Last Updated on October 8, 2026

Grow Light Distance Chart for Seedlings (LED, Fluorescent, HID)

Hang fluorescent tubes 2 to 4 inches above the tops of your seedlings, LED shop lights 2 to 6 inches, LED grow panels at the height the maker lists (most LEDs sit 12 to 24 inches from the leaves), and HID lamps several feet up. Run them 12 to 16 hours a day and turn them off at night. Those are starting points. The light at the leaf drops fast as you raise a fixture, so the real answer for your light comes from a reading taken at the top leaves, and the seedlings themselves tell you when to move it.

Grow light distance chart

My original guide to starting seeds indoors gives 2 to 4 inches for grow lights, which is the right gap for tubes and low-power LED shop lights. A high-output LED panel at 2 inches is a different story, which is why panels get their own row.

How many hours of light do seedlings need?

Run grow lights 12 to 16 hours a day for vegetable seedlings. That is the range University of Minnesota Extension gives, and the one in my seed starting guide. Purdue Extension and the University of Maryland narrow it to 14 to 16 hours. Do not leave them on around the clock, because many plants need some dark period each night to develop properly. K-State’s Johnson County office does allow up to 24 hours at the seedling stage, but Minnesota and Vermont extension both say to keep a dark period. A plug-in timer handles it, and if you use a heat mat, keep it off the light timer.

Hours and distance work together: a light that sits a little high can make up some of the difference with more hours. The measuring section below puts numbers on that trade.

Why a PPFD reading beats any rule of thumb

Every distance in the chart is a guess about how much light reaches the leaf. PPFD (photosynthetic photon flux density) measures it, in micromoles per square meter per second. Three things make the guess unreliable:

• Light falls off fast with distance. University of Missouri Extension measured a two-tube fixture of standard 40-watt fluorescent tubes, with a reflector, at 700 foot-candles 6 inches below the tubes, 400 at 1 foot and 180 at 2 feet. Raising that light from 6 inches to 2 feet cut the light about three quarters.
• Watts are not light. Virginia Cooperative Extension says it is not recommended to use lumens, foot-candles, lux or watts when measuring light levels for plants. Watts tell you what a fixture draws from the wall, not what lands on the seedling.
• Every fixture spreads light differently. The brightest spot under a fluorescent fixture is directly beneath the center of the tubes. And as lights get closer to plants, fewer plants are evenly lit.

That is why Vermont Extension says to follow the height recommendations from the light’s manufacturer. If your light did not come with PPFD readings at a stated height, measure.

Measure it: find the height that hits your DLI target

UNH Extension lays out the most accurate way to set a light: decide how much daily light (DLI) the seedlings need, decide how many hours you will run the light, work out the intensity that takes, then use a light meter to find the distance where the lamp delivers it.

1. Pick a DLI target. Purdue’s Vegetable Crops Hotline, writing for greenhouse transplant growers, puts most vegetable transplants at 10 to 15 mol per square meter per day, and tomato, pepper and eggplant at 15 to 20. Virginia’s crop table lists seedlings lower, at 5 to 10, and notes the right DLI depends on the plant and its age.
2. Pick your hours. 16 is a common choice.
3. Work out the PPFD you need. PPFD = DLI ÷ (hours × 0.0036). For 12 mol over 16 hours: 12 ÷ (16 × 0.0036) = 208. The table below has the common combinations, and my DLI calculator does any other.
4. Measure at the top leaves. Hold the sensor where the leaves are, under the middle of the light and again at the edge of the tray, and move the light until the reading matches. Standalone quantum sensors run from under $100 to over $1,000, and phone apps that use the phone’s light sensor are typically the cheapest option. UNH found those apps “seem pretty good.”

PPFD in µmol per square meter per second, worked from DLI ÷ (hours × 0.0036) and rounded.

Signs the light is too far or too close

Light is not the only cause of leggy seedlings. Most vegetables prefer night temperatures of 60 to 65°F with days about 10°F warmer, and warmer than that gives leggy plants. Crowded seedlings compete for light and stretch too. If the light is close and the stems still stretch, check those next.

Raising schedule: keep the gap as seedlings grow

The distance in the chart is a gap between the light and the top leaves, not a height above the tray. Seedlings close that gap from below, and every inch they grow puts the leaves in brighter light, as the Missouri readings above show. So the light moves up with them.

1. Before sprouts appear. Most seeds do not need light until the plants have emerged; soil temperature matters more at that stage. Seeds that need light to germinate are the exception: UNH’s seed starting fact sheet suspends fluorescent lights 6 to 12 inches above those seeds for 18 hours a day.
2. The day they come up. Set the starting gap for your light from the chart and start the timer.
3. Every time you water. Check the gap with a ruler at the tallest seedling and raise the light back to it. Hanging lights from chains makes that easy. The Mars Hydro TS1000 I reviewed and the Spider Farmer SF-1000 both came with ratchet hangers for the same job.
4. Mixed trays. Crops that grow at different speeds will not share one gap for long. Set the light for the tallest plants. If a fixture cannot move, raise the plants on stands, shelves or boxes instead, and the same trick lifts short seedlings up to the light.
5. Dim instead of raising, if you can. On the Viparspectra P1000 I unboxed, a dimming knob on the driver lets you adjust the light through the growing stages instead of moving it. LED output is linearly related to PPFD, so 50 percent power gives about half the light.
6. Before transplant. Seedlings raised under lights still need to be hardened off before they go outside. My guide to transplanting seedlings into the garden starts that about 10 to 14 days before planting out.

Wattage and rated coverage tell you the footprint a light is built for, not the right height. The TS1000, the light I put in my grow room for the fall transplants, draws 150 watts and covers a 3 x 3 foot space. That is 36 x 36 inches, room for three standard 10 x 20 inch trays side by side.

Quick questions

How close should grow lights be to seedlings?

Keep fluorescent tubes 2 to 4 inches above the tops of the seedlings, as University of Minnesota Extension recommends. University of Maryland Extension keeps fluorescent and LED fixtures 2 to 6 inches above seedlings. LED grow panels go at the height the maker lists, often 12 to 24 inches, and HID lamps go several feet up.

How many hours of light do seedlings need?

12 to 16 hours a day. University of Minnesota Extension gives 12 to 16, and Purdue Extension and University of Maryland Extension narrow it to 14 to 16. Turn the lights off at night, because many plants need some dark period each night to develop properly.

How far should an LED grow light be from seedlings?

It depends on the light. Low-power LED shop lights and strips work 2 to 6 inches above seedlings. Iowa State University Extension says most LED lights can be positioned 12 to 24 inches from the foliage, and high-output panels should go at the height the maker lists or where a light meter reads the PPFD you need.

How do I know if my grow light is too close or too far?

Seedlings that stretch, lean toward the light or turn pale need more light, so lower the light or run it longer. Bleached leaves, or upper leaves that turn yellow or brown and burn first, mean the light is too close or too strong, so raise it.

Can I leave grow lights on 24 hours for seedlings?

It is better not to. University of Minnesota Extension advises against leaving lights on continuously because many plants need some dark period each night to develop properly. Use a timer and run 12 to 16 hours instead.

Seeds under lights now, garden dates next

How long seedlings spend under lights depends on the crop and your last frost. My seed starting calendar counts the weeks back from that date, and you can look up your frost dates by zip code. Plan My Garden is free to start: enter your zip code, draw your beds, pick tomatoes, peppers and everything else you want to grow, and it places each plant at square-foot spacing and builds a planting schedule (start indoors, transplant and direct sow dates) from your frost dates.

Related: DLI Calculator · Step-by-step seed starting guide · Vegetable Seed Germination Temperature Chart