How many grow lights does a 4-foot wire shelf need?
For a 48 x 24 inch shelf, two 4-foot fixtures is the usual answer. Shallower shelves may need only one row.
The rule we use
The University of Saskatchewan shows two 4-foot fixtures over a 24 x 48 inch shelf, and says fruiting plants grown to maturity may want up to four. Our calculator turns that into a planning rule: one row of lights per 12 inches of shelf depth, with a 5 percent tolerance. It is a planning rule, not a standard, and the calculator lets you change it.
Common shelf sizes
| Shelf (length x depth) | 4 ft bars | Power at 40 W each | 2 ft bars | Power at 10 W each | 1020 trays that fit |
|---|---|---|---|---|---|
| 48 x 14 in | 2 (2 rows x 1) | 80 W | 4 (2 rows x 2) | 40 W | 2 |
| 48 x 18 in | 2 (2 rows x 1) | 80 W | 4 (2 rows x 2) | 40 W | 2 |
| 48 x 24 in | 2 (2 rows x 1) | 80 W | 4 (2 rows x 2) | 40 W | 4 |
| 36 x 18 in | 2 (2 rows x 1) | 80 W | 4 (2 rows x 2) | 40 W | 1 |
| 48 x 36 in | 3 (3 rows x 1) | 120 W | 6 (3 rows x 2) | 60 W | 6 |
Calculated with the shelf calculator using its default tray size of 21 x 11 inches. The watts per fixture are placeholders, so read the number off the fixture you buy.
Reading the table
- A 24 inch deep shelf gets two rows. A 14 inch deep shelf gets one row.
- A shelf that is exactly 4 feet long takes one 4-foot bar per row. If your fixture is a few inches longer than the shelf, it can still hang over the ends.
- For a 36 inch long shelf, a 4-foot bar overhangs by 12 inches. Two 2-foot bars cover it with 12 inches to spare, which the calculator flags as a note.
- Deeper than about 30 inches is hard to reach and to light evenly. Use more rows or a shallower shelf.
Check the shelf height
Wire shelving usually has adjustable shelf heights. Leave enough room between the tray and the fixture above for the hanging distance in our distance guide, plus the height the seedlings will reach.
Sources
- University of Saskatchewan: Indoor garden, Little Green Thumbs style
- Oklahoma State University Extension: Lighting can be tricky when starting seeds indoors (January 2024)