A dimmable bar-style LED is the top pick for most home cannabis grows, and the two distances that carry the workload are 18 to 24 inches above the canopy in veg and 12 to 18 inches in flower. Every fixture below is judged on four criteria: the stage of growth, the intensity that reaches the leaves, the heat pushed toward the canopy, and the amount of weekly adjustment the light asks of you.
how many watts for cannabis grow light
LED bar fixtures
Bar-style LEDs spread diodes across a rectangle instead of clustering them under one bulb. That even footprint is what lets you hang the fixture short without scorching the middle of the canopy.
full spectrum led grow lights for weed
Hanging heights by stage
- Clones and seedlings: 24 to 36 inches, dimmed to 25 to 40 percent
- Vegetative: 18 to 24 inches, dimmed to 50 to 75 percent
- Flower: 12 to 18 inches at full power
Pros
- One fixture covers seedling through harvest when the driver dims
- Low radiant heat, so the light can sit close to the tops
- Even coverage across a square tent means fewer hot spots
Cons
- Cheap units misstate wattage, which makes any chart useless
- Fixed hangers force you to raise plants instead of raising the light
- Full power at a short distance can bleach upper leaves
Best for: a 2x4 or 4x4 tent where one dimmer has to serve every stage and headroom is limited to about 6 feet.
Best Grow Light for Autoflower Cannabis: Picks Compared
HID lamps (MH and HPS)
Metal halide for veg and high pressure sodium for flower still deliver proven results, but the bulb throws heat down along with light, so the chart has to be read against wattage.
Hanging heights by stage
- 400W metal halide, veg: 18 to 24 inches
- 600W HPS, flower: 18 to 24 inches
- 1000W HPS, flower: 24 to 36 inches
Pros
- Established spectrum that growers know how to read
- Replacement bulbs cost little compared with a new fixture
- Strong penetration into a tall canopy
Cons
- Canopy heat demands an air-cooled hood or a tall room
- Distance changes with reflector shape, not just wattage
- Ballast, hood, and bulb add up in space and weight
Best for: a basement or spare room with real exhaust, cool ambient air, and 7 feet or more of ceiling height.
CMH and ceramic metal halide
A single 315W CMH runs both stages, which makes it the simplest chart to memorize.
Hanging heights by stage
- Vegetative: 18 to 24 inches
- Flower: 12 to 18 inches
Pros
- Full spectrum supports veg and flower without a bulb swap
- Better color rendering than HPS, so leaf problems show up early
- One fixture fits a 3x3 footprint
Cons
- Less efficient per watt than current LED bars
- Still needs airflow across the hood
- Bulbs age and shift output over time
Best for: a 3x3 tent where you want one light, one distance, and no stage changes.
T5 and CFL for propagation
Fluorescent tubes and compact lamps belong close to the plant, which is the opposite of every fixture above.
Hanging heights by stage
- T5 tubes: 4 to 8 inches above seedlings and clones
- CFL bulbs: 2 to 6 inches, adjusted as plants stretch
Pros
- Low heat at short distance
- Cheap entry point for seeds and mother plants
- Easy to layer several tubes for wider trays
Cons
- Weak for flowering, so a second light is usually needed
- Small coverage per tube
- Output drops as tubes age
Best for: a propagation shelf or a mother room that never reaches flower.
The chart is a starting point, not a rule
Distance only matters because intensity falls with the square of the distance from the lamp. Move a light from 24 inches to 12 inches and the canopy receives roughly four times the intensity, which is why a chart gives ranges instead of single numbers. Two checks keep you honest: hold the back of your hand at canopy height for 30 seconds, and read a PPFD meter or a phone light app at the tops. If the hand test is uncomfortable or the reading climbs past your target, raise the fixture or dim it.
Why convenience decides the fixture
A chart earns its keep only when it can be read in seconds. The fixture that wins is the one whose dimmer and hanger cover every stage, so the answer to "how high is the light" is one number and one percent, not a bulb swap and a ladder. Look for a dimmer with detents, a hangar you can move with one hand, and a stated wattage you can trust. Convenience is not a luxury in a grow room. It is the reason the chart gets followed at all.
How to use this chart in week one
- Pick the row that matches your fixture type and stage.
- Set the fixture at the top of the listed range, not the bottom.
- Check canopy temperature and leaf response after 24 hours.
- Lower by 2 inches or raise power by 10 percent, then wait another two days.
- Write down the height and percent that worked, and reuse it next run.