No single number works for every setup, and any chart that hands you one is hiding the two things that matter most. How far grow lights from plants? Far enough that the leaves do not burn, and close enough that seedlings do not stretch. The gap that does both depends on the fixture type and the plant stage. Seedlings under a shop fluorescent tube need one grow light distance. A mature fiddle leaf fig under an LED panel needs another. You get three numbers here instead of one, and each is scoped to a situation.
Late February, and my seed trays under the basement stairs smell like wet peat. The fluorescent tube I set over them hangs so low that I duck under it to reach the back row, and the tallest tomato seedlings almost brush the cool plastic housing. That gap is 4 to 6 inches (10 to 15 cm), which is what fluorescent fixtures want over seedlings. The same tube over a grown pothos would hang two or three times higher.
Light does not fade in a slow, even line as it travels. Intensity drops off fast as distance from the source grows, so a few extra inches of height cost your plants more light than you would guess. A fixture that looks bright to your eye at chest level can still leave leaves pale and stretched. The trade runs the other way too. Pull the fixture closer and the usable light under the center of the panel climbs. It also spreads less evenly across the tray, so plants at the corners fall behind the ones in the middle.
The three numbers do not blend into an average. Iowa State University Extension puts LED fixtures 12 to 24 inches (30 to 61 cm) above foliage, and fluorescent fixtures 6 to 12 inches (15 to 30 cm). Seedlings under fluorescent are the tight case at 4 to 6 inches above the plant tops. Iowa State scopes each range to its own fixture type, which is why the LED range starts about where the fluorescent range ends.
Treat the range for your setup as a starting height and let the plants correct you from there. Grow light distance is a setting you revisit, not one you fix once. Leaves that burn under the brightest part of the fixture are asking for more space, so raise it a couple of inches and watch them for a week. Seedlings that stretch want the opposite move. Bring the fixture closer, or leave it running longer each day, because distance and run time both feed the same daily light total.
Plan the hanging height grow light rig around chain and S-hooks instead of a fixed bracket. Cheap chain lets you drop the fixture an inch at a time as the seeds come up, then walk it back up week by week as the plants climb toward the tube. A dimmer gives you a second lever on an LED. Output tracks the dial in a straight line, so half power gives you close to half the light, and you can soften a fixture without moving it at all.
Measure from the top leaves rather than the soil line, because that gap closes on its own every week while the plants grow. Check the grow light distance on the same day you water and the height stays honest. Start at 12 inches (30 cm) for an LED over established plants, drop a fluorescent tube down near the seedlings, and let the leaves tell you where to go next.
Read the full article: Grow Lights for Indoor Plants: What Really Works
