Understanding Indoor Light for Houseplants
Light is the single biggest factor determining whether your houseplant thrives or slowly declines—yet it's also the most commonly misjudged. Our eyes adapt to dim rooms, making us terrible at estimating actual light levels. A space that feels “bright” to you might deliver only 50 foot-candles to your plant, while a sunny windowsill could blast 5,000. This calculator replaces guesswork with numbers your plants actually respond to.
The Four Factors That Determine Your Light Level
1. Window Direction
In the Northern Hemisphere, south-facing windows deliver 5-10x brighter light than north-facing windows. East windows get gentle morning sun; west windows get harsher afternoon rays. North windows provide consistent but low light year-round.
2. Distance from Window
Light doesn't fade gradually—it drops off dramatically. Move just 3 feet from a bright window and you might lose 50% of available light. At 8-10 feet, even a south-facing window delivers only low-light conditions.
3. Obstructions
Trees, neighboring buildings, overhangs, and dirty windows can slash incoming light by 20-60%. A south window blocked by a large oak might deliver less light than an unobstructed north window. Sheer curtains reduce light by 20-40%.
4. Window Size
Larger windows don't just let in more light—they project it deeper into the room, expanding the usable growing area. Two medium windows can outperform one small window even if they face a “worse” direction.
What Foot-Candles Mean for Your Plants
Not all houseplants need the same light, and foot-candles give you an objective way to match plants to locations. Low light (25-250 fc) suits ZZ Plants, Snake Plants, and Pothos—species evolved for forest floors. Medium light (250-1,000 fc) works for most tropical houseplants: Philodendrons, Calatheas, Monsteras, and Peace Lilies. These are your workhorses—tolerant of some variation but happiest with consistent, moderate brightness.
Bright indirect light (1,000-5,000 fc) is what Fiddle Leaf Figs, Birds of Paradise, and Rubber Trees crave—the intensity near a sunny window without harsh midday rays. Finally, direct sun (5,000+ fc) is reserved for succulents, cacti, most herbs, and a handful of tropical sun-lovers. Putting a low-light plant in direct sun scorches leaves; putting a succulent in a dim corner guarantees leggy, weak growth. Match the plant to the light, not the other way around.
Signs Your Plant Is Getting Wrong Light
When a plant doesn't get enough light, it tells you—if you know what to look for. Stems stretch toward the nearest window and become leggy and weak. New leaves emerge smaller than older ones. Variegated plants lose their patterns, reverting to solid green to maximize chlorophyll. Growth slows to a crawl or stops entirely. If your plant hasn't produced new growth in months and you're watering correctly, light is almost always the culprit.
Too much light causes different damage. Leaves develop pale, bleached patches or brown, crispy spots—especially on the side facing the window. The plant may wilt during peak sun hours even when the soil is moist, as it can't transpire fast enough. Colors fade from vibrant green to washed-out yellow-green. If you're seeing scorch marks or faded foliage, move the plant back from the window or add a sheer curtain to diffuse the intensity.
Why Measuring Light Matters
Studies suggest that 60-70% of houseplant failures trace back to light problems—usually placement in conditions far dimmer than the plant requires. The challenge? Human eyes are spectacularly unreliable light meters. Our pupils adjust, our brains compensate, and what feels like a “well-lit room” often measures below 100 foot-candles—barely enough for the most shade-tolerant species. Using a calculator (or a light meter app) removes the guesswork. You get an objective number, match it to your plant's requirements, and dramatically improve your odds of success.
Frequently Asked Questions
How do I know which direction my window faces?
Use your phone's compass app while standing at the window, facing outside. The direction shown is your window's orientation. Alternatively, note where the sun rises (east) and sets (west)—if your window gets morning sun, it faces east; afternoon sun means west; all-day sun in the Northern Hemisphere means south.
Can artificial light replace natural light?
Yes, but most standard room lighting is far too weak. A typical ceiling fixture delivers 20-50 foot-candles—low light at best. Grow lights work because they output 200-2,000+ foot-candles at close range. Position grow lights 6-12 inches from the plant and run them 10-14 hours daily. Check out our grow light guide for recommendations.
What about sheer curtains—do they count as obstructions?
They do. Sheer curtains typically reduce incoming light by 25-40%, depending on fabric weight. That might drop a bright-light spot into medium territory. This isn't necessarily bad—sheers can protect sensitive plants from harsh direct sun—but factor them into your calculations using the obstruction setting.
Do seasons affect indoor light levels?
Significantly. In winter, the sun sits lower in the sky, enters at a sharper angle, and days are shorter. A spot getting 800 foot-candles in June might receive only 300 in December. Many tropical houseplants struggle in winter—it's usually a light issue, not temperature. Consider moving plants closer to windows during winter months.
Why doesn't my plant thrive in “bright indirect light”?
Because “bright indirect light” is vague to the point of uselessness—it could mean 400 foot-candles or 2,000. Your Fiddle Leaf Fig sulking in “bright indirect” might actually be getting low-to-medium light. Instead of relying on subjective descriptions, measure the actual foot-candles. If your “bright” spot measures below 500 fc, it's not bright enough for most bright-light plants.
Ready to find the perfect plants for your light conditions? Browse our low-light plants, medium-light plants, or bright-light plants. Need help choosing? Take our plant quiz to get personalized recommendations based on your actual conditions.