Hang LED grow bars and fluorescent tubes 2–4 inches above the seedlings, and raise them as the plants grow. LED panels are much brighter and hang higher, so follow the maker's seedling height.
Open this example in the Grow Light Planner
Why the distance depends on the light
Light spreads out and gets weaker the farther it travels. Low-output lights, such as fluorescent tubes and budget LED bars, have to hang close to give seedlings enough. Extension guides put them 2–4 in above the seedlings, with 2–3 in ideal (Starting Seeds Indoors; Starting Vegetable Seeds Indoors).
LED panels are far brighter, especially at the center. Hung a few inches over seedlings, a panel would light a small spot very strongly and leave the edges dim. They hang higher so the light spreads over the trays. No extension guide gives a seedling height for panels, so use the maker's recommendation. The BedMath planner assumes about 24 in for a 100 W panel.
Hanging height by fixture
| Light | Height above the seedlings | Basis |
|---|---|---|
| Fluorescent tubes or 2-ft LED bars | 2–4 in | Extension guides: 2–4 in, 2–3 in ideal |
| 100 W LED panel, 2 × 2 ft | about 24 in | Planner default; use the maker's seedling height |
Whatever the light, keep the same gap as the seedlings grow by raising the lights. Adjustable hangers (rope ratchets or chain) make that easy.
Example: 4 trays of tomatoes under panels
Four 1020 trays of tomatoes on a 48 × 24 in shelf, lit by 2 × 2 ft LED panels. The planner gives:
Two panels are the fewest that cover the trays, and at full power they give far more light than tomato seedlings need. The planner suggests dimming them to about 30% if they're dimmable.
Now switch to LED bars. The same trays need 10 bars (100 W in all), hung 2–4 in above the seedlings. That fits a standard wire shelving unit, where 30 in of headroom per shelf usually doesn't.
The takeaway: bars suit stacked shelves; panels suit a single tall shelf or a frame.
Run your own numbers
The free BedMath Grow Light Planner works this out for your trays, shelves, crop, and lights, and shows every formula and source. This link opens it with the example above filled in.
Sources
Published 2026-11-03. The numbers in the examples come straight from the planner.