Most indoor plants need 250–500 foot candles (approximately 2,500–5,400 lux) of light to thrive — roughly the level you’d find 4–6 feet from a south-facing window on a clear day. Herbs and fruiting edibles need more: 500–1,000+ foot candles. True low-light plants like pothos and snake plants survive at 50–150 foot candles but grow slowly at those levels.
The problem is that “low light,” “medium light,” and “bright indirect” are marketing terms, not measurements. Here’s what each actually means in foot candles, how to check your own home’s light levels, and when to add a grow light rather than fight your windows.
What “Low,” “Medium,” and “Bright” Light Actually Mean
Foot candles (fc) measure the light intensity your plant receives at leaf level — not what the sun provides outside, but what actually reaches the plant. A foot candle is the illuminance of one candle at one foot away. Plants don’t experience your window’s theoretical output; they experience what arrives after passing through glass, curtains, and whatever’s between them and the source.
| Light Category | Foot Candles | Lux | Typical Home Location |
|---|---|---|---|
| Very low / dim | 25–50 fc | 270–540 lx | Interior rooms away from windows; north-facing rooms in winter |
| Low light | 50–150 fc | 540–1,600 lx | North-facing windows; 6–8 ft from a south or east window |
| Medium / indirect | 150–500 fc | 1,600–5,400 lx | 3–5 ft from a south or east window; near a west window |
| Bright indirect | 500–1,000 fc | 5,400–10,800 lx | 1–3 ft from a south or east window; filtered through a sheer curtain |
| Direct sun (indoors) | 1,000–2,000 fc | 10,800–21,500 lx | Unobstructed south or west window sill; morning east window |
Light Requirements by Plant Type
Use this reference table to match your plant to its actual light needs — and to decide whether your current space delivers it or whether a grow light is worth adding.
| Plant Type | Min. Light (fc) | Ideal Light (fc) | Daily Hours | Grow Light Needed? |
|---|---|---|---|---|
| Pothos, ZZ plant, snake plant | 50 fc | 100–200 fc | 10–14 hrs | Only in very dark rooms |
| Peace lily, Chinese evergreen | 75 fc | 150–300 fc | 10–14 hrs | Rarely; only in deep interior |
| Heartleaf philodendron | 100 fc | 200–400 fc | 12–14 hrs | Sometimes in winter |
| Herbs (mint, parsley, chives) | 200 fc | 400–800 fc | 14–16 hrs | Yes, in most interior spaces |
| Herbs (basil, cilantro, dill) | 400 fc | 800–1,200 fc | 14–16 hrs | Yes — needs full-sun equivalent |
| Seedlings (first 4 weeks) | 100 fc | 200–400 fc | 14–18 hrs | Yes — consistent levels are critical |
| Leafy greens (lettuce, spinach) | 200 fc | 400–600 fc | 12–16 hrs | Yes for productive growth |
| Fruiting plants (tomato, pepper) | 500 fc | 1,000–2,000 fc | 14–18 hrs | Yes — full grow light setup needed |
How Much Light Does Your Home Actually Deliver?
Most people overestimate their indoor light levels significantly. A south-facing window on a clear summer day provides 1,000–2,000 foot candles at the sill. But at just 4 feet from that window, levels drop to roughly 150–300 foot candles — medium indirect. At 8 feet away, you’re at 50–100 foot candles even on a sunny day.
Windows lose approximately 50% of available light through glass alone. Outdoor obstructions — buildings, trees, roof overhangs, awnings — reduce it further. In winter, the sun’s lower angle means south-facing windows that delivered 1,500 fc in July may provide only 300–400 fc in December. North-facing rooms in winter can drop to 25–50 foot candles, which is too dim for nearly any plant to sustain meaningful growth.
How to measure it: Inexpensive lux meters are widely available at low cost and let you measure actual foot candles at plant level. Divide the lux reading by 10.76 to convert to foot candles. Take readings at the same time of day over a few days to account for cloud cover. This removes the guesswork entirely and tells you exactly what your plants are working with.
When to Add a Grow Light
Consider adding a grow light when:
- Your herbs or edibles are leggy, pale, or not producing despite good care
- It’s winter and your window light has dropped measurably below summer levels
- You’re starting seeds indoors and need consistent, even intensity throughout the tray
- Your room measures below 150 foot candles but you want to grow herbs or leafy greens productively
- Your plant shows signs of light deficiency (yellowing from the top, etiolation, stalled growth)
For specific grow light recommendations at every budget, our grow lights buying guide covers 11 verified picks with CPSC-screened products and primary-source specs. To understand the signs your plant specifically needs more light, see our guide on when to use a grow light.
Frequently Asked Questions
How much light does a pothos need?
Pothos (Epipremnum aureum) survives at 50 foot candles and thrives between 100–200 foot candles of indirect light. It tolerates low light better than most houseplants but grows significantly faster in medium indirect light (200–400 fc). A grow light is rarely needed unless the room genuinely has no windows or remains below 50 fc year-round.
How many hours of light do indoor plants need per day?
Houseplants: 10–14 hours. Herbs and edibles: 14–16 hours. Seedlings: 14–18 hours. All plants benefit from some dark period (6–10 hours) for respiration and metabolic processes. Running a grow light 24 hours a day stresses most species and is not recommended. A plug-in timer makes maintaining the schedule automatic.
What is “bright indirect light” in foot candles?
Bright indirect light is typically 500–1,000 foot candles — the light level near a south or east-facing window but not in the direct beam of sunlight. This is often achieved 1–3 feet from the window glass on a sunny day, or through a sheer curtain. Most flowering houseplants and tropical aroids thrive in this range.
Can a grow light fully replace sunlight?
For most herbs, leafy greens, and houseplants: yes. A grow light set at the right distance and run for the appropriate daily hours provides all the photosynthetically active radiation these plants need. Most indoor edibles max out their photosynthetic rate at 1,000–2,000 foot candles — well within what a good LED panel delivers. Outdoor full sun provides 10,000+ fc, which is far more than most indoor crops can use.
Do grow lights help plants in a north-facing room?
Yes. North-facing rooms in winter can drop to 25–50 foot candles — too low for most plants to grow meaningfully. Even a basic LED grow light strip raises effective levels to 100–300 foot candles, which is sufficient for shade-tolerant houseplants and some herbs. For basil or cilantro in a north-facing room, a more powerful LED panel is needed to reach the 400–800 fc these herbs require.
How do I know if my plant is getting too much light?
Signs of excessive light: bleached or whitened patches on the upper surface of leaves (particularly facing the light source), brown crispy leaf margins, leaves curling upward, or wilting despite adequate moisture. Unlike light deficiency (which develops gradually), light overexposure typically appears within 1–3 days of moving a plant too close to a strong light source. Raise the fixture 4–6 inches and monitor for one week.
General growing information is provided for educational purposes — verify specific plant requirements with a qualified horticulturist if you have particular concerns about your plants.