How Far Should a Grow Light Hang? A Distance Chart by Wattage

Hang a grow light too far away and it does almost nothing — light intensity falls off fast with distance, not gradually. Hang it too close and you scorch leaves or cook seedlings. Most of the advice online skips straight past this and just says “hang it above the plants,” which isn’t a distance.

This is a starting-point chart, not a measured spec sheet for any specific fixture — see the callout below for why, and how to get an exact number for the light you actually own.

The starting-distance chart

Actual wattage	Starting distance	Typical use
5–15 W	15–30 cm (6–12 in)	Single clip-on, one or two small pots
15–30 W Most windowsill setups	30–45 cm (12–18 in)	Multi-head clip-on, a small tray
30–50 W	45–60 cm (18–24 in)	Small panel over a shelf
50 W+	60 cm+ (24 in+)	Panel over a full grow tent or rack

General starting points based on common LED grow-light guidance, not a measurement of any specific product. Real output varies by fixture — always verify with a lux meter before committing to a permanent setup.

Every product in our clip-on grow light comparison falls in the 10–20W actual-draw range, which is why 15–30cm shows up so often as the practical answer for a windowsill.

Why distance matters more than people expect

Light intensity doesn’t fall off in a straight line as you move a fixture away — it falls off with the square of the distance (the inverse-square law). Double the distance from a light source and you don’t get half the light at the leaf — you get roughly a quarter.

That’s the reason “just hang it somewhere above the plants” fails so often. A light that’s genuinely doing the job at 20cm might be doing almost nothing useful at 40cm, even though 40cm still looks like it’s “right there” to the eye.

Signs you’re too far away

• Stems stretch and lean toward the light (legginess)
• Leaves stay small and pale
• Growth is slow even though you’re running the light 12+ hours a day

If you’re seeing these and you’re already at the recommended distance for your wattage, the fixture itself may simply not be putting out enough light — worth cross-checking against the published specs in the grow light comparison.

Signs you’re too close

• Leaf edges curl, bleach pale, or develop dry brown patches directly under the light
• The soil surface dries out unusually fast
• New growth near the light looks stunted compared to growth further away

Move the light up in small steps (5cm / 2in at a time) and give the plant a few days to respond before judging the result — plants don’t correct their growth instantly.

Measuring it properly with a lux meter app

1. Turn on only the grow light (block out other light sources if you can)
2. Open a free lux meter app on your phone
3. Hold the phone flat, sensor facing up, exactly where the top leaves sit
4. Read the number, then move the light up or down and re-measure

Match what you get against the light targets already laid out by crop in 15 vegetables that grow without direct sunlight — that article has specific lux targets for over a dozen crops, so you can tune distance to the plant you’re actually growing rather than a generic “grow light” number.

The one-line version

Start at 15–30cm for a typical clip-on light, watch the plant for a week, and adjust in small steps based on what you see — or skip the guesswork entirely and measure it with a lux meter app on day one.