Pillar Guide
Cannabis Grow Lights: Sizing, Height and Schedules
Light is the single biggest lever on yield. This hub covers wattage per square foot, PPFD and DLI targets, hanging height, and why a $24 timer is the highest-return purchase in a grow room.
Watts per square foot is the fast answer
Use 30-40 actual wall watts per square foot of canopy for a full-power flowering room. A 4x4 tent is 16 square feet, so 450-600 watts. Under-lighting is the most common reason for airy, disappointing flower; over-lighting bleaches the tops.
Efficacy matters as much as wattage. A modern fixture at 2.5-2.8 µmol/J delivers far more usable light per watt — and far less heat — than a cheap blurple panel at 1.5.
PPFD and DLI: intensity versus daily total
PPFD is how much light lands on the canopy right now. DLI is how much lands over the whole day. A plant in veg can take a lower PPFD for 18 hours and end up with a similar daily total to a flowering plant at high intensity for 12 hours.
Aim for a DLI of roughly 25-35 mol/m²/day in veg and 35-45 in flower. Above about 45 you need CO2 supplementation to get any further benefit.
Timers are non-negotiable
Photoperiod plants flower because of uninterrupted darkness. A missed schedule or a light leak can trigger re-vegging or hermaphroditic flowers, which ruins a crop that was otherwise going fine.
A dual-outlet digital timer removes the human factor completely: lights on and off at the same second every day, whether you are home, asleep, or on holiday.
Run the numbers
Gear for this stage
Budometry 480W Full-Spectrum LED
$329Dimmable quantum board covering a 4x4 tent from seedling through late flower.
Digital Light Timer (Dual Outlet)
$24Locks in 18/6 veg and 12/12 flower photoperiods so your plants never get confused.
Every guide in this topic
Lighting
How Far Should Your LED Be From Cannabis Plants?
Lighting
18/6 vs 20/4 vs 24/0: The Best Veg Light Schedule
Lighting
How Many Watts Do You Need Per Square Foot?
Lighting
Light Burn vs Nutrient Burn: How to Tell Them Apart
Lighting
Light Leaks in Flower: Why They Ruin Harvests
Lighting
What 'Full Spectrum' Actually Means on an LED Grow Light
Lighting
LED vs HPS for a Home Grow: An Honest Comparison
Lighting
Setting Up a Grow Light Timer (and Not Ruining Your Flip)
Lighting
The Flowering Stretch: Managing Weeks 1-3 of 12/12
Lighting
Grow Light Coverage: Matching a Fixture to Your Tent Size
Frequently asked questions
- How many watts per square foot for cannabis?
- 30-40 real wall watts per square foot of canopy for flowering, using a fixture with an efficacy of at least 2.3 µmol/J. That is roughly 150W for a 2x2 and 450-600W for a 4x4.
- How far should an LED be from cannabis plants?
- Roughly 30-36 inches for seedlings, 24-30 inches in vegetative growth, and 18-24 inches in flower, adjusted for fixture power. If the top leaves taco upward or bleach white, raise the light.
- What light schedule should I use?
- 18 hours on and 6 hours off for vegetative growth, then 12 on and 12 off to trigger flowering in photoperiod plants. Autoflowers can stay at 18/6 or even 20/4 for their entire life.