Cannabis is one of the hungriest plants for light you can grow indoors. The difference between a light that's "fine" and one that's right is the difference between airy, larfy buds and dense ones — and it's decided before a seed goes in. Here are the numbers that matter, and how to turn them into a light you can actually buy.
The two numbers that matter
PPFD — photosynthetic photon flux density — is how much usable light lands on the canopy each second, in µmol/m²/s. It's what the plant experiences. Every serious light maker publishes a PPFD map; that map is worth more than any wattage on the box.
DLI — daily light integral — is PPFD added up over the hours the light is on, in mol/m²/day. It's why a 12-hour flowering day needs a brighter light than an 18-hour veg day to deliver the same total.
Targets for cannabis:
| Stage | PPFD (µmol/m²/s) | Hours | DLI (mol/m²/day) |
|---|---|---|---|
| Seedlings & clones | 200–400 | 18 | 13–26 |
| Vegetative | 400–600 | 18 | 26–39 |
| Flowering | 700–1000 | 12 | 30–43 |
| Flowering with added CO₂ | 1000–1500 | 12 | 43–65 |
Two things to notice. Flowering wants nearly double the intensity of veg — so a light that seemed plenty for a bushy veg plant is suddenly the limiting factor after the flip. And above about 1000 µmol/m²/s, a plant in normal room air can't use more light: it's CO₂-limited, not light-limited. Without supplemental CO₂, pushing past that just makes heat and bleaches the tops.
Turning that into watts
PPFD maps are the right way; a watts-per-square-foot rule gets you close for modern full-spectrum white LED — the Samsung-diode bar or board style:
- Vegetative only (mothers, clones): 20–25 W per sq ft
- Full grow through flower: 30–40 W per sq ft, and closer to 40 for dense flower
| Tent | Floor | LED wattage to aim for |
|---|---|---|
| 2×2 | 4 sq ft | 100–150 W |
| 2×4 | 8 sq ft | 200–300 W |
| 3×3 | 9 sq ft | 250–300 W |
| 4×4 | 16 sq ft | 400–500 W |
| 5×5 | 25 sq ft | 600–700 W |
That's actual wattage drawn from the wall — "power draw" or "actual power" on the spec sheet. Never the "1000W" in the product name; that's a marketing number, explained in What LED wattage really means.
Why "coverage" claims are optimistic
A light listed for "4×4 coverage" usually means it keeps plants alive over 4×4 in veg. Its flowering coverage — the area it holds at 700+ PPFD — is a size down. A light sold for flowering a 4×4 should be drawing 400 W or more; if it's 250 W it's a 3×3 flowering light with an ambitious label. Read the PPFD map at 18 inches and look at the corners of your tent size: if they read half of the centre, the outer plants will stretch and flower thin.
Hanging height, dimming and the light-stress signs
Bars and boards run best 12–24 inches above the canopy. Closer is more intense over a smaller patch; further spreads it. A dimmer lets you start seedlings at 30–40% and step up as the plant grows — far better than hanging the light two feet away and throwing light at the walls. Dimming is not optional; buy a light that has it.
Push intensity gradually in flower, and watch for stress: leaves cupping or "tacoing" upward, tops bleaching pale, purple stems near the light. Back off 10–20% or raise the light a few inches. Cannabis will tell you within two days.
Spectrum, briefly
For cannabis, a full-spectrum white LED with a few deep-red (660 nm) diodes does everything. Blue-heavy light keeps veg compact; red-heavy light drives flowering — white LEDs cover both. UV and far-red bars are refinements that can add a little potency and reduce stretch; they're worth it after the basics, not instead of them. Old purple "blurple" panels grow plants but make inspecting for pests and deficiencies miserable, and are a generation behind on efficiency.
What to actually buy
For a 2×2, a 100 W dimmable board or bar: flowers one plant properly and runs cool.
For a 2×4, one 200–250 W bar light, or two 100 W lights side by side. Two spread across the rectangle better and let you dim one for a mother plant or seedlings while the other flowers.
For a 4×4, a 400–480 W bar light. This is where diode efficiency matters for the electricity bill: a modern high-efficacy light can draw 100 W less than an older one for the same output, and over a 12-hour flowering day for nine weeks that's real money. The size classes are compared in Which LED for your tent.
The one-line version
Multiply your tent's floor area by 30–40 W, buy a dimmable full-spectrum white LED that actually draws that much, and check its PPFD map holds 700+ at the corners of your tent at 18 inches. Everything else is a refinement.
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