How Far Should a Grow Light Be From Plants?
There is no single magic number, because hang height depends on two things: how hot and intense your light is, and what stage your plants are in. A high-power LED panel running at 300 watts throws far more usable light (and heat) than a 24-watt T5 tube, so it belongs much higher above the canopy. As a rough starting frame, low-power LEDs sit around 12 to 18 inches, high-power LEDs around 18 to 24 inches, T5 fluorescents just 4 to 10 inches, and HPS/HID lamps 18 to 36 inches because of their heat output. This calculator scales those baselines by your wattage, footprint, and growth stage so you are not guessing.
Intensity, Photoperiod, and DLI
Plants do not just respond to height. They respond to PPFD (photosynthetic photon flux density, measured in micromoles per square meter per second) and to how many hours that light runs. Seedlings want a gentle ~200 PPFD for 14 to 16 hours; vegetative plants thrive near ~400 PPFD for 16 to 18 hours; flowering and fruiting plants push toward ~600 PPFD, usually on a 12-hour cycle. Multiply intensity by run time and you get the Daily Light Integral (DLI), the total photon dose your plants receive each day.
DLI (mol/m²/day) = PPFD × hours × 3600 ÷ 1,000,000
Why Distance and Hours Work Together
Light intensity falls off with the square of the distance, so raising a fixture from 12 to 18 inches can cut the PPFD at the canopy by more than half. That is why this tool estimates the PPFD your wattage actually delivers across your footprint, compares it to the stage target, and tells you whether to drop the light closer, raise it higher, or leave it be. If your fixture is underpowered for the area, you can compensate slightly with a longer photoperiod, but you cannot run flowering plants on 18 hours without disrupting their cycle, so height and wattage do most of the work.
Frequently Asked Questions
How do I know if my grow light is too close?
Watch the top leaves nearest the light. Bleaching, crispy or curling tips, and a faded yellow-white cast at the canopy are classic signs of light burn, even when the leaf is not hot to the touch. If you see this, raise the fixture a few inches and recheck after a couple of days.
Why are my seedlings tall and leggy?
Leggy, stretched stems mean the light is too far away or too weak, so seedlings reach toward it. Lower the light to the bottom of the recommended range or add wattage, and keep it on 14 to 16 hours a day. A small fan also encourages sturdier stems.
How many hours a day should I run a grow light?
It depends on the stage. Seedlings and leafy greens do well at 14 to 18 hours, while flowering and fruiting plants typically need a 12-hour on / 12-hour off cycle to trigger and hold bloom. Always give plants a dark period; running lights 24 hours a day stresses most species and rarely improves growth.
Does light type really change the hang height that much?
Yes. Fluorescent and CFL bulbs run cool and low-intensity, so they can sit just a few inches above seedlings. HPS and HID lamps radiate intense heat and must stay 18 to 36 inches away to avoid scorching, while LEDs land in between based on their actual power draw. That is why this calculator asks for both type and wattage.
Practical Guide for Grow Light Height & Duration Calculator
Start by measuring the actual power draw of your fixture, not the inflated marketing wattage on the box. A panel sold as 1000W often draws only 150 to 200W from the wall, and that real number is what determines how much usable light reaches your plants. Enter the honest figure so the intensity estimate and hang height recommendation stay realistic.
Adjust height as plants grow. The recommended distance is measured from the lamp to the top of the canopy, not to the soil, so a fixed light effectively gets closer every week as plants stretch upward. Raise the fixture or lower the shelf to hold the gap steady, and re-run the numbers when you transition a tray from seedling to vegetative spacing.
If you cannot hit the target intensity, fix the footprint before the photoperiod. Spreading one light over too many square feet thins the PPFD fast, so consolidate trays under the brightest center of the beam or add a second fixture. Stretching the daily hours can help leafy crops a little, but it cannot rescue flowering plants that need a strict 12-hour dark period.
Quick Checklist
- Measure from the lamp to the top of the canopy, then re-check every week as plants grow.
- Use your light's real wall-draw wattage, not the box marketing number.
- Give every stage a dark period; never run lights 24 hours straight.
- Watch the top leaves for bleaching (too close) or stretching (too far) and adjust a few inches at a time.