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