Marijuana grow light wattage for a 4x4 tent
Use 480 to 640 watts of LED actual draw for a 4x4 tent. That equals 30 to 40 watts per square foot across 16 square feet. For HID, a 600-watt HPS lamp is the common choice. For CMH, use one 630-watt fixture or two 315-watt fixtures.
marijuana grow light wattage for 4x4 tent
Tent area math
A 4x4 tent covers 16 square feet. Multiply 16 by your target watts per square foot.
how far should grow lights be from cannabis plants
- 25 W/sq ft = 400 W. Low light. Vegetative growth and small plants.
- 30 W/sq ft = 480 W. Minimum for flower with LED.
- 35 W/sq ft = 560 W. Midpoint for flower.
- 40 W/sq ft = 640 W. Upper target for flower without CO2.
These numbers use fixture draw at the wall. They do not use equivalent wattage on the box.
LED actual draw vs equivalent wattage
LED makers sometimes label a 400-watt fixture as a 1000-watt equivalent. The useful number is actual power draw. Check the spec sheet for watts at the wall. A 4x4 tent at 30 to 40 watts per square foot needs 480 to 640 real watts. A fixture that draws 450 watts is below the flower target.
HID and CMH wattage
A 600-watt HPS lamp is a common match for a 4x4 tent. It draws 600 watts plus ballast loss. A 1000-watt HPS in a 4x4 tent gives about 62 watts per square foot. That adds heat and can bleach plants if distance is wrong. A 315-watt CMH covers a smaller area. Two 315-watt CMH fixtures give 630 watts for a 4x4 tent.
PPFD and DLI check
Wattage is a starting point. PPFD and DLI are the measured light numbers. Cannabis flower targets often fall in the 600 to 1,000 µmol/m²/s PPFD range. DLI for flower is often 30 to 45 mol/m²/day without CO2. A 480-watt LED at 2.5 µmol/J gives about 1,200 µmol/s PPF. Spread over 16 square feet, that is about 800 µmol/m²/s PPFD at canopy level before wall losses. A 640-watt LED at 2.5 µmol/J gives about 1,600 µmol/s PPF, or about 1,075 µmol/m²/s PPFD before losses.
Hang height and coverage
Use the fixture footprint. A 4x4 tent needs light across 16 square feet. Bar LEDs spread light across the canopy. A single board concentrates light in the center. Start LED fixtures 18 to 24 inches above the canopy. Start HPS and CMH lamps 24 to 36 inches above the canopy. Move fixtures up if leaf edges curl or tops bleach. Move them down if stems stretch.
Heat and ventilation
LED fixtures put less heat into the air than HPS. They still need airflow. A 4x4 tent with 600 watts of LED needs an exhaust fan and carbon filter rated for the room volume. HPS adds more heat. Use an air-cooled hood or a sealed fixture in small rooms. Keep the driver outside the tent when the room runs warm.
Common wattage mistakes
- Using equivalent watts instead of actual draw.
- Buying a 4x4 tent light with 200 real watts and expecting flower density.
- Putting a 1000-watt HPS in a 4x4 tent without heat control.
- Ignoring PPFD at the corners. Center readings can be 30% higher than edge readings.