How Far Should a Grow Light Be From Plants? The Inverse Square Law, Explained

There’s a specific kind of frustration in indoor growing: your plants are stretching and leggy, or their leaves are bleached and curling, and you’ve done everything the seed packet said. Nine times out of ten the culprit isn’t the nutrients or the water — it’s the single number nobody printed on the light’s box: how far the fixture should hang above the canopy. That distance is not a minor tweak. Because of a piece of basic physics, moving a light a few inches closer or farther can double or halve the amount of light your plants actually receive. Get the hang height right and a modest fixture grows healthy, compact plants; get it wrong and even an expensive light either starves or scorches them. The Grow Light Distance Calculator does the arithmetic for you, but the reasoning behind it is worth carrying in your head.

The inverse square law is why small moves matter so much

Light from a fixture spreads out as it travels. The same beam of photons that covers one square foot near the lamp is spread across four square feet at twice the distance, so any given leaf catches a quarter as much. That relationship has a name — the inverse square law — and it’s exact: intensity falls off with the square of distance. The formula the calculator uses is I₂ = I₁ × (d₁ / d₂)², where I₁ is the measured intensity at some reference distance d₁ and I₂ is what you get at your new distance d₂.

The squared term is the part that surprises people. Doubling the distance doesn’t cut intensity in half — it cuts it to a quarter. Moving from 12 inches to 18 inches (a 50% increase) drops intensity to about 44% of what it was. That’s why “just raise it a bit” is such an unreliable instruction: a bit is a lot. It also explains why the edges of your grow tent are always dimmer than dead center, and why a plant six inches taller than its neighbors can be getting nearly twice the light. When you understand that the relationship is squared rather than linear, the whole business of setting hang height stops feeling like guesswork.

PPFD is the number that matters — not watts, not lux

Plants don’t eat watts. What drives photosynthesis is the number of usable photons hitting a given area each second, measured as PPFD — Photosynthetic Photon Flux Density — in micromoles per square meter per second (µmol/m²/s). PPFD only counts photons in the 400–700 nm range that chlorophyll can actually use, which is why the wattage on the box tells you almost nothing about growing power. A 100-watt fixture tuned for plant-usable wavelengths can outgrow a 150-watt one that wastes output on green and infrared.

The target PPFD depends on what stage your plants are in, and the calculator bakes in sensible ranges:

• Seedlings and clones: 200–400 PPFD. Young plants have small root systems and can’t use much light; blast them and you bleach the tender new growth.
• Vegetative growth: 400–600 PPFD. The plant is building leaves and stems and can handle — and wants — considerably more.
• Flowering and fruiting: 600–1000+ PPFD. This is where the plant cashes in all that light into buds or fruit, and it can use everything you can reasonably give it (with CO₂ supplementation, even more).

Enter your fixture’s rated PPFD at its reference distance, tell the tool your growth stage, and it reports the PPFD landing at the canopy plus whether that’s too dim, in range, or too hot — and it solves for the ideal distance to hit the middle of your target band.

Why a cheap lux meter will lie to you (a little)

Proper PAR meters that read PPFD directly are expensive, so many growers reach for a cheap lux meter or a phone light-meter app instead. That can work, but only if you know the catch: lux and PPFD are not the same thing, and the conversion depends on your light’s spectrum. Lux is weighted for human eyes, which are most sensitive to green — exactly the wavelengths plants use least. For typical white LEDs, roughly 1 PPFD ≈ 54 lux; for HPS (high-pressure sodium) lamps the ratio is closer to 1 PPFD ≈ 82 lux because of their yellow-heavy output. Use the wrong multiplier and your reading can be off by 30% or more. The calculator shows an approximate lux column alongside PPFD precisely so you can sanity-check a cheap meter, but treat those lux numbers as ballpark. If you’re serious, borrow or rent a real quantum sensor to calibrate once, then trust your distances.

Reading the distance table and dialing it in

The tool also prints a small table of PPFD at several fixed distances — 6, 12, 18, 24 and 36 inches — computed straight from the inverse square law using your fixture’s rated output. That table is the fastest way to see the trade-off at a glance: you’ll notice the intensity plummet between the near rows and flatten out at the far ones. Use it to pick a starting height, then watch the plants. Leaves that cup, bleach, or point away from the light are telling you it’s too close; plants that stretch tall with long gaps between leaf nodes are reaching for light that’s too far. Adjust in small steps — remember, an inch matters more than it feels like it should — and give the plants a day or two to respond before moving again.

One nuance the raw physics leaves out: real fixtures aren’t perfect point sources. A wide LED panel or a fixture with a good reflector spreads light more evenly and behaves a little gentler than the strict inverse square curve near the lamp, while a bare bulb follows it almost exactly. So use the calculated distance as a strong starting point, not gospel, and let the canopy have the final word.

Set the height once, then let the plants confirm

Indoor growing rewards getting the fundamentals right, and hang height is as fundamental as it gets. Feed your fixture’s rated PPFD and reference distance into the Grow Light Distance Calculator, pick your growth stage, and it will tell you exactly how high to hang the light to hit the intensity your plants want — no more squinting at wattage numbers or trusting a lux app blindly. Once the light is dialed, the other half of healthy indoor growth is feeding: if you’re running hydroponics or mixing liquid nutrients, the Fertilizer / Nutrient Dilution Calculator gets your PPM and EC right, and the Appliance Electricity Cost Calculator will tell you what all those photons are costing you on the power bill.