Cannabis Grow Light Calculator — DLI & PPFD Estimator

Estimate the PPFD at your canopy, calculate Daily Light Integral (DLI), and find out whether your grow light setup delivers optimal intensity for seedlings, veg, or flower. Free, instant results — no sign-up required.

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Technology of your fixture

In feet

In feet

Inches above canopy

Photoperiod length

Current plant stage

Your Grow Light Results

Estimated PPFD—
Daily Light Integral (DLI)—
Coverage Area—
Watts per Sq Ft—
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What Is DLI and Why It Matters for Cannabis

Daily Light Integral (DLI) measures the total amount of photosynthetically active light a plant receives in a 24-hour period, expressed in mol/m²/day. While PPFD (Photosynthetic Photon Flux Density) tells you how intense the light is at any given moment, DLI tells you the cumulative dose your plants actually get. Think of PPFD as the speed of a faucet and DLI as the total water collected in a bucket over the day.

Cannabis is a high-light crop. Getting the right DLI at each growth stage drives photosynthesis, terpene production, and cannabinoid development. Too little light means stretchy, airy buds; too much causes bleaching and heat stress. The formula is straightforward: DLI = PPFD × 3600 × hours of light / 1,000,000.

Optimal Light Levels for Each Cannabis Growth Stage

Different growth stages have different light appetites. Use the table below as a reference when dialling in your setup.

Stage PPFD Range (µmol/m²/s) DLI Target (mol/m²/day) Light Hours Notes
Seedling / Clone 200 – 400 12 – 18 18 – 24 h Keep lights high; fragile roots need gentle intensity
Vegetative 400 – 600 25 – 40 18 h Gradually increase PPFD as plants grow; supercrop or train to keep canopy even
Early Flower 600 – 800 35 – 50 12 h Transition slowly over 1 – 2 weeks; watch for stretch
Late Flower 800 – 1,000 45 – 65 12 h Maximum intensity for dense buds; supplement CO2 above 1,000 PPFD

LED vs HPS vs CMH — Comparing Grow Lights

Choosing the right light technology affects your electricity bill, heat management, and ultimately your harvest quality. Here is a quick side-by-side comparison.

Light Type Efficiency (µmol/J) Heat Output Lifespan Upfront Cost Best For
LED 2.0 – 3.0 Low – Moderate 50,000 – 100,000 h $$$ – $$$$ All stages; best long-term value and efficiency
HPS 1.5 – 1.9 High 10,000 – 24,000 h $ – $$ Flower; proven performer, low entry cost
CMH / LEC 1.7 – 2.1 Moderate 15,000 – 20,000 h $$ – $$$ Full cycle; excellent spectrum and efficiency balance
CFL / T5 0.7 – 1.1 Low 8,000 – 15,000 h $ Seedlings and clones; low heat, low intensity

How to Measure Light in Your Grow Tent

The most accurate way to measure PPFD is with a quantum PAR meter such as the Apogee MQ-500 or the more budget-friendly Photone app for smartphones. Position the sensor at canopy height and take readings at multiple points across your grow space to create a coverage map.

  • PAR meters — professional-grade accuracy; essential for dialling in commercial setups. Price range: $100 – $500.
  • Phone apps (Photone, PPFD Meter) — surprisingly decent for hobbyists. Accuracy within 10 – 15% of dedicated meters after calibration.
  • Coverage mapping — divide your canopy into a 3×3 or 4×4 grid, take a PPFD reading at each point, and average them. This reveals hot spots and dim edges.

Light distance guidelines:

  • LEDs (200 – 300W): 18 – 24 in. during veg, 12 – 18 in. during flower
  • LEDs (400W+): 24 – 30 in. during veg, 16 – 22 in. during flower
  • HPS (600 – 1000W): 24 – 36 in. year-round; use a hand test for heat
  • CFL / T5: 2 – 6 in. from canopy (low heat, low intensity)

Frequently Asked Questions

How many watts per square foot do I need for cannabis?
For modern LEDs, aim for 30-40 watts per square foot during flower and 20-30 watts per square foot during veg. HPS lights are less efficient, so you will need 40-60 watts per square foot. These are rough guidelines — actual light output depends on the specific fixture's efficiency rating.
What is the ideal DLI for flowering cannabis?
Flowering cannabis performs best with a DLI between 40 and 65 mol/m²/day. Most commercial growers target around 45-55 mol/m²/day during peak flower. Going above 65 can cause light stress, bleaching, and diminishing returns. Use a 12/12 light schedule and adjust PPFD upward to hit your DLI target within fewer hours.
Can you give cannabis too much light?
Yes. Light stress (photoinhibition) occurs when PPFD exceeds roughly 1,000-1,200 µmol/m²/s for most cultivars, especially without supplemental CO2. Symptoms include bleached or white leaf tips, taco-curling leaves, and foxtailing in flower. If you add CO2 to 1,200-1,500 ppm, plants can handle up to 1,500 PPFD.
How high should I hang my grow light?
It depends on the light's power and beam angle. As a general starting point: LEDs 18-24 inches above the canopy during veg, 12-18 inches during flower. HPS fixtures run hotter and need 24-36 inches. Always check the manufacturer's recommendation and use a PPFD meter or this calculator to verify intensity at canopy level.
Do I need different lights for veg and flower?
Not necessarily. Full-spectrum LEDs work well for both stages — you simply adjust intensity and photoperiod. However, some growers prefer bluer spectra (5,000-6,500K) for compact veg growth and warmer spectra (2,700-3,500K) to boost flowering. Many modern LEDs include a veg/bloom switch or dimmable spectrum control.
How do I calculate PPFD from watts?
You cannot convert watts to PPFD directly because efficiency varies by fixture. The formula uses an efficiency factor (measured in µmol per joule): PPFD ≈ (Wattage × Efficiency) / Coverage Area in m². Top-tier LEDs achieve 2.5-3.0 µmol/J, while HPS averages around 1.7 µmol/J. This calculator applies those factors automatically.

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