How much light does my houseplant need?
Almost every popular plant guide talks about "bright indirect," "medium," and "low" light without giving you numbers. That's a problem: plants don't care about labels — they care about photons. The two measurements worth knowing are foot-candles (the old US unit, still used in most plant-care books) and lux (the international SI unit). One foot-candle is about 10.76 lux.
Here's what each named zone actually means in numbers, and the kinds of houseplants that grow versus merely cling on in each:
| Zone | Foot-candles | Lux | Plants that thrive here |
|---|---|---|---|
| Direct sun | 1,000+ | 10,750+ | Succulents, cacti, citrus, bird of paradise, jade plant |
| Bright indirect | 400 – 1,000 | 4,300 – 10,750 | Monstera, fiddle leaf fig, rubber plant, alocasia |
| Medium | 100 – 400 | 1,075 – 4,300 | Philodendron, calathea, spider plant, peperomia |
| Low | 25 – 100 | 270 – 1,075 | Snake plant, ZZ plant, peace lily, cast iron plant |
| Very low | < 25 | < 270 | Almost nothing photosynthesises sustainably here. Survival rather than growth. |
One pattern we see constantly: most beginner plant deaths are slow declines from cumulative low-light stress, not dramatic overwatering. A plant placed in light it can't photosynthesise in will gradually lose leaves over months — long after anyone remembers what changed. Light is the single most under-diagnosed cause of houseplant decline. (We explore this in our review of Darryl Cheng's The New Plant Parent, which makes the case better than any beginner book in print.)
What does "bright indirect light" actually mean?
"Bright indirect" is the phrase every plant tag uses and nobody defines. In numbers, it's roughly 400 to 1,000 foot-candles: enough to cast a sharp shadow but with no direct sunbeam falling on the leaves. In practice, it's the position one or two feet back from a south or west-facing window — close enough to catch the brightness, far enough that the sun's disc isn't visible from the plant's perspective.
A few practical heuristics, borrowed from Cheng:
- Sharp shadow on the wall? Bright (indirect or direct depending on where the beam falls).
- Soft, blurry shadow? Medium.
- No shadow at all? Low.
- Can you read a book comfortably without artificial light? Probably bright enough for most foliage plants.
- Can you see the sun's disc through the glass when you look up from where the plant sits? That's direct, not indirect.
The catch for UK readers, and anyone above about 50° latitude: most rooms in winter, even south-facing, only just clear the threshold for "medium" — let alone "bright indirect." This isn't a flaw in your home; it's the difference between Northern European winter light and the Californian baselines most plant-care books are written against. The calculator above makes that difference visible.
How this calculator works
We didn't want to be one more "south-facing window = bright" tool. The model takes four inputs and runs them through real solar geometry and real cloud cover:
- Location — your map pin gives latitude and longitude, which drive the sun's angle and the local cloud climatology.
- Month — drives sun elevation (the same window gets dramatically different light in January and June) and selects the right cloud-cover average.
- Window direction — determines whether the sun is in front of the glass or behind it. Our tool catches the moments when there's no direct sun at all (north-facing in northern winter, south-facing in southern winter), which generic tools miss.
- Obstructions and distance — sheers, trees, blinds and feet from the glass each pull the result down.
The math, briefly:
- Solar altitude and azimuth at local solar noon on the 15th of the chosen month, using NOAA's declination + equation-of-time formulas.
- Atmospheric attenuation via the Kasten-Young air-mass formula × Bird simplified clear-sky transmission (τ ≈ 0.7).
- Monthly cloud cover for the chosen location, averaged from the past three years of daily ERA5 reanalysis data via the Open-Meteo archive API (free, no key, cited).
- Window incidence: cosine of the angle between the sun vector and the window normal. Diffuse sky added in for the half-sky a vertical window sees.
- Indoor falloff: 1 / (1 + 0.35d + 0.04d²) where d is feet from the glass — gentler than pure inverse-square because the window acts as an extended source, not a point.
- Total irradiance in W/m² × 93 lm/W (sunlight's photopic luminous efficacy) gives lux, then ÷ 10.764 gives foot-candles.
We use solar noon rather than a full-day integral because it's the moment popular plant-care advice describes when it talks about "bright" or "low" light. A future enhancement will integrate over the full day (DLI — daily light integral) for growth-rate predictions rather than placement decisions.
Worked examples
The same window, in two cities or two months, can land in three different light zones. A few to anchor intuition:
Manchester · January · south-facing · 2 ft · unobstructed
~80 foot-candles. "Low light" by Cheng's thresholds. The classic UK winter problem: even a south-facing window in central Manchester at noon delivers light that most plant books would call too dim for the species they recommend. Most popular houseplants survive but don't grow here.
Manchester · June · same window
~1,170 foot-candles. Direct sun. The same window, six months later: enough to scorch most foliage plants unless you filter with a sheer curtain. A monstera or pothos would do better a foot back, or behind sheers.
Sydney · June · south-facing · 2 ft · unobstructed
~150 foot-candles. No direct sun. The tool flags this — and it's correct. In southern-hemisphere winter the sun sits in the north, so a south-facing window in Sydney in June sees only diffuse skylight. Calathea, philodendron and the medium-light foliage plants do well here; succulents won't.
London · October · east-facing · 4 ft · sheer curtains
~180 foot-candles. Medium. Morning direct sun, gentled by the sheer, then diffuse for the rest of the day. The calathea / spider plant / boston fern zone.
Frequently asked
Why does the result change so much by month?
The sun is significantly lower in winter than summer. At London's latitude (51.5° N) the noon sun reaches 62° above the horizon in June but only 15° in December. Lower sun means more atmosphere for light to pass through, less reaching your window, and — for vertical glass — a different geometric projection onto the window plane.
Is "bright indirect light" the same everywhere?
No. The phrase describes a range of foot-candles (400 – 1,000). What achieves that varies wildly by location and season. A south-facing window in Singapore delivers bright indirect almost year-round. A south-facing window in Edinburgh delivers it for maybe three months. The popular plant-care advice that says "all monsteras need bright indirect" is true but unhelpful unless you know what bright indirect means in your home.
Why do two cities at the same latitude give different numbers?
Cloud cover. The calculator pulls the past three years of daily cloud-cover averages from Open-Meteo's ERA5 archive for the chosen location and month. A consistently cloudy place returns lower numbers than a clear one at the same latitude — which is why Manchester and Vancouver come back darker than London and Paris despite similar sun geometry.
My plant looks unhappy. Is it the light?
Often, yes — and the symptoms are subtle. Light-related decline usually shows as small new leaves, leggy stretched stems, pale washed-out colour, or the slow loss of bottom leaves over weeks. If you're seeing those, cross-check the yellow-leaves guide and the leggy-growth guide. Move the plant gradually rather than in one leap — a sudden jump to a much brighter spot can scorch leaves acclimated to shade.
How accurate is the calculator?
Within roughly 15% under clear-sky conditions; less certain in diffuse or cloudy conditions because year-to-year cloud variance is real. The solar geometry is precise; the inputs aren't. Treat the readout as a calibration aid for plant placement, not a measurement instrument. For high-stakes positioning, supplement with a free lux-meter app like Photone (uses your phone's camera) and trust the plant's response above either reading.
Can I use this calculator outside the UK?
Yes — the map covers the whole world, and the cloud-cover data and solar geometry both work everywhere. The tool was built in the UK and our worked examples lean UK-shaped, but the model itself is universal. Northern Canadian winters and Singaporean monsoons both work fine.
Pair with
- Light Guide — how to assess indoor light without a calculator: windows, obstructions, the eye-test.
- Library: The New Plant Parent (Cheng) — where the foot-candle thresholds we use came from, and our UK calibration of them.
- Library: How Plants Work (Chalker-Scott) — the plant-physiology primer that grounds every science claim on this site.
- Diagnose: leggy growth — if your plant is stretching, the calculator above probably explains why.