Formulas and assumptions
How to plan grow lights for seed starting
- Count your trays. The planner assumes standard 1020 flats, about 11 × 21.25 in.
- Measure a shelf. Width and depth decide how many flats fit per shelf, and so how many shelves you light.
- Pick the crop. Seedlings need a daily light integral of roughly 10-15 mol per square meter per day (Michigan State; UNH assumes 12). Lettuce and spinach need shorter days (Wisconsin).
- Choose bars or panels. LED bars hang close, about 2-4 in above the leaves (Minnesota), so they suit stacked shelves. Panels hang higher and need more headroom.
- Set a timer for 12-16 hours a day, leaving at least 8 hours of darkness (Minnesota, Maryland).
- Add heat mats for warm-season crops, on a thermostat rather than the light timer (Minnesota), and remove them once seeds sprout.
- Raise the lights as seedlings grow, keeping the same gap above the leaves.
FAQ
How many hours a day should grow lights run for seedlings?
Most guides say 12-16 hours a day (Minnesota), with at least 8 hours of darkness (Maryland). Lettuce and spinach should get no more than about 11 hours, or they may bolt (Wisconsin). The planner uses 16 hours for most crops and 11 for lettuce and greens.
How far above seedlings should grow lights hang?
For fluorescent tubes and LED bars, no more than about 4 in above the seedlings, and as close as 2 in (Minnesota). Raise them as the plants grow. LED panels are much brighter at the center, so they hang higher; follow the maker's seedling height.
How many grow lights do I need per seed tray?
It depends on the light's output. The planner turns the crop's daily light target into the light level needed over your trays, then divides by each fixture's output. Budget LED bars rarely publish their output (PPF), so the planner assumes one; if your box lists PPF, enter it for a better answer.
Do I need a heat mat?
Not always. Warm-season crops such as tomatoes and peppers germinate best in warm soil, around 85 °F at the optimum (Oregon State), and a heat mat speeds that up (Minnesota). Plug mats into a thermostat, not the light timer, and take them away once seeds sprout.
How much does it cost to run the lights?
Watts × hours per day ÷ 1,000 gives kilowatt-hours per day. Multiply by your electricity price and by the days under lights. The planner shows it per day, per month, and for the season, with heat mats listed separately.
Can I use a shop light instead of a grow light?
Yes, if it's bright enough and close enough. UNH found that output varies widely: a good fixture might reach the target in 8-10 hours a day, while a weak one would need more hours than a day has (UNH). Enter the fixture's watts and PPF if you know them.
Sources
- Michigan State University Extension (Runkle), Daily Light Integral Defined and DLI requirements. Seedlings in plugs: about 6-10 mol/m²/day early and 10-15 later; a common greenhouse minimum for transplants is 10-12.
- UNH Extension, Growing Seedlings Under Lights. Assumes 12 mol/m²/day for seedlings; hours needed vary widely by fixture.
- University of Minnesota Extension, Starting Seeds Indoors. Lights no more than 4 in above seedlings (2 in ideal), 12-16 hours a day, potting mix at 65-75 °F, and don't plug heat mats into the light timer.
- University of Maryland Extension, Starting Vegetable Seeds Indoors. Lights 2-3 in above seedlings, 8 hours of darkness, and weeks from sowing to transplant by crop.
- University of Wisconsin-Madison Extension. Spinach and lettuce should get no more than 11 hours of light, or they may bolt.
- Oregon State University Extension, Soil Temperature Conditions for Vegetable Seed Germination. Optimum germination soil temperatures by crop.
- U.S. Energy Information Administration, Short-Term Energy Outlook. 18.2 cents/kWh forecast US residential average for 2026.
- Product specs for defaults: 1020 flat footprint about 21.1-21.25 × 10.8-11.25 in (True Leaf Market, Grit); 10 × 20.75 in heat mat at 18 W (Vivosun); 100 W 2 × 2 ft panel at 2.5 µmol/J (Spider Farmer SF1000 listing).