Grow Light Distance Calculator – PAR & Foot‑Candles to Plant

PPFD (Photosynthetic Photon Flux Density) measures the amount of photosynthetically active photons hitting a surface per second, expressed in µmol/m²/s. It's the most meaningful metric for growers because it quantifies the light plants can actually use for photosynthesis. Unlike lux or foot‑candles — which are weighted to human vision — PPFD directly correlates with plant growth rate and yield potential.

The Inverse Square Law states that light intensity decreases proportionally to the square of the distance from the source. If you double the distance, PPFD drops to ¼ of the original value. For example: 800 µmol/m²/s at 12 inches becomes just 200 µmol/m²/s at 24 inches. This is why small adjustments in hanging height can dramatically affect plant light exposure — and why this calculator uses the formula d₂ = d₁ × √(I₁/I₂) to find your optimal distance.

Converting foot‑candles or lux to PPFD requires a conversion factor that depends on the light spectrum. Approximate factors:

• LED full‑spectrum: 1 fc ≈ 0.18 µmol/m²/s | 1 klux ≈ 16 µmol/m²/s
• HPS: 1 fc ≈ 0.13 µmol/m²/s | 1 klux ≈ 12 µmol/m²/s
• Metal Halide: 1 fc ≈ 0.15 µmol/m²/s | 1 klux ≈ 14 µmol/m²/s
• Fluorescent: 1 fc ≈ 0.16 µmol/m²/s | 1 klux ≈ 15 µmol/m²/s
• Natural daylight: 1 fc ≈ 0.20 µmol/m²/s | 1 klux ≈ 18.5 µmol/m²/s

This calculator applies the correct factor automatically based on your selected light type.

Seedlings and clones are delicate and require gentle light: 100–250 µmol/m²/s. Too much light at this stage causes stress, wilting, and bleaching. Start at the lower end (~150) and gradually increase as roots establish. Keep the light 24–36 inches away for most LED panels, or use dimming if available.

During peak flowering, cannabis thrives at 800–1000 µmol/m²/s without CO₂ supplementation. With CO₂ enrichment (1200–1500 ppm), plants can utilize 1000–1500 µmol/m²/s for maximum yields. Exceeding 1000 µmol/m²/s without added CO₂ often leads to diminishing returns — the plant cannot process the extra photons efficiently and may show signs of light stress (bleached buds, curled leaves, foxtailing).

LED distance varies by wattage and lens design. General guidelines:

• 100–150W LED panels: 12–18 inches for veg, 10–14 inches for flower
• 240–300W LED panels: 18–24 inches for veg, 14–18 inches for flower
• 450W+ LED panels: 24–30 inches for veg, 18–24 inches for flower

Always check manufacturer PPFD charts and use this calculator with your specific values for precise placement. LED lights with lenses or secondary optics can have more focused beams — requiring slightly greater distance.

Yes, absolutely. Placing lights too close causes:

• Light burn / bleaching: Top leaves and buds turn white or yellow
• Heat stress: Leaf edges curl upward, tips brown
• Excessive transpiration: Plants dry out too quickly
• Photoinhibition: Photosynthesis actually decreases above ~1000–1200 µmol/m²/s (without CO₂)

If your calculated distance is under 8 inches, consider dimming your light or raising it and accepting slightly lower PPFD for safety.

DLI (Daily Light Integral) is the total amount of photosynthetically active photons delivered over a full day, measured in mol/m²/day. Formula: DLI = PPFD × hours × 3600 ÷ 1,000,000. For example, 600 µmol/m²/s × 18 hours = 38.9 mol/m²/day. Cannabis typically needs: seedlings 12–16, veg 20–30, flower 30–45 mol/m²/day. This calculator estimates DLI based on your PPFD result and light schedule.

As you raise the light, coverage area increases but intensity decreases (inverse square law). The coverage diameter depends on the beam angle: Coverage Diameter = 2 × Distance × tan(Beam Angle ÷ 2). A 100° beam at 18 inches covers roughly a 43‑inch diameter circle. This calculator estimates coverage based on your beam angle input — helping you plan your grow space efficiently.

Adjust height weekly during vigorous growth phases. As plants grow taller, the distance to the light decreases — and PPFD increases exponentially. A plant growing 3 inches closer to the light can experience a 30–50% increase in PPFD. Regular adjustments maintain consistent light levels and prevent light stress. During the flowering stretch, check every 2–3 days.