Grow Light Calculator
Calculate recommended grow light wattage, lumens, and hanging height for your growing area and plant type.
Supports feet and meters.
Grow light coverage calculates the area a grow light can effectively illuminate for plant growth, based on wattage, light intensity, and the light’s mounting height. Using too little light produces weak, stretched plants; too much wastes electricity.
The Formula:
Coverage area (sq ft) = Wattage / Recommended watts per square foot
Wattage Requirements by Grow Stage:
| Stage | Watts per sq ft (HID/HPS) | Watts per sq ft (LED) |
|---|---|---|
| Seedlings / clones | 15–25 W | 10–15 W |
| Vegetative growth | 25–35 W | 20–30 W |
| Flowering / fruiting | 35–50 W | 30–40 W |
LED Efficiency Note: LEDs are ~30% more efficient than HPS. A 300W LED delivers roughly the same output as a 400W HPS lamp.
Worked Example:
You have a 600W LED grow light. Growing flowering tomatoes:
Coverage = 600 / 35 = 17.1 sq ft
A grow tent of 4 ft × 4 ft = 16 sq ft — a good match.
Mounting Height Guidelines:
| Light Type | Seedling Height | Veg Height | Flower Height |
|---|---|---|---|
| LED 300W | 30–45 cm | 45–60 cm | 30–45 cm |
| LED 600W | 45–60 cm | 60–75 cm | 45–60 cm |
| HPS 1000W | 45–60 cm | 60–90 cm | 45–60 cm |
Daily Light Integral (DLI):
DLI = PPFD (μmol/m²/s) × photoperiod (hours) × 3600 / 1,000,000
Most vegetables need 20–30 mol/m²/day DLI. Fruiting crops (tomatoes, peppers) need 30–40.
Practical Tips:
- Use a light meter (lux or PAR meter) rather than relying on wattage alone
- Rotate plants weekly for even coverage near edges
- Dimming a large light is better than raising it — it maintains even PPFD distribution
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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