LED Watt Density Calculator — W/m²

Electrical watts per area only. This does not calculate PPFD or hanging height. Use the fixture’s PPFD chart and measured canopy light.

LED wattage ÷ Area (m²) = W/m².

150 W ÷ 0.36 m² = 416.7 W/m².

Hanging your light is a balancing act. Too close, and you bleach your leaves with "light burn." Too far, and your plants stretch and under-produce.

Signs of Incorrect Distance

Too Close (Light Stress):

Bleaching: The top leaves turn white or pale yellow while the rest of the plant is green.
Tacoing: Leaf edges curl upward to protect themselves from intensity.
Stunted Growth: The plant stays very short with extremely tight node spacing.

Too Far (Light Hunger):

Stretching: Long, thin stems between leaves.
Pale Color: Lack of light prevents strong chlorophyll production.
Slow Growth: The plant lacks the energy to build biomass.

Common Mistakes

Using manufacturer "coverage area" claims at face value. Many marketing coverage claims are measured at PPFD levels too low for flowering — check the PPFD-at-distance chart in the spec sheet, not just the square-footage number.
Ignoring canopy unevenness. A single-point LED bar mounted too low creates a hot center and dim edges; wider light bars or multiple smaller fixtures at a slightly greater height give more even coverage.
Not re-adjusting height as plants grow. A height set correctly for a 15 cm seedling becomes far too close once the canopy has grown 20 cm taller — recheck distance every 1–2 weeks during active vegetative growth.
Using the hand test on LEDs. As noted above, LEDs run cool to the touch even at damaging photon intensities — always calculate distance from actual PPFD, not perceived heat.

Pro Tip: Use the "Hand Test" (With Caution)

While the "hand test" (if it's too hot for your hand, it's too hot for the plant) worked for old HPS bulbs, it does not work for LEDs. LEDs are often cool to the touch but emit enough photon pressure to kill leaf tissue. Always use a PAR meter app or follow manufacturer specs.

FAQ

How close can I put my LED to the plants?

Electrical watts per area only. This does not calculate PPFD or hanging height. Use the fixture’s PPFD chart and measured canopy light.

What happens if the light is too far away?

Your plants will "stretch" (etiolation), becoming tall and weak as they search for more intensity. Your DLI will drop significantly.

Do seedlings need the same distance as flowering plants?

Electrical watts per area only. This does not calculate PPFD or hanging height. Use the fixture’s PPFD chart and measured canopy light.

What PPFD should I target at canopy level?

200–400 µmol/m²/s for seedlings/leafy greens, 400–600 for vegetative growth, and 600–900+ for fruiting crops under high-end LEDs.

Does hang height affect DLI or just PPFD?

Both. PPFD is the instantaneous intensity at canopy level; DLI is PPFD accumulated over your photoperiod. Raising or lowering the light changes PPFD directly, which changes your total DLI for the same number of hours.