Grow Light Calculator

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:

These are the figures this calculator uses. LED wattage is the number that matters now; the HID column is kept because plenty of people still run those lamps and want to compare.

LED Efficiency Note: LEDs draw roughly a third less power than HPS for the same usable light output. A 400W LED does the work of a 600W HPS lamp.

Worked Example:

A 4 ft × 4 ft tent, 16 sq ft, growing flowering tomatoes at 40–60 W/sq ft:

Wattage = 16 × 40 to 16 × 60 = 640 to 960 W of LED

Flowering runs a 12-hour photoperiod, so at the top of that range: 960 W × 12 h × 30 days ÷ 1,000 = 346 kWh a month. That is the number worth checking before buying the light, because on many tariffs the electricity costs more over a season than the fixture did.

Mounting Height Guidelines:

Daily Light Integral (DLI):

DLI = PPFD (μmol/m²/s) × photoperiod (hours) × 3600 ÷ 1,000,000

DLI is the total light a plant receives in a day, and it is the honest measure. PPFD alone tells you nothing without the hours, which is why a bright light on a short day can underfeed a crop that a dimmer light on a long day satisfies.

Change the photoperiod above and watch the DLI move. That is the point of the input: a 600 PPFD fixture on a 12-hour flowering schedule delivers 25.9, right at the bottom of what a fruiting crop wants, and the same fixture on 18 hours delivers 38.9, which is more than the plant can use and 50% more electricity for nothing.

Practical Tips:

• Use a PAR meter rather than relying on wattage. Wattage is what the light draws from the wall; PPFD is what reaches the leaf, and cheap fixtures lose a great deal between the two.
• Lumens are a measure of brightness to a human eye, which peaks in the green a plant reflects. Treat any lumen figure on a grow light as marketing.
• Rotate plants weekly. The centre of any fixture is far brighter than the corners.
• Dimming a large light beats raising it. Raising it evens out the intensity but throws more of the light at your walls.