Calculator
Use one method, then refine with overlap and height.
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Example data table
| Scenario | Area | Method | Fixture | Target | Output (typical) |
|---|---|---|---|---|---|
| Small tent | 120 × 60 cm | Target PPFD | 1 × 300 µmol/s | 350 PPFD | 1 fixture, 1×1 layout, spacing matches tent |
| Medium rack | 200 × 100 cm | Target PPFD | 4 × 300 µmol/s | 350 PPFD | 4 fixtures, 2×2 layout, ~50×100 cm spacing |
| Focused reflector | 100 × 100 cm | Beam angle | 90° beam, 45 cm height | — | ~90 cm footprint, center spacing reduced by overlap |
Formula used
Beam-angle footprint
- Diameter:
D = 2 × H × tan(θ/2) - Area:
A = π × (D/2)² - Spacing:
S = D × (1 − overlap)
H is hanging height, θ is beam angle.
Intensity-driven (PPFD)
- Total PPF:
PPFₜ = PPFD × Area ÷ U - Fixtures:
N = ceil(PPFₜ ÷ PPFᶠ) - Spacing:
Sx = W ÷ cols,Sy = L ÷ rows
U is utilization; PPFᶠ is fixture PPF.
Height adjustment from a PPFD reading
H₂ = H₁ × √(PPFD₁ ÷ PPFD₂)
This is a practical approximation. Real fixtures are not point sources.
How to use this calculator
- Measure your grow area length and width accurately.
- Choose a method based on your best available information.
- Enter target PPFD, then fixture PPF or watts and efficacy.
- Use overlap to smooth the canopy and reduce edge dimming.
- If you own a meter, enter a PPFD reading to refine height.
- Submit and use the suggested spacing as your starting layout.
Tip: After installing, verify with a PPFD meter or plant response. Adjust height slowly to avoid light stress.
Practical guidance
When to raise lights
If leaves curl, bleach, or dry quickly, raise the fixture and increase airflow. Keep overlap moderate to reduce hotspots across the canopy.
When to lower lights
If stems stretch and nodes space out, lower the fixture gradually. Increase PPFD carefully while maintaining good watering and nutrient balance.