The grow light distance calculator tells you how far to hang your grow light, how much area it covers, and how many hours per day to run it — based on your light type, wattage, and your plant's specific light needs.
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How to Use the Grow Light Distance Calculator
Proper grow light placement is the difference between thriving plants and plants that survive but never grow. Too close, and you risk light burn — bleaching and crisping the leaves closest to the light. Too far, and the light intensity drops below the plant's minimum threshold for photosynthesis. The inverse square law means doubling the distance reduces light intensity to one quarter.
Step 1: Select Your Light Type and Wattage
Choose between LED, fluorescent (T5/T8 tubes), or HPS (High-Pressure Sodium). Each type has different heat output and intensity characteristics that affect placement distance. Enter the actual wattage — not the "equivalent" marketing wattage. For LED panels, use the actual power draw listed on the product specifications, not the maximum theoretical wattage.
Step 2: Select Your Plant's Light Needs
Choose your plant from the dropdown to auto-set light level, or select manually. Low-light plants (snake plants, ZZ plants, pothos) need 25–100 foot-candles. Medium-light plants (philodendrons, monsteras, peace lilies) need 100–300 foot-candles. High-light plants (succulents, herbs, citrus) need 300–600+ foot-candles.
Step 3: Interpret the Results
The recommended distance is the optimal hanging height for your specific setup. The PPFD estimate gives you a rough sense of light intensity at plant level. The coverage area shows how much floor space the light effectively illuminates at the recommended distance. If you entered growing area dimensions, the calculator also tells you how many lights you need for even coverage.
Signs Your Grow Light Distance Is Wrong
Too close: pale or bleached patches on upper leaves, curling leaves, or very slow growth in high-light plants. Too far: stretching (leggy growth), deep green but slow growth, or plants leaning toward the light. Adjust by 2-3 inches at a time and observe the plant's response over 2-3 weeks.
FAQ
How far should an LED grow light be from plants?
LED grow light distance depends on wattage and plant light needs. A general rule for LED: distance (inches) = √(wattage) × 1.5 for high-light plants, × 2.5 for low-light plants. For example, a 100W LED should be 15 inches from high-light plants, 25 inches from low-light plants. Enter your specific values in the calculator above for an exact recommendation.
How many hours should grow lights be on?
Most houseplants need 12-16 hours of grow light per day to supplement natural light. Low-light tolerant plants (snake plants, ZZ plants) are fine with 12 hours. Medium-light plants (pothos, philodendrons) do best with 14 hours. High-light plants (succulents, herbs, peppers) want 16 hours. Always provide a dark period — plants need 6-8 hours of darkness to complete photosynthesis cycles.
What is the difference between LED, fluorescent, and HPS grow lights?
LED grow lights are most efficient (lowest heat per watt), can be placed closer to plants, and last 50,000+ hours. Fluorescent lights (T5/T8 tubes) produce minimal heat and are ideal for seedlings and low-light plants. HPS (High-Pressure Sodium) lights produce more heat and require greater distance, but are powerful for large coverage areas. LEDs are best for houseplants.
How do I calculate grow light coverage area?
Coverage area depends on light intensity and distance. The light cone spreads at approximately 60° from the fixture. The formula is: coverage radius = distance × tan(30°) ≈ distance × 0.577. Coverage area = π × radius². A 100W LED at 24 inches produces approximately 8-10 sq ft of effective coverage for medium-light plants.
What is PPFD and why does it matter for grow lights?
PPFD (Photosynthetic Photon Flux Density) measures the amount of light usable for photosynthesis reaching the plant surface, in micromoles per square meter per second (μmol/m²/s). Low-light plants need 50-150 μmol/m²/s. Medium-light houseplants need 150-400 μmol/m²/s. High-light plants need 400-600+ μmol/m²/s. The calculator provides a rough PPFD estimate based on wattage and distance.
How many grow lights do I need for my growing area?
The calculator above estimates how many lights are needed based on your growing area dimensions and the coverage area of your selected light. As a rough rule, a 100W LED covers approximately 4×4 feet (16 sq ft) for medium-light plants. A 200W LED covers roughly 4×5 feet. Multiple smaller lights often provide more even coverage than a single large light.
Is this tool free?
Yes, completely free with no signup required. All calculations run in your browser.