PPFD / DLI Calculator
Is your grow light strong enough, and are your plants getting enough total daily light? Enter your canopy PPFD and photoperiod — get your DLI, a stage-target status check, and a clear answer on whether to raise your fixture, extend your lights, or stand pat. Works for cannabis, microgreens, herbs, vegetables, and bonsai.
Check your light intensity and daily light total
Estimate PPFD from fixture
Estimate, not truth
Wattage-based PPFD is a calculated estimate — real canopy readings from a quantum meter (Apogee SQ-500) are the only reliable measurement. Use this estimate as a planning starting point, not a verification.
Your light right now
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Enter your PPFD and photoperiod to see your DLI and stage-target check.
Common PPFD/DLI mistakes
- Using a lux meter or phone app — lux reads total visible light, not the PAR wavelengths plants use. Conversion factors are fixture-specific and wrong for LEDs. Use a quantum meter (Apogee, LI-COR) or estimate from fixture specs.
- Measuring at ceiling or fixture height — PPFD should be read at the canopy, where the plant actually is. Center-of-canopy and edge readings can differ 30–50% on the same fixture.
- Ignoring photoperiod when comparing DLI — 600 PPFD × 18h = 38.9 DLI is very different from 600 PPFD × 12h = 25.9 DLI. Always cite both numbers together.
- Chasing maximum PPFD without CO2 — plants hit a photosynthesis ceiling around 800–1000 PPFD at ambient CO2. More light without more CO2 just wastes electricity and risks bleaching.
Daily light integral
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mol/m²/day
Pick a species and stage to see your target check.
Your PPFD
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Stage PPFD target
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Stage DLI target
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Photoperiod
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Hang-height recommendation
Measure, enter, adjust
Measure PPFD at the canopy (not the ceiling)
PPFD is a point-in-time intensity reading. It has to be taken where the plant actually lives — at the top of the canopy, pointed straight up at the fixture. A meter at ceiling height or fixture height reads dramatically higher than what your plants see. A quantum meter (Apogee SQ-500 or equivalent) is the reliable measurement. Phone apps and lux meters give false numbers for LEDs — the conversion factors they use are fixture-spectrum-dependent and usually wrong. If you don't own a meter, use this tool's Pro-mode fixture estimator as a planning starting point and verify later.
Pick species and stage accurately
Species and stage drive every number downstream. Cannabis at flower wants 6× the PPFD of microgreens in their growing stage. Lettuce wants less than half of what a tomato in fruiting needs. Getting the stage right matters too — early flower (600–900 µmol/m²/s) has a meaningfully lower ceiling than mid flower (700–1000), and mis-picking pushes the tool to approve PPFD levels that will actually bleach your top colas. If you're partway between stages, pick the later one — plants adapt to more light better than they recover from over-light damage.
Set your photoperiod to match your actual timer
DLI is PPFD × hours ÷ a constant. Change the hours, the whole answer changes. A 500 µmol/m²/s canopy at 18/6 veg produces a DLI of 32.4 mol/m²/day — solid for mid-veg cannabis. The same 500 µmol/m²/s at 12/12 flower only produces 21.6 DLI — well below the 35+ flower target. That's not a lighting problem, it's a photoperiod problem — your flower plants need higher PPFD to compensate for fewer hours. The photoperiod input isn't a suggestion; enter your actual timer setting.
Adjust by the smaller of the two numbers
The tool checks both PPFD (intensity) and DLI (daily total). If they disagree, fix the one flagged as out-of-range. Low PPFD + OK DLI (long photoperiod at weak intensity) → raise intensity, don't extend hours further. OK PPFD + low DLI (strong intensity at short photoperiod) → extend photoperiod by 1–2 hours, don't crank the fixture harder. Both high → raise the fixture 2–4 inches and wait 30 minutes before re-reading; plants adapt slowly to intensity drops. Both low → upgrade the fixture. No amount of photoperiod extension compensates for an under-spec'd light.
Don't trust lux meters or phone apps
Lux measures total visible light; PPFD measures the PAR wavelengths plants actually use. The conversion factor between them is spectrum-specific — sunlight ≈ 0.0185, modern white LED ≈ 0.015, blurple LED ≈ 0.022, HPS ≈ 0.0122. A phone app reading 40,000 lux under a white LED means ~600 PPFD; the same reading under an old blurple means ~880 PPFD. Cheap "PAR meter" apps are guessing. If you're serious about light management, a real quantum meter is a $350–$450 one-time purchase that pays for itself the first time it saves you from bleaching a flower room.
CO2 changes the math
The targets in this tool assume ambient CO2 (400–450 ppm) — standard home-grow conditions. If you're supplementing CO2 to 1200–1500 ppm, plants can use ~20% more PPFD before hitting photoinhibition. That means cannabis flower targets shift from 700–1000 to roughly 850–1200 µmol/m²/s, and DLI targets shift from 35–55 to 42–65. Without CO2, pushing PPFD past 1000 just wastes electricity — the photosynthesis curve flattens. Don't chase max PPFD unless you've also committed to CO2, sealed-room airflow, and the higher heat load those bring.
How PPFD and DLI actually work
Plants don't eat lumens. They eat photons — specifically the photosynthetically active radiation (PAR) between 400 and 700 nanometers. PPFD measures the rate those photons hit a square meter of canopy each second. DLI measures the total that accumulates over a day. Every other lighting metric — lumens, lux, foot-candles, "wattage equivalent" — is either irrelevant to plants or actively misleading. For the full treatment of light-and-cannabis science, our Grow Lights learning module is the next stop after this tool.
PPFD
Instant intensity
Photons of PAR hitting one m² per second. Units: µmol/m²/s. Think of it as the rate plants photosynthesize at this exact moment. Point-in-time reading.
DLI
Daily total
Photons accumulated per m² across the whole day. Units: mol/m²/day. Think of it as the meal plants get. Intensity × time ÷ 1 million.
PPE
Fixture efficiency
Photons produced per joule of electricity. Units: µmol/J. This is how you tell a good fixture from a bad one — 2.7 µmol/J is solid, 1.5 µmol/J is wasted electricity.
Why lux meters lie about PPFD
A lux meter measures total visible light weighted by the human eye's sensitivity curve — which peaks at green (555 nm) because that's where we see best. Plants don't care where we see best. Plants use blue (around 450 nm) and red (around 660 nm) most efficiently, which are exactly the wavelengths lux meters underweight. The result: two fixtures with identical lux readings can deliver wildly different PPFD.
Most online "lux to PPFD calculators" use a single conversion factor (usually 0.0185) that's only correct for direct sunlight. Apply it to an LED and you'll be off by 20–40%. Apply it to an old-gen purple LED and you'll be off by more than 50%. The actual conversion factors per fixture type:
| Light source | Lux → PPFD factor | Example: 40,000 lux reads as |
|---|---|---|
| Direct sunlight | × 0.0185 | 740 µmol/m²/s |
| Modern white LED (Samsung LM301) | × 0.015 | 600 µmol/m²/s |
| HPS (high-pressure sodium) | × 0.0122 | 488 µmol/m²/s |
| CMH / ceramic metal halide | × 0.014 | 560 µmol/m²/s |
| Old-gen blurple LED (red+blue only) | × 0.022 | 880 µmol/m²/s |
| T5 fluorescent | × 0.015 | 600 µmol/m²/s |
Same 40,000 lux reading, values ranging from 488 to 880 PPFD depending on which fixture. If your tool or app doesn't ask which fixture you have, its conversion is wrong. That's why this calculator refuses lux input — there's no honest single-factor answer, and most growers don't know their fixture's spectrum well enough to pick the right one.
The DLI formula and why it matters more than PPFD alone
The DLI calculation is one line:
DLI = PPFD × hours × 3600 ÷ 1,000,000
The 3600 converts hours to seconds. The 1,000,000 converts micromoles to moles. That's it. But the implications are huge — same plant, same fixture, same intensity, different photoperiod, entirely different daily light total:
| PPFD (µmol/m²/s) | Photoperiod | DLI (mol/m²/day) | Stage fit |
|---|---|---|---|
| 500 | 18h veg | 32.4 | Solid cannabis veg |
| 500 | 12h flower | 21.6 | Too low for flower |
| 800 | 12h flower | 34.6 | Low-end flower |
| 900 | 12h flower | 38.9 | Ideal flower |
| 200 | 16h microgreens | 11.5 | Ideal microgreens |
| 1200 | 12h flower (CO2) | 51.8 | Commercial flower with CO2 |
A grower running 500 PPFD and thinking "I've got plenty of light for flower" has missed 40% of the DLI their plants need. The fixture is fine; the photoperiod + intensity combination isn't. This is the exact failure mode most home grows hit — plants look OK under the lights, but yield comes in 30% below expectations because cumulative daily light was short the whole cycle.
Why wattage-based PPFD estimates are a ceiling, not a reading
You'll see this formula everywhere: PPFD = fixture PPF ÷ canopy area in m². It's mathematically correct for the average PPFD if every photon the fixture emits hit the canopy uniformly. In reality:
Fixtures lose 10–15% of photons to reflective tent walls and absorption. Beam angles concentrate light at the center, where PPFD can be 25% higher than average; edges and corners can be 35% below. Hang height matters — a fixture at 24" produces roughly 85% of its 18" PPFD at the same canopy point. A real canopy's PPFD is not one number — it's a range from center-hotspot to edge-cold-corner.
That's why this calculator's Pro-mode fixture estimator returns center, average, and edge PPFD separately, plus a uniformity coefficient (edge ÷ center). Industry standard for a well-distributed canopy is uniformity ≥ 0.70. Below 0.60, you'll see uneven growth across the tent — tall center plants, stunted corners. A calculator that returns a single PPFD number from fixture wattage is hiding this variance.
Fixture efficiency (PPE) — how to shop for lights
PPE (µmol/J) is the one spec that actually matters when comparing fixtures. A 320W LED with 2.7 PPE produces more usable light than a 600W HPS with 1.7 PPE, at half the electricity. Modern quality fixtures beat older ones by 50%+ on this metric. Here's what you should expect from each fixture class:
| Fixture type | Typical PPE | Notes |
|---|---|---|
| Modern LED (Samsung LM301, Osram, Cree diodes) | 2.5–3.2 µmol/J | Current best. Quality commercial fixtures hit 3.0+. Cheap Amazon LEDs often test at 1.8–2.2 despite marketing claims. |
| CMH / LEC (ceramic metal halide) | 1.8–2.1 µmol/J | Good full-spectrum alternative to HPS. Better CRI, lower efficiency than LED. |
| Old-gen LED (blurple, red+blue only) | 1.5–2.2 µmol/J | Spectrally narrow. Modern white LEDs have passed them on efficacy and spectrum. Still being sold — avoid. |
| HPS (high-pressure sodium) | 1.6–1.9 µmol/J | Efficient red spectrum, weak blue. Was commercial standard until modern LEDs. Generates significant heat. |
| T5 fluorescent | 1.2–1.6 µmol/J | Fine for clones and seedlings. Way too inefficient for flowering — wastes electricity. |
The math of fixture upgrade: a 600W HPS at 1.7 PPE produces 1,020 µmol/s of total PPF. A 400W modern LED at 2.8 PPE produces 1,120 µmol/s — more usable light on 200 watts less electricity. That's why every commercial cannabis operation switched from HPS to LED over the last decade. The PPE spec tells you which fixture actually delivers photons per dollar of electricity.
The photosynthesis curve: diminishing returns after 800 PPFD
Cannabis photosynthesis rate climbs linearly with PPFD up to about 600–800 µmol/m²/s, then the curve flattens dramatically. At ambient CO2, pushing PPFD to 1000 only gains ~15% more photosynthesis over 800. Past 1000 PPFD without CO2 supplementation, gains are negligible and bleaching risk rises sharply. The extra 500W of electricity to push PPFD from 800 to 1500 produces almost no yield benefit in a standard home grow.
CO2 changes the curve. With CO2 enriched to 1200–1500 ppm, the photosynthesis ceiling shifts upward — plants can effectively use PPFD up to about 1500 µmol/m²/s before the curve flattens. This is why commercial flower rooms run both high PPFD and CO2 supplementation: each enables the other. Running just one is wasted electricity or wasted CO2. The targets in this tool's species selectors assume ambient CO2.
Stage targets across species
Different crops evolved under different light regimes and have wildly different optimum PPFD/DLI combinations. The tool's species selector loads the right targets automatically, but here's the reference for quick comparison:
| Species & stage | PPFD (µmol/m²/s) | DLI (mol/m²/day) | Photoperiod |
|---|---|---|---|
| Cannabis — seedling/clone | 100–300 | 10–18 | 18h |
| Cannabis — veg | 300–600 | 25–35 | 18h |
| Cannabis — early flower | 600–900 | 26–39 | 12h |
| Cannabis — mid flower | 700–1000 | 30–43 | 12h |
| Cannabis — late flower/ripening | 600–900 | 26–39 | 12h |
| Microgreens — growing | 100–200 | 6–14 | 14h |
| Herbs — growing/harvest | 200–400 | 10–20 | 14h |
| Vegetables — leafy (lettuce, spinach) | 200–350 | 12–17 | 16h |
| Vegetables — fruiting (tomato, pepper) | 400–800 | 20–30 | 14h |
| Bonsai — tropical | 150–400 | 8–20 | 14h |
| Bonsai — temperate | 100–300 | 4–13 | 12h |
All values assume ambient CO2 (400–450 ppm). CO2 supplementation at 1200–1500 ppm shifts upper bounds ~20% higher. Cannabis targets verified against Fluence application guides, Rodriguez-Morrison et al. 2022 (Industrial Crops and Products), and Migro light science research. Microgreens and vegetables targets verified against Purdue University (Boldt & Lopez), Penn State Extension, Virginia Tech, and Missouri Extension.
Common PPFD and DLI questions
What's the difference between PPF, PPFD, and DLI?
PPF (Photosynthetic Photon Flux) is the total PAR photons a fixture emits per second — a fixture-level spec, measured in µmol/s. It doesn't account for how those photons are distributed. PPFD (Photosynthetic Photon Flux Density) is the photon rate hitting a single square meter of canopy per second — a point-in-space reading measured in µmol/m²/s. PPFD is what your plants actually experience. DLI (Daily Light Integral) is the total photon accumulation per m² across a day — PPFD × photoperiod seconds ÷ 1,000,000, measured in mol/m²/day. When you shop for a light, look at PPF. When you measure what's hitting your plant, that's PPFD. When you plan your crop's daily light meal, that's DLI.
Why does my calculated PPFD seem higher than my meter reading?
The wattage-based estimate in this tool (Pro mode) calculates a theoretical ceiling — what PPFD would be if every photon the fixture emits hit the canopy uniformly. Real canopies lose 10–15% to reflective wall absorption, 20–35% to edge/corner falloff, and more to fixture beam angle. A real meter reading at canopy center typically comes in 20–30% below the wattage estimate, with edges lower still. The estimate is useful for planning (sizing fixtures before you buy) but the meter is the truth once the setup exists. If your meter reading is 40%+ below the estimate, something's wrong with the fixture — dying diodes, reflective failure, or dramatically under-spec'd wattage.
Can I use a lux meter or phone app instead of a PAR meter?
No, and this is the #1 mistake home growers make. Lux measures visible light weighted by the human eye's green-peaked sensitivity curve — plants care about blue and red, which lux meters underweight. The lux-to-PPFD conversion factor varies with fixture spectrum: sunlight ≈ 0.0185, modern white LEDs ≈ 0.015, HPS ≈ 0.0122, old blurple LEDs ≈ 0.022. Two fixtures showing identical 40,000 lux can deliver anywhere from 488 to 880 PPFD. Phone apps usually apply the sunlight factor to everything, producing 20–50% errors under LED. A real quantum meter (Apogee SQ-500 or LI-COR) runs $350–$450 — the only investment that pays for itself the first time it prevents a bleaching event.
What DLI do cannabis plants need at each stage?
Cannabis DLI targets at ambient CO2 (400–450 ppm): seedling/clone 10–18 mol/m²/day (typically from 100–300 PPFD × 18h), veg 25–35 mol/m²/day (300–600 PPFD × 18h), early flower 26–39 mol/m²/day (600–900 PPFD × 12h), mid flower 30–43 mol/m²/day (700–1000 PPFD × 12h), and late flower 26–39 mol/m²/day (600–900 PPFD × 12h, slightly reduced to avoid bleaching during ripening). CO2-supplemented commercial grows push ~20% higher on each bound. Below the minimum DLI, growth stalls. Above the maximum without CO2, you get photoinhibition and leaf damage before any yield benefit.
What's the ideal photoperiod for each growth stage?
For cannabis: seedlings and clones tolerate 18–24 hours with no flowering risk (they're not light-period-sensitive until sexually mature). Veg runs 18/6 standard, with some growers pushing 20/4 for faster growth at higher electricity cost. The 12/12 flower trigger is the universal cannabis convention — reducing photoperiod to 12 hours signals the plant to flower. Some commercial growers run 13/11 or 11/13 late in flower for slight yield/potency shifts, but 12/12 is the baseline. For non-photoperiodic species: microgreens run 14–16h, leafy vegetables 16h, fruiting vegetables 14h, herbs 14h. Running 24-hour lighting saves fixtures at the cost of higher DLI variance and no photosynthetic benefit past the plant's light compensation point.
Is more DLI always better?
No — there's a hard ceiling for each species, and exceeding it causes photoinhibition rather than more yield. Cannabis photosynthesis climbs linearly up to about 600–800 PPFD, then flattens dramatically. At ambient CO2, pushing past 1000 PPFD produces almost no additional growth but significantly raises bleaching risk and electricity cost. CO2 supplementation to 1200–1500 ppm shifts the ceiling upward — plants can effectively use up to ~1500 PPFD before the curve flattens — but the CO2 supplementation setup costs $500+ and requires sealed-room airflow management. For most home grows, the sweet spot is PPFD 700–900 at 12h for flower (DLI 30–39), which sits just past the photosynthesis knee without wasting electricity. Chasing maximum PPFD without CO2 is a losing trade.
Why does my grow light spec sheet list µmol/s but I need µmol/m²/s?
The spec sheet is listing PPF (total photons the fixture emits per second) — a fixture-level spec. What you need is PPFD (photons hitting each square meter of canopy per second) — a canopy-level measurement. To convert roughly: divide PPF by canopy area in square meters, then discount 15–25% for fixture-to-canopy losses. A fixture rated 850 µmol/s over a 4×4 ft canopy (1.49 m²) produces a theoretical maximum of 570 µmol/m²/s average, landing around 430–480 µmol/m²/s in real conditions. Most fixture spec sheets also provide a PPFD map at specific hang heights — use that if available; it's more accurate than the divide-by-area approximation.
How do I measure canopy PPFD accurately?
Take multiple readings, not one. Hold the quantum meter sensor at the top of the canopy pointed straight up — not tilted at the fixture. Read the center of the canopy, then each corner, then the midpoints between. For a 4×4 tent that's 9 readings minimum. Average the five center/corner readings for overall PPFD. Note the center-to-edge ratio — if it's below 0.70, your fixture's coverage is uneven and plants at the edges will underperform. Always read with your grow lights at full intensity and stable (give LEDs 5 minutes to warm up). Retake readings whenever you adjust hang height, add plants, or change stages — canopy PPFD drifts as plants grow closer to the fixture.
Fixtures, meters, and everything to dial in your light
The difference between a grow that hits its PPFD targets and one that doesn't is usually the fixture — and sometimes the meter that tells you how the fixture is actually performing. Everything here is spec-sheet verified for real PPE numbers, not marketing-inflated "equivalent wattage" claims.
LED Grow Lights
Modern LEDs running Samsung LM301 or Osram diodes hit 2.5–3.2 µmol/J — more usable light per watt than any other fixture type. Our LED selection skews toward the 2.7+ PPE range. A 320W quality LED outperforms a 600W HPS at half the electricity cost.
PPFD / PAR Meters
The $350 purchase that pays for itself the first time it catches an underlit canopy or saves you from bleaching a top cola. Apogee and LI-COR quantum meters — the only reliable way to verify what this calculator estimates.
CMH / LEC Fixtures
Ceramic metal halide at 1.8–2.1 PPE. Not quite LED-efficient but delivers excellent full-spectrum light with high CRI. Popular with growers who want natural-looking color in the room and a single fixture that handles both veg and flower.
HPS & HID Fixtures
HPS at 1.6–1.9 PPE, lower than modern LED but still the choice for growers with large rooms where HPS per-watt cost beats LED upfront. Red-heavy spectrum shines for flower. Generates serious heat — pair with adequate ventilation.
Timers & Light Controllers
DLI depends on photoperiod consistency — a drifting timer or a 3-hour power outage turns a 38 DLI target into a 32 DLI reality. Digital timers, relays, and smart controllers that actually stay accurate month-to-month.
Hangers & Light Accessories
Ratchet hangers to dial in the last 2 inches of PPFD. Reflective Mylar and panda film to improve edge uniformity. Rope ratchets, Y-splitters, and everything else that makes fixture positioning an adjustment instead of a wrestling match.