With budget 2-ft LED bars, plan on about 2½–3 bars per 1020 tray: four trays of tomatoes need 10 bars. The exact count depends on each light's output, so check the PPF on the box if it's listed.
Open this example in the Grow Light Planner
How the count works
The number of lights comes from how much light the trays need each day, divided by what each light puts out. Here are the steps the planner follows, with 4 trays of tomatoes.
- Set a daily light target. Michigan State puts seedlings at 10–15 mol/m²/day (Daily Light Integral Defined), and UNH assumes 12 (Growing Seedlings Under Lights). The planner uses 12 for tomatoes.
- Turn it into a light level. Over 16 hours a day, that takes 208 µmol/m²/s.
- Measure the trays. Four 1020 trays cover 935 sq in, or 0.60 m².
- Work out the light needed. 208 × 0.60 ÷ 0.85 = 148 µmol/s. The 0.85 is the planner's assumption for the share of light that lands on the trays.
- Divide by each light's output. A 10 W bar at 1.8 µmol/J gives 18 µmol/s, so 8.2 bars, rounded up to 9.
- Round up to whole rows. 2 bars end to end span the trays, so 9 bars become 5 rows of 2: 10 bars. The rows also have to be no more than 6 in apart, so edge seedlings aren't left in the dark.
bars = ⌈ target × area ÷ 0.85 ÷ output per bar ⌉, rounded up to whole rows
Worked example: 4 trays of tomatoes
Rounding up to whole rows gives a little more light than the target, which is the safe side for seedlings.
Bars per tray count (tomatoes)
Same 48 × 24 in shelves, 2-ft 10 W LED bars, and 16 hours of light.
| Trays | LED bars | Light wattage | Shelves | Delivered mol/m²/day |
|---|---|---|---|---|
| 1 | 3 | 30 W | 1 | 17.5 |
| 2 | 6 | 60 W | 1 | 17.5 |
| 3 | 8 | 80 W | 1 | 15.6 |
| 4 | 10 | 100 W | 1 | 14.6 |
| 6 | 16 | 160 W | 2 | 15.6 |
| 8 | 20 | 200 W | 2 | 14.6 |
| 12 | 30 | 300 W | 3 | 14.6 |
That works out to about 2½–3 bars per tray. One tray needs 3: a single 2-ft bar spans the tray, and its light target works out to 2.1 bars, which rounds up to 3.
If your box lists PPF
Budget LED bars rarely publish their light output, so the planner assumes 1.8 µmol/J. If your box or spec sheet lists PPF (in µmol/s), enter it with the watts under Your fixture in the planner for an exact count. Brighter bars mean fewer bars, down to the minimum that covers the trays.
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. Worked examples come straight from the planner's current math.