Grow Light Calculator
Room size and plant type to total wattage and fixture count.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Grow Light Calculator
Getting the right amount of light is the single most important factor in indoor gardening. Too little and plants stretch, yield drops, and flowering stalls. Too much and you waste electricity and risk light burn. Here is how to use this calculator:
- Select your plant type. Each category maps to a watt-per-square-foot range based on the plant's light requirements. Seedlings and leafy greens need the least light (20-30 W/sq ft). Fruiting and flowering crops like tomatoes need the most (40-60 W/sq ft).
- Enter your room dimensions. Measure the length and width of the growing area in feet. This should be the actual canopy footprint, not the full room size.
- Read your results. The calculator shows total LED wattage (actual draw, not equivalent), the number of 300-watt fixtures needed, the recommended hanging height, and the appropriate light spectrum type.
All wattage figures are for actual LED draw, not marketing "equivalent" wattage. A fixture labeled "600W HPS equivalent" typically draws only 250-320 watts from the wall. Always verify a fixture's actual watt draw on its spec sheet.
About the Grow Light Calculator
The watts-per-square-foot (W/sq ft) method is the most widely used guideline for sizing indoor grow lights. It is a starting point, not an exact science. Actual light requirements vary by strain, growth stage, distance from the canopy, and the light's PAR (photosynthetically active radiation) efficiency. High-quality LED fixtures with broad-spectrum output can often achieve equivalent results at the lower end of the W/sq ft range.
For hanging height, LEDs should be kept farther away than HID lights because their intensity drops off sharply with distance. During the seedling stage, hang lights higher (18-24 inches) to avoid bleaching tender leaves. Lower the fixture as plants mature and can handle higher intensity. All calculations run entirely in your browser. No data is stored or sent anywhere.
Frequently Asked Questions
How many watts do I need per square foot for a grow light?
For seedlings and leafy greens, 20-30 true watts of LED per square foot is sufficient. Herbs and houseplants do well at 30-40 watts. High-light crops like tomatoes and peppers need 40-50 watts. Flowering and fruiting plants at peak production benefit from 50-60 watts per square foot. These are actual watt draw figures, not the inflated "equivalent" numbers printed on many fixture boxes.
What is the difference between actual watts and equivalent watts on grow lights?
Equivalent wattage is a marketing comparison to older HID (high-intensity discharge) bulbs, not a measure of how much electricity the fixture uses. A "1000W HPS equivalent" LED panel typically draws only 400-600 watts from the wall. Always use actual watt draw when sizing your electrical circuit and calculating light coverage. The spec sheet or the back of the fixture will list the true wattage.
How high should I hang my grow light above plants?
Hanging height depends on the fixture's power and the plant's growth stage. For seedlings, hang LED panels 18-24 inches above the canopy to prevent light stress on tender seedlings. Vegetative plants can handle lights at 18-24 inches. During flowering and fruiting, lower the light to 12-18 inches to maximize intensity where it counts. Always watch for light bleaching (white or yellow patches on upper leaves), which signals the light is too close.
Can I use a regular LED bulb instead of a grow light?
Standard white LED bulbs lack the blue and red wavelengths that plants use most efficiently for photosynthesis. They can keep seedlings alive and support low-light houseplants, but they will not produce the yields or growth rates of a purpose-built grow light. Full-spectrum grow LEDs are tuned to deliver light in the 400-700 nm range, with emphasis on the blue range (400-500 nm) for vegetative growth and red range (600-700 nm) for flowering. For any serious growing, a dedicated grow light is worth the investment.