Grow Light Distance Calculator (PPFD by Height)
Estimate how light intensity changes as you raise or lower the lamp.
Result
The inverse-square law is an approximation for real fixtures. Always follow the manufacturer's recommended distance and verify with a PAR meter.
History
What it is
This estimator uses the inverse-square law to predict how light intensity (PPFD) changes when you raise or lower a grow light. Light intensity falls with the square of distance, so small changes in hanging height have a large effect at the canopy.
Who should use it
Growers setting hanging height for LED or HID fixtures who know the PPFD at one distance (from the spec sheet or a meter) and want to estimate it at a new height without re-measuring.
How to use it
- Enter a known PPFD value and the distance it was measured at.
- Enter the new distance you are considering.
- Read the estimated PPFD at the new distance.
- Compare the result against your crop's target PPFD/DLI and set the height accordingly.
Example calculation
Worked example
If a fixture gives 1,000 µmol·m⁻²·s⁻¹ at 12 inches, raising it to 18 inches drops intensity to about 444 µmol·m⁻²·s⁻¹ — because (12 ÷ 18)² = 0.44. Lowering the light back toward 12 inches restores the higher intensity.
Formulas used
PPFD_new = PPFD_known · (d_known / d_new)² (inverse-square law)
How to read your result
The inverse-square law is an idealised point-source model. Real LED panels with lenses and wide boards fall off more gently, so treat the result as an upper-bound guide and confirm with a meter where possible.
Recommended ranges
| LED power | Approx. distance |
|---|---|
| < 300 W | 12 – 18 in (30 – 45 cm) |
| 300 – 600 W | 18 – 24 in (45 – 60 cm) |
| 600 W+ | 24 – 36 in (60 – 90 cm) |
Common mistakes to avoid
- Treating the estimate as exact for a large LED board — broad fixtures deviate from a point source.
- Setting height by heat on the back of your hand instead of by PPFD/DLI targets.
- Forgetting that edges of the footprint get far less light than the centre.
- Hanging too close after switching from HID to LED, causing light bleaching.
Frequently asked questions
How far should LED grow lights be from plants?
It depends on wattage and crop, but commonly 12–24 inches for mid-power LEDs in flower and more in veg. Set distance by target PPFD, not guesswork.
What is the inverse-square law for grow lights?
Intensity is proportional to 1 ÷ distance². Double the distance and intensity drops to about a quarter. It is exact for point sources and approximate for panels.
Why did my PPFD drop so much when I raised the light?
Because intensity falls with the square of distance — a modest height increase produces a much larger intensity drop.
Does this work for HID and LED?
It is most accurate for compact/point-like sources (HID bulbs, small COBs) and an approximation for large LED quantum boards.
How do I find PPFD at canopy?
Use a PAR/PPFD meter at canopy height, or start from the manufacturer's PPFD-at-distance chart and adjust with this calculator.