August 2026: extension.illinois.edu, extension.msstate.edu, extension.arizona.edu, yardandgarden.extension.iastate.edu, growlightmeter.com, floraplantcare.com, apps.apple.com, terraforums.com, staugorchidsociety.org, libanswers.nybg.org, en.wikipedia.org, hgic.clemson.edu, missouribotanicalgarden.org, pmc.ncbi.nlm.nih.gov, houseplantjournal.com, epicgardening.com, plantophiles.com, houseplantsnook.com, soltech.com.
Look up your own reading
Type your reading, or tap one of the presets. It tells you which tier the number lands in and what lives there.
Your light reading
Tier boundaries follow the foot-candle brackets interior growers have used for decades, converted to lux. They are conventions, not physics: treat readings near a line as belonging to both sides of it.
Where did your number come from?
The tier lookup is only as honest as the reading you feed it, and readings go wrong in predictable ways. A number taken at eye level, by the window, at noon, describes the brightest moment of the best spot in the room. The plant lives somewhere else: lower, further in, and all day long.
If you are using a phone as the meter, lay the phone down flat at the spot the pot actually occupies, screen up, at pot height. Do not point it at the window; that measures the window, not the spot. And treat a single reading as a snapshot. The same shelf can read 2,000 lux on a clear morning and 400 on an overcast afternoon, which is why serious estimates work from the typical day rather than the lucky one.
The same reading is not the same answer
Here is the point of this whole page in one worked example. Take a spot 1.5 metres in from a north-facing window. Under typical clear conditions that estimates to about 2,200 lux, which lands just inside the high tier: a pothos or a monstera there is comfortable, with room to spare. That is an estimate for a clear day. The readings people logged a metre or so from a window, across every direction, hour and sky, have a middle of 526, which is one more reason to put a meter on the spot rather than trust the direction.
Now put a croton in the same spot. Same light, same 2,200 lux, and the verdict drops to a caution, because a croton is a high-tier plant and the number that flattered the pothos runs thin for it.
Worth a try
It can work here with a little care.
A little dim, but workable
Comfortable temperature
Humidity suits it
Airflow suits it
Everything else on that card is green. The temperature suits it, the humidity suits it, the air suits it. Light is the one dimension you cannot judge by standing in the room, and it is the one that quietly decides whether this plant grows or merely persists.
The numbers that look wrong
Below about 50 lux you are not in a low-light spot, you are in a no-light spot. When we measured six points across one ordinary living room, the dark corner read 18 lux. No tier contains that number. Tolerance gets a tough plant through a dim winter; it does not get any plant through a corner the light never reaches. That 18 comes from a six-spot walk across one ordinary living room.
Above about 30,000 lux you are reading direct sun. That is not an error, it is just a different question. Direct sun readings swing enormously with the hour and the season, and for most foliage plants the concern flips from not enough to too much, too fast. The sill plants in the very-high tier are the ones built for it.
And if your reading seems to disagree with a plant that is visibly thriving, believe the plant. A reading is one moment; the plant is the average of every hour it has stood there.
About this page
The tier boundaries are conventions, the foot-candle brackets long used in interior growing, converted at one foot-candle to 10.76 lux and rounded, which is why the lookup flags readings near a boundary instead of pretending the line is sharp. The library shares come from our curated care data, where each species carries a light tier.
The indoor readings are measured, and only measured: our open dataset counts a phone’s built-in light sensor, white paper over a phone camera, or a separate light meter, and leaves out the app’s own estimates and grow lights. The 18 lux corner is from one room on one clear day. The worked example is a real engine result from the same scoring that runs in the app and the free spot checker; its light figure is an estimate, and we say so where it appears.
By GrowSpot · Published August 6, 2026 · Last updated October 3, 2026
