Grow Light Calculator
Calculate grow light wattage, coverage, and number of fixtures for your indoor garden. Includes recommended PPFD, photoperiod, and electricity cost.
Quick Answer
Vegetables need about 35 watts per square foot of quality LED light. A 4×4 area needs roughly 560 watts total. Leafy greens need less (25W/sq ft), fruiting plants need more (50W/sq ft). Hang LEDs 18-24 inches above the canopy.
Growing Space
Your Light Fixture (optional)
Square coverage per fixture (e.g., 4 = 4x4 ft)
Results
560W
Total Watts Needed
16 ft²
Growing Area
1
Fixtures Needed
500 µmol
Target PPFD
14h
Photoperiod
18-24 inches
Hanging Height
235 kWh
Monthly Usage
$37.63
Monthly Cost
About This Tool
The Grow Light Calculator helps indoor gardeners determine the right amount of artificial lighting for their growing space. It calculates total wattage needed, number of fixtures, optimal hanging height, and monthly electricity cost based on your room size and plant type.
Understanding PPFD
PPFD (Photosynthetic Photon Flux Density) measures the amount of usable light reaching your plants in micromoles per square meter per second. Seedlings need about 200 PPFD, leafy greens 200-400, vegetables 400-600, and fruiting plants 600-900+. PPFD is the most accurate way to measure grow light effectiveness, superior to lumens or lux which are weighted for human vision, not plant growth.
LED vs Other Grow Lights
Modern LED grow lights are the most efficient option, converting about 2.5-3 micromoles of light per watt of electricity. Older HPS (high pressure sodium) lights produce about 1.7 micromoles per watt and generate much more heat. Fluorescent T5 lights work well for seedlings and leafy greens but lack the intensity for fruiting crops. LED costs more upfront but saves significantly on electricity and cooling.
Hanging Height and Coverage
Closer lights deliver more intense light to a smaller area. Higher lights spread coverage but reduce intensity. Start at the manufacturer-recommended height and adjust based on plant response. Signs of too much light include bleached or curling leaves. Signs of too little light include leggy stems and pale foliage. Most LED grow lights perform best at 18-24 inches for vegetables.
Photoperiod
Most vegetables and herbs grow best with 14-16 hours of light per day. Some flowering plants require specific day lengths to trigger blooming. Leafy greens are flexible and grow under most schedules. Always provide at least 6-8 hours of darkness for plant rest and metabolism. Use a timer to maintain consistent schedules.
Frequently Asked Questions
How many watts per square foot do I need?
How high should I hang my grow light?
How many hours of light do indoor plants need?
Do grow lights use a lot of electricity?
Can I use regular LED bulbs as grow lights?
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