PPFD Calculator
Calculate the light intensity your plants actually get — wattage isn’t enough, PPFD (photosynthetic photon flux density) is what counts.
Calculated with PlantClue
Why PPFD matters more than watts
What is PPFD?
PPFD (photosynthetic photon flux density) measures the actual amount of light that hits one square meter per second — in µmol/m²/s. Plants only use light in the 400–700 nm range (PAR) for their photosynthesis rate. A light’s wattage only tells you how much power it draws, not how much of it reaches your plants as usable light.
Why distance is key
PPFD drops with the square of the distance (inverse-square law). A light at 12 in (30 cm) delivers about four times as much PPFD as at 24 in (60 cm). At the same time, the evenly lit area gets bigger as you raise the light. Finding the right distance is the easiest way to optimize PPFD without buying new hardware.
Estimate vs. measurement
This calculator gives you an informed estimate. For precise values you need a PAR meter (Apogee MQ-500, LI-COR LI-190). The calculated values help you find the right ballpark and avoid big mistakes (too dark, too bright) — before you spend money or risk damage.
PPFD Chart: Target Values, Light Efficiency & Coverage
The calculator above uses these values internally for its estimate. For your exact wattage, light type and distance, use the calculator — here you get the quick overview.
| Growth stage | Target PPFD range |
|---|---|
| Seedling / clone | 200–350 µmol/m²/s |
| Vegetative stage | 400–600 µmol/m²/s |
| Early flower | 600–800 µmol/m²/s |
| Late flower | 700–1,000 µmol/m²/s |
| Light type | Efficiency (µmol/J) |
|---|---|
| LED premium (Samsung LM301 / Osram) | 2.6 |
| LED mid-range | 2.0 |
| LED budget (unknown chips) | 1.5 |
| CMH / LEC | 1.35 |
| HPS / HID | 1.05 |
| T5 fluorescent | 0.75 |
| Light distance | Coverage |
|---|---|
| ≤ 8 in (20 cm) | 60 % |
| 8–12 in (21–30 cm) | 68 % |
| 12–16 in (31–40 cm) | 74 % |
| 16–20 in (41–50 cm) | 80 % |
| 20–24 in (51–60 cm) | 84 % |
| 24–30 in (61–75 cm) | 87 % |
| > 30 in (75 cm) | 88 % |
Coverage: a simplified estimate of the share of light output that lands on your grow area — real values depend heavily on the light’s reflector and optics.
Download the PPFD chart as a PDFWhat can go wrong when you dial in PPFD
- Treating watts as PPFD — a 300 W budget LED can deliver less PPFD than a 200 W premium LED with better chips.
- Hanging the light too close — too much PPFD (light burn) shows up as white or brown spots on the top leaves.
- Measuring only the center — PPFD is always higher in the middle under the light than at the edges. Several measuring points give you the real picture.
- Blindly trusting manufacturer specs — many cheap LEDs are advertised with peak PPFD under ideal lab conditions, not the real-world value in your tent.
PPFD — what growers want to know
What is PPFD and why does it matter?
PPFD (photosynthetic photon flux density) measures the number of light particles (photons) that hit one square meter of surface per second — in µmol/m²/s. It’s the key factor for your plants’ photosynthesis rate, not the light’s wattage.
How much PPFD do cannabis plants need?
Seedlings and clones handle 200–350 µmol/m²/s. In the vegetative stage, 400–600 µmol/m²/s is ideal. In early flower, 600–800 µmol/m²/s is ideal, and in late flower 700–1,000 µmol/m²/s. Above 1,000 µmol/m²/s is usually counterproductive without added CO2.
Why is my calculated PPFD only an estimate?
PPFD depends on many factors: reflector shape, tent color, light angle, heat and the fixture’s actual PPF rating. Without a meter (PAR meter), every calculated value is an informed estimate. For precise measurements we recommend an Apogee or LI-COR meter.
What’s the difference between PPFD and DLI?
PPFD is the light intensity at a given moment (how bright). DLI (daily light integral) is the total amount of light over a day (how long × how bright). DLI = PPFD × hours of light × 3,600 ÷ 1,000,000. Together, both values give you the full picture.
Which light type has the best PPFD per watt?
Premium LEDs (Samsung LM301B/H, Osram) now deliver 2.4–2.8 µmol/J — far more than HPS (about 1.0–1.1 µmol/J) or CMH (about 1.3 µmol/J). Budget LEDs without brand-name chips often land at 1.4–1.6 µmol/J, despite attractive wattage claims.
