Indoor Plant Light Calculator
Determine indoor light level from window direction and distance.
Returns bright, medium, or low classification with plant recommendations and grow light specs.
Indoor plant light and watering needs are quantified through light intensity (foot-candles or lux), photoperiod (hours of light per day), and watering frequency based on pot size, medium, and environmental conditions.
Light intensity conversion: 1 foot-candle (fc) = 10.764 lux 1 lux = 0.0929 fc
Light requirements by category:
- Low light: 50–250 fc (550–2,700 lux) — north-facing windows, hallways
- Medium light: 250–1,000 fc (2,700–10,750 lux) — east/west windows, 3–4 ft from south window
- Bright indirect: 1,000–2,000 fc (10,750–21,500 lux) — 1–2 ft from south window, diffused
- Direct sun: 2,000–10,000+ fc — unobstructed south-facing window or grow light
Artificial grow light formula: PPFD needed (μmol/m²/s) × DLI (daily light integral) = Light Hours
DLI = PPFD × 3,600 × Light Hours ÷ 1,000,000
Typical plant DLI targets:
- Low-light houseplants: 1–3 mol/m²/day
- Medium plants (pothos, philodendron): 3–8 mol/m²/day
- High-light (succulents, herbs): 8–20 mol/m²/day
- Fruiting vegetables indoors: 20–40 mol/m²/day
Watering frequency estimate: Days Between Watering = Pot Volume (liters) × Evapotranspiration Factor ÷ Daily Water Loss (ml/day)
Simplified rule:
- Small pot (4″): water every 5–7 days in summer, 10–14 days in winter
- Medium pot (6–8″): every 7–10 days summer, 14–21 days winter
- Large pot (10–14″): every 10–14 days summer, 21–30 days winter
Finger test: insert finger 1–2 inches into soil. Dry = water. Moist = wait.
Popular houseplants — light and water cheat sheet:
- Pothos: low–medium light; water when top 2″ dry
- Snake plant (Sansevieria): low light; water every 2–6 weeks
- Monstera deliciosa: bright indirect; water when top 2–3″ dry
- Peace lily: low–medium light; water when leaves begin to droop slightly
- Fiddle-leaf fig: bright indirect; water weekly in summer
- ZZ plant: low light; water every 2–4 weeks (drought-tolerant)
Worked example — grow light sizing: You want to grow herbs (basil, mint) indoors. Target DLI: 15 mol/m²/day. Grow light PPFD: 200 μmol/m²/s.
Hours needed = (15 × 1,000,000) ÷ (200 × 3,600) = 15,000,000 ÷ 720,000 = 20.8 hours
That’s too long — upgrade to a 400 PPFD light: 15,000,000 ÷ (400 × 3,600) = 10.4 hours per day. Much more practical.