💡 Grow Light Calculator
Calculate optimal wattage, PPFD, and coverage for LED, HPS, and CMH grow lights
Calculate Light Requirements
Length × Width in feet (e.g., 4ft × 4ft = 16 sq ft)
Different technologies require different wattages
Flowering requires more intense light
US average: $0.12/kWh
Your Lighting Recommendations
Energy Cost Estimates
💡 Lighting Tips
- Maintain proper distance between light and canopy (12-24" for LED, 18-36" for HPS)
- Monitor plant response - light stress shows as bleached or curled leaves
- Use a light meter or PAR meter to verify actual PPFD at canopy level
- Consider adding supplemental lighting for corners and edges
Understanding Grow Lights
Wattage vs. PPFD
Wattage measures electricity consumption, not light output. PPFD (Photosynthetic Photon Flux Density) measures actual usable light reaching your plants in μmol/m²/s. Modern LEDs produce more PPFD per watt than older HPS lights. Always prioritize PPFD over wattage when comparing lights.
Light Requirements by Stage
Vegetative stage: 300-600 PPFD for 18 hours daily. Flowering stage: 600-1000 PPFD for 12 hours daily. Seedlings need only 200-400 PPFD. More light generally means bigger yields, but only if plants have adequate nutrients, CO₂, and water. Too much light causes stress and bleaching.
LED vs. HPS vs. CMH
LED: Most efficient (30-40W/sq ft), low heat, long lifespan (50,000+ hours), higher upfront cost. HPS: Proven technology (50-60W/sq ft), high heat output, lower upfront cost, needs replacement every 12-18 months. CMH/LEC: Balanced spectrum (40-50W/sq ft), moderate heat, better spectrum than HPS, middle ground on cost.
Coverage and Penetration
Light intensity decreases with distance (inverse square law). Ensure your light can adequately cover your entire grow space. Multiple smaller lights often work better than one large light for even coverage. Consider plant height and canopy density when positioning lights.
How to Use This Calculator
- Measure your grow space dimensions and calculate square footage (length × width)
- Enter your grow space size in square feet
- Select your light type: LED, HPS, or CMH/LEC
- Choose your growth stage: vegetative (18 hours light) or flowering (12 hours light)
- Optionally enter your electricity cost per kWh to see operating costs
- Click "Calculate" to see recommended wattage, PPFD targets, and energy costs
- Use the recommendations to size your lighting system appropriately
Frequently Asked Questions
How accurate is the grow light calculator?
This calculator uses industry-standard wattage-per-square-foot guidelines: 30-40W/sq ft for LED, 50-60W/sq ft for HPS, and 40-50W/sq ft for CMH. Actual needs may vary based on light quality, efficiency, and plant genetics. These recommendations ensure adequate light intensity for healthy cannabis growth.
Is this tool free to use?
Yes! This tool is completely free with no hidden costs, subscriptions, or limitations. Use it as often as you need.
Do I need to create an account?
No account needed. The tool works entirely in your browser without requiring signup or personal information.
Does this work on mobile devices?
Yes, this tool is fully responsive and works on smartphones, tablets, and all desktop browsers.
Is my data private?
Absolutely. All processing happens locally in your browser. Your data never leaves your device and is not stored on any server.
What is PPFD and why does it matter?
PPFD (Photosynthetic Photon Flux Density) measures the amount of usable light reaching your plants, measured in μmol/m²/s. Cannabis needs 300-600 PPFD for vegetative growth and 600-1000 PPFD for flowering. Higher PPFD generally means bigger yields, but requires proper nutrients and CO₂.