Grow Light Coverage & PPFD Calculator

Grow Light Coverage & PPFD Calculator

Calculate average PPFD at canopy level, daily light integral (DLI), and how many fixtures you need to hit your target — for any grow space and any light. Need help choosing the right fixture? Talk to a Grow Expert.

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PPFD & Light Coverage Calculator

Enter room dimensions and fixture details. Switch to Advanced to set canopy efficiency and target growth stage.

How to use this calculator
  1. Enter room dimensions — length and width in feet.
  2. Enter your fixture count and the PPF rating from your fixture's spec sheet (not lumen output).
  3. Set your photoperiod — hours the lights are on per day (18 hrs veg / 12 hrs flower).
  4. Advanced: adjust canopy efficiency % for your wall type and select a growth stage for a target comparison.
  5. Tap Calculate PPFD — results show average PPFD, DLI, status chip, and a fixture count suggestion.
  6. Open the heatmap below to see a top-down view of estimated light distribution across your canopy.

Formula: avg PPFD = (fixtures × PPF / area in m²) × (efficiency / 100)

DLI formula: DLI (mol/m²/day) = PPFD × photoperiod hours × 3600 / 1,000,000

Target PPFD ranges: Seedling 100–300 · Veg 400–600 · Flower 600–1,000 · Flower + CO₂ 800–1,500 µmol/m²/s

PPFD & DLI Quick Reference

Target PPFD and DLI values by growth stage — Hydrobuilder grower reference guide
Growth Stage Target PPFD (µmol/m²/s) Target DLI (mol/m²/day) Typical Photoperiod Notes
Seedling / Clone 100–300 6–12 18–20 hrs High intensity causes stress; start low and ramp
Vegetative 400–600 20–30 18 hrs Most LED bar fixtures at standard height
Flowering 600–1,000 25–40 12 hrs High PPFD drives yield; ensure adequate airflow
Flowering + CO₂ 800–1,500 35–55 12 hrs CO₂ at 1,200–1,500 ppm unlocks photosynthesis above 1,000 µmol/m²/s
Propagation (T5) 50–150 3–8 18–24 hrs Soft light only; root development is priority
Greens / Herbs 200–400 12–20 16–18 hrs Lettuce, basil, and leafy crops need less intensity than fruiting plants

Frequently Asked Questions

What is PPFD and why does it matter for growing?
PPFD (Photosynthetic Photon Flux Density) measures the amount of light that actually reaches your plant canopy, expressed in µmol/m²/s. Unlike lumen ratings — which measure human-perceived brightness — PPFD measures the wavelengths plants use for photosynthesis. Too low and plants stretch or yield poorly; too high causes light stress, bleaching, and nutrient lockout. Hitting the right PPFD for your growth stage is one of the highest-leverage adjustments you can make.

What is DLI and how is it calculated?
DLI (Daily Light Integral) is the cumulative amount of photosynthetically active light delivered over an entire day, measured in mol/m²/day. It's calculated as: DLI = PPFD × photoperiod hours × 3600 / 1,000,000. A DLI of 20–40 mol/m²/day is typical for flowering cannabis, while seedlings only need 6–12. DLI is especially useful when comparing different photoperiods — a 12-hour flowering plant may need higher PPFD to match the DLI of an 18-hour veg plant.

What's the difference between PPF and PPFD?
PPF (Photosynthetic Photon Flux) is the total light output of a fixture in µmol/s — how much PAR light the fixture produces per second in all directions. PPFD is what arrives at canopy level after accounting for mounting height, room size, and fixture spacing. A fixture with high PPF but poor mounting height or spacing can deliver inconsistent PPFD across your canopy. Always compare fixtures using PPF from the spec sheet, then use this calculator to find the PPFD your plants actually receive.

How many grow lights do I need for my space?
Fixtures needed = (target PPFD × grow area in m²) / (PPF × canopy efficiency). This calculator automates that math and shows a fixture suggestion alongside the result. A common rule of thumb for flowering cannabis is 600–900 µmol/m²/s; at 85% efficiency, a 1,000 µmol/s PPF fixture covers roughly 1.4 m² to 2.0 m². Use the heatmap to check uniformity — edge falloff matters more than averages when maximizing yield.

Does mounting height affect PPFD?
Yes — significantly. Light intensity follows the inverse square law: doubling the distance between the fixture and canopy reduces PPFD by approximately 75%. Most LED fixtures publish PPFD maps at a specific mounting height (commonly 18–24 inches). The canopy efficiency percentage in Advanced mode accounts for this real-world falloff. Always follow manufacturer mounting height recommendations and verify with a PAR meter at multiple canopy points for accurate readings.

What is a good watts-per-square-foot target?
For modern high-efficiency LEDs, target 30–50 watts/ft² for flowering (using wall plug watts, not equivalent claims). Older HPS systems needed 50–60 watts/ft² to reach the same PPFD because of lower efficacy. However, watts/ft² is a rough heuristic — PPF and PPFD are far more meaningful because they measure actual photons delivered, not energy consumed. Use watts/ft² as a sanity check, and PPFD as your primary metric.
💡 Top-Down Light Distribution Heatmap Heatmap ▾
Low PPFD
Target Zone
High PPFD
Excess (hotspot)

Estimated top-down PPFD distribution across your canopy based on current inputs. Fixtures are positioned in an even grid. Hover or tap any point to see the estimated PPFD at that location. Run the calculator first to update this map with your values.

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