Grow Light Lab

There is no single correct distance — it depends on the light intensity your plants need (PPFD target) and how powerful your fixture is. A common starting point is 24–36 inches for seedlings, 18–24 inches for vegetative growth, and 12–18 inches for flowering with a typical 200–400W LED, but you should verify with a meter or the hanging-distance calculator above.

The physics behind it is the inverse-square law: halve the distance and intensity roughly quadruples. That is why moving a light just a few inches makes such a big difference.

Seedlings want gentle light — about 100–300 µmol/m²/s (PPFD). With a full-power LED that usually means hanging it 24–36 inches above the canopy, or dimming the driver to 25–50% and keeping it around 18–24 inches. Watch the seedlings: stretching (legginess) means too far, bleached or curling cotyledons means too close.

Yes. Too much light causes light burn: bleached white or yellow leaf tops, leaves folding upward like tacos, and crispy edges even when watering is fine. Most plants max out around 800–1000 PPFD without supplemental CO₂; pushing past that wastes electricity and stresses the plant. If you see symptoms, raise the light 6 inches and reassess after a few days.

Divide lumens by the illuminated area in square meters to get lux, then multiply by a spectrum-dependent conversion factor: PPFD ≈ (lumens ÷ m²) × K. For a white full-spectrum LED, K ≈ 0.016, so 20,000 lumens spread over 2 m² gives 10,000 lux ≈ 160 PPFD. Use the spectrum converter above — it shows the honest range, not a fake single number.

Multiply lux by a conversion factor K that depends on the light source: PPFD = lux × K. Common approximate values are sunlight 0.0185, white LED 0.016, metal halide 0.0143, and HPS 0.0122. Because a lux meter weights light the way human eyes see it (green-heavy), the result is always an estimate — typically ±15–25% — never lab-grade.

PAR is the range of light plants use (400–700 nm). PPF is how much PAR a fixture emits per second (µmol/s) — a property of the lamp. PPFD is how much of it lands on your canopy per square meter per second (µmol/m²/s) — a property of your setup. DLI (daily light integral) is PPFD accumulated over the whole photoperiod (mol/m²/day) — the total daily dose. Lux and foot-candles, by contrast, measure brightness for human eyes and are the wrong unit for plants.

DLI = PPFD × photoperiod (hours) × 0.0036. Example: 300 PPFD × 18 hours × 0.0036 = 19.44 mol/m²/day. The 0.0036 is just unit conversion (3600 seconds per hour ÷ 1,000,000 µmol per mol). DLI is the number experienced growers plan around, because plants respond to the total daily dose, not just instantaneous intensity.

Rearrange the formula: PPFD = DLI ÷ (hours × 0.0036). To hit a DLI of 20 with an 18-hour photoperiod you need 20 ÷ (18 × 0.0036) ≈ 309 PPFD; with only 12 hours you would need ≈ 463 PPFD. Shorter days demand stronger light — the reverse solver in the DLI tab does this math for any combination.

A practical rule of thumb for modern LEDs: 10–20 W/ft² for seedlings, 25–35 W/ft² for vegetative growth, and 35–45 W/ft² for flowering or fruiting. These assume LED efficacy around 2.0–2.7 µmol/J. Older HPS setups need roughly 50–65 W/ft² for the same result because they convert electricity to plant-usable light less efficiently.

Multiply tent area by 35–45 W/ft² for flowering with LED: 2×2 ft → 140–180W, 3×3 ft → 315–405W, 4×4 ft → 560–720W, 5×5 ft → 875–1125W. For seedlings or leafy greens you can roughly halve those numbers. The wattage sizer tab computes this for any custom tent size and growth stage.

Widely used targets: seedlings/clones 100–300, leafy greens 250–450, herbs 300–500, microgreens 150–350, fruiting crops like tomato and pepper 600–900, and low-light houseplants 50–200 µmol/m²/s. These are canopy-level values — measure or estimate at the top of the plants, not at the fixture.

Leafy greens and herbs do well at DLI 12–20; fruiting crops like tomatoes and peppers want DLI 20–35; seedlings and microgreens are happy around DLI 6–14; low-light houseplants survive on DLI 2–9. Remember DLI couples intensity with day length — 400 PPFD for 12 hours (17.3 DLI) is not the same grow as 400 PPFD for 18 hours (25.9 DLI).

Exactly, only for an ideal point source — and a grow light is not one. The inverse-square law is a good approximation when the hang distance is larger than the fixture itself (a small panel hung 24 inches up). For large LED bars hung close to the canopy, intensity falls off more slowly and the edges stay brighter than the law predicts. Treat the hanging-distance result as a planning estimate, then confirm with a PAR meter or the manufacturer's PPFD chart.

A lux meter measures light weighted by the human eye's sensitivity, which peaks in green — exactly where plants are least efficient. Two fixtures can read the same 10,000 lux yet deliver very different PPFD if one is green-heavy and the other is red/blue-heavy ("blurple"). Without knowing the spectrum, any conversion is ±15–25% at best. That is why this tool shows a range instead of pretending one number is exact.

Yes, because their spectra differ. Approximate factors: HPS ≈ 0.0122, metal halide ≈ 0.0143, cool-white fluorescent ≈ 0.0135, white full-spectrum LED ≈ 0.016, and sunlight ≈ 0.0185 PPFD per lux. A lux meter reading under HPS therefore converts to less plant-usable light than the same reading under sunlight — another reason to measure PPFD directly when precision matters.

A calculator is enough for planning — choosing fixture wattage, tent size, photoperiod, and a starting hang height. A PAR meter (or a phone app used carefully) is for verification: confirming what your specific fixture, reflector, and tent actually deliver at canopy level, since real-world losses from walls, lenses, and aging diodes are invisible to math. Use both: plan with the calculator, verify with the meter, adjust once.