Grow
Grow Light & DLI Calculator
Find out how much light your seedlings really get each day, and whether your shop light is enough.
Daily light integral calculator
From the manufacturer's PPFD map, measured at the height your leaves sit.
16 h
Seedlings do best at 14–18 hours. Leave at least 6 hours of dark.
14.4
mol/m²/day (DLI)
250
PPFD µmol/m²/s
🌿 Vegetable seedlings wants 10–14 DLI (200–300 PPFD for 14–16 hours).
You can shorten the photoperiod to 15.6 hours or raise the fixture and save power.
Most stretching problems are a DLI under 10, not a watering problem.
Light targets by crop
| Crop / stage | DLI | PPFD | Hours | Notes |
|---|---|---|---|---|
| 🌱 Germination / just sprouted Seedlings | 4–6 | 100–200 | 14–16 h | Light the moment the first sprout breaks the surface, or you get leggy stems in 48 hours. |
| 🌿 Vegetable seedlings Seedlings | 10–14 | 200–300 | 14–16 h | Most stretching problems are a DLI under 10, not a watering problem. |
| 🍅 Tomato / pepper starts Seedlings | 14–20 | 250–400 | 16–18 h | Peppers want the high end and warm roots; tomatoes will take everything you give them. |
| 🥦 Brassica starts Seedlings | 12–16 | 200–350 | 14–16 h | Keep them cool and bright or they get soft and floppy. |
| 🧅 Onion / leek starts Seedlings | 12–16 | 200–300 | 14–16 h | Long days early on are fine — they bulb on daylength outdoors, not indoors. |
| 🌾 Microgreens Greens | 6–12 | 150–250 | 12–16 h | Blackout dome for 3–4 days first, then light. More light = more color, less stretch. |
| 🥬 Lettuce & salad greens Greens | 12–17 | 200–300 | 14–18 h | Above ~20 DLI lettuce turns bitter and can tipburn. |
| 🥬 Spinach / chard / kale Greens | 12–18 | 200–350 | 14–16 h | Kale takes more light than lettuce without complaining. |
| 🌿 Basil Herbs | 16–22 | 250–450 | 16–18 h | Pale, thin basil almost always means not enough light. |
| 🌿 Cilantro, parsley, dill Herbs | 10–16 | 180–300 | 14–16 h | Cilantro bolts fast under heat plus very long days. |
| 🌿 Mint, chives, oregano Herbs | 10–14 | 150–300 | 14–16 h | Forgiving — happy on a bright shelf. |
| 🪴 Rosemary, thyme, sage Herbs | 18–25 | 300–500 | 14–16 h | Mediterranean herbs sulk indoors under weak light. |
| 🍅 Tomato — fruiting indoors Fruiting | 22–30 | 400–700 | 14–16 h | Fruiting indoors is expensive light. Under 20 DLI you get flowers but poor set. |
| 🌶️ Pepper — fruiting indoors Fruiting | 20–30 | 400–600 | 14–16 h | Needs warmth as much as light — keep it above 70°F. |
| 🍓 Strawberry Fruiting | 17–22 | 300–500 | 14–16 h | Day-neutral varieties do best under lights. |
| 🥒 Cucumber (compact indoor) Fruiting | 20–30 | 400–600 | 14–16 h | Huge appetite for light and water; needs hand pollination indoors. |
| 🥕 Radish / baby turnip Roots | 12–17 | 200–350 | 12–14 h | Too little light gives all tops and no root. |
| 🥕 Carrot (deep container) Roots | 14–18 | 250–400 | 14–16 h | Long crop indoors — expect 70+ days. |
| 🪴 Low-light houseplants Houseplants | 2–6 | 50–150 | 10–12 h | Pothos, snake plant, ZZ. Easy to overdo. |
| 🌴 Bright-light houseplants Houseplants | 6–12 | 150–300 | 12–14 h | Monstera, ficus, most aroids. |
How high to hang it
Fluorescent T5 / CFL
2–4 in above the leaves
Low intensity — it must be close or seedlings stretch.
LED shop light (40–50 W)
6–10 in
Sweet spot for seedling trays; raise as they grow.
LED grow bar / panel (100 W+)
12–24 in
Start high, lower slowly and watch for bleaching.
HPS 400 W+
18–30 in
Heat is the limit, not intensity.
Light troubleshooting
Tall, thin, floppy stems (leggy)
DLI too low or light too far away
Lower the light or add hours. Aim for 12–16 DLI on seedlings and run a small fan.
Purple or reddish leaf undersides
Cold roots or phosphorus uptake, sometimes very high light
Warm the tray to 65–75°F before changing the light.
Pale, bleached, or crispy leaf tips
Too much light, too close
Raise the fixture 4–6 inches or dim it, then watch for a week.
Seedlings lean toward one side
Uneven coverage from a single-point light
Rotate the tray daily and add reflective sides.
Growth stalls with plenty of light
Temperature, water, or nutrients — not light
Check root-zone temp and start a quarter-strength feed.
Good to know
- • DLI = PPFD × hours × 0.0036. That's the whole formula.
- • PPFD is intensity right now; DLI is the total the plant collected all day.
- • A bright south window in winter is often only 3–6 DLI — not enough for tomato starts.
- • Plants need dark hours. Never run seedling lights 24/7; 14–18 hours is the useful range.
- • Doubling the distance from a small light cuts intensity to roughly a quarter.
- • Watts on the box means nothing. Photon output (µmol/J) is what grows plants.