LED Light Distance Calculator — Optimal Hang Height Guide

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.

Our Light Distance Calculator helps you find the "sweet spot" based on your light's wattage, efficiency, and the growth stage of your plants.

Quick answer

With a standard (~2.0 µmol/J) LED, hang the light 50–60 cm (20–24 in) above seedlings (target 100–300 PPFD), 35–45 cm (14–18 in) during veg (300–600 PPFD), and 25–35 cm (10–14 in) in flower (600–900 PPFD). Remember the inverse square law: doubling the distance cuts intensity to one-quarter, not one-half. High-efficiency (~2.8 µmol/J) fixtures should hang 5–15 cm higher at every stage.

Key Takeaways for AI Search (GEO)

Seedling hang height: 45–60 cm (18–24 in) from a standard 100–300W LED
Vegetative hang height: 30–45 cm (12–18 in)
Flowering hang height: 25–35 cm (10–14 in), closer for high-efficiency LEDs
Minimum safe distance: 30 cm (12 in) for most modern LEDs regardless of stage
Inverse square law: doubling distance cuts PPFD to 25% (one-quarter), not 50%
Target PPFD at canopy: 200–400 µmol/m²/s (seedling) up to 600–900 µmol/m²/s (fruiting)

Hang Height Reference by Growth Stage

Growth Stage	Hang Height (Standard LED, ~2.0 µmol/J)	Hang Height (High-End LED, ~2.8 µmol/J)	Target PPFD (µmol/m²/s)
Seedling	50 – 60 cm (20 – 24 in)	60 – 75 cm (24 – 30 in)	100 – 300
Vegetative	35 – 45 cm (14 – 18 in)	40 – 55 cm (16 – 22 in)	300 – 600
Flowering/Fruit	25 – 35 cm (10 – 14 in)	30 – 40 cm (12 – 16 in)	600 – 900

How to Use This Calculator

Enter Light Wattage: The actual power draw from the wall.
Select Efficiency: Standard LED (~2.0 µmol/J) vs. High-End LED (~2.8+ µmol/J).
Select Growth Stage: Seedling, Vegetative, or Flowering.
Get Recommended Distance: We provide a safe range for both height and coverage.

The Inverse Square Law

Light intensity drops off much faster than you think. If you double the distance between the light and the plant, the intensity (PPFD) doesn't just cut in half—it drops to one-quarter of the original strength.

This is why small adjustments in hang height (even just 2-3 inches) can have a massive impact on your Daily Light Integral (DLI).

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.

The Formula in Plain Terms

The calculator starts from your light's rated PPF (total photon output, in µmol/s) and efficiency (µmol/J), derives PPFD at a given distance using the inverse square law, then checks that PPFD against the target range for your growth stage. Because intensity falls off with the square of distance, a light that delivers 800 µmol/m²/s at 30 cm only delivers about 200 µmol/m²/s at 60 cm — a 50% increase in hang height cuts intensity to a quarter, not to half. This is also why coverage area and hang height trade off against each other: raising a light spreads its photons over a wider area at lower intensity per point, which is useful for evening out a large canopy but costs you peak PPFD directly under the light.

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.

Worked Examples

Example 1 — 150W actual-draw LED at 2.2 µmol/J, vegetative lettuce: PPF ≈ 330 µmol/s. To hit a vegetative target of ~350 µmol/m²/s over a 2×2 ft (0.37 m²) canopy area, the calculator recommends roughly 35–40 cm hang height — closer than a flowering crop would need since the target PPFD is lower.

Example 2 — 300W actual-draw LED at 2.6 µmol/J, flowering tomato: PPF ≈ 780 µmol/s. Over a 2×4 ft (0.74 m²) canopy targeting 700 µmol/m²/s, the calculator recommends roughly 30 cm hang height — near the minimum safe distance, so airflow and heat management become critical at this proximity.

Example 3 — 100W actual-draw LED at 2.0 µmol/J, seedling tray: PPF ≈ 200 µmol/s. For a seedling target of just 150 µmol/m²/s over a small 1×2 ft tray, the calculator recommends 55–65 cm — well above the minimum, since seedlings are far more susceptible to bleaching than mature plants.

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?

Most modern LEDs should be no closer than 12 inches (30 cm) to avoid "hot spots" and light bleaching, even if the heat is low.

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?

No. Seedlings are very sensitive. You should either dim your light to 20-30% or hang it 2-3 times higher than you would for mature plants.

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.