The rule behind every distance chart
Light from a small source spreads out as it travels. Double the distance and the same energy covers four times the area, so the intensity at any point drops to a quarter. Triple the distance and it falls to a ninth. This is the inverse-square rule, and it is why a fixture that seems bright at 15 cm is weak at 60 cm.
Real fixtures are not perfect point sources. A long LED bar spreads light along its length, a reflector concentrates it, and walls bounce some back. So the rule gives you the shape of the falloff, not an exact number for your room. That is still useful: it tells you that small height changes matter far more than most people expect.
A chart built from one measurement
Measure the light at the canopy with the fixture where it is now. Write down that reading and the distance. Then use the table to estimate what happens if you move the light. The example assumes 200 units at 30 cm; substitute your own reading and the ratios stay the same.
| Distance from canopy | Relative intensity | Example reading | What it means |
|---|---|---|---|
| 15 cm | 4.0x | 800 | Strong; check for heat and scorch |
| 23 cm | 1.8x | 360 | Meaningfully brighter than the baseline |
| 30 cm | 1.0x | 200 | Your starting point |
| 45 cm | 0.44x | 89 | Less than half; fine for low-light plants |
| 60 cm | 0.25x | 50 | A quarter; often too little for active growth |
| 90 cm | 0.11x | 22 | Barely above room light |
Two things fall out of that table. First, moving a fixture from 30 cm to 15 cm quadruples the light, which is usually more than enough of a change. Second, moving it from 45 cm to 60 cm costs half the light for a small gain in coverage. Adjust in small steps and re-measure.
How to measure at the canopy
A dedicated PAR meter reads the light plants use and is the right tool if you are tuning a serious setup. A lux meter reads human-visible light and is a rough proxy: good enough to compare two positions, not good enough to convert into a precise plant number. Phone apps vary a lot between models, so treat their readings as a relative gauge.
Hold the sensor at leaf height, flat, facing the fixture. Take several readings across the canopy rather than one at the centre. The edge of a square tray can get a third of the light the middle gets, and a plant at the edge will grow accordingly.
If you have a PAR reading in micromoles per square metre per second, the grow light DLI and timer calculator turns it plus your schedule into daily light integral. That is the number that matters for a plant’s day, and it lets you compare a strong light run for eight hours with a weaker one run for twelve.
Heat matters as much as distance
Close is not automatically better. A fixture that runs hot can damage leaves long before the light becomes useful, and a leaf pressed against a warm diffuser will scorch. Feel the back of your hand at canopy height under the light for a minute. If it feels uncomfortably warm, the plant will too.
Incandescent and halogen bulbs waste most of their energy as heat and are a poor choice for foliage. Modern LED fixtures run cooler but still warm up. Keep a gap even with LEDs, and never let a leaf touch the diffuser.
Coverage shrinks as the fixture drops
Lower means brighter over a smaller area. If you have a wide shelf, one fixture close to the middle will light the middle well and leave the ends in the dark. Two fixtures at a moderate height usually beat one powerful fixture at the same height for an even canopy.
Match the spread to the plants. A single small plant wants the fixture close and centred. A tray of seedlings or cuttings wants even light across the whole tray, which usually means more height and more fixtures. Our grow light guide covers fixture types and what their labels do and do not tell you.
Then set the timer
Distance controls intensity. The timer controls duration. Most houseplants do well with a long enough dark period, and running a light around the clock is not an advantage. Sixteen hours on and eight off is a common schedule for active growth; twelve to fourteen hours suits many foliage plants.
Use the running cost calculator before you commit to a long schedule; the electricity adds up and our worked cost examples show how to read a bill. Then check the plant’s response over a few weeks, not a few days.
Sources and further reading
Published by AI Planter Editorial Team. Read our editorial standards and tool limitations.
Frequently asked questions
Does doubling the distance really cut light to a quarter?
For a small source in open air, yes, approximately. Long fixtures and reflective walls flatten the curve somewhat, so measure rather than trusting the ratio exactly.
How far should a grow light be from houseplants?
Start at whatever distance gives a usable reading at the canopy, often 20 to 45 cm for a modest LED fixture, then adjust. The plant’s response and the heat at leaf height decide.
Can I use a phone app as a light meter?
For comparing positions, roughly. Do not treat the number as a calibrated PAR reading; apps differ widely between phones.
Do I need a PAR meter?
Only if you are tuning light seriously. A lux meter plus the inverse-square chart is enough for most indoor setups.
Is more light always better?
No. Too much light causes scorch, bleached patches and heat damage. Match the level to the species and give a dark period.