Hydroponic Grow Light Calculator
Choosing the wrong grow light is one of the most common beginner mistakes. Too weak and your plants stretch, stay pale, and never fruit. Too strong and you get leaf burn, bleaching, and wasted electricity.
This free hydroponic grow light calculator takes your grow space size and crop type and tells you exactly how many watts you need, the right PPFD target, daily light hours, and how high to hang your light. It works for LED panels, countertop kits, and dedicated grow tents.
Step 1 of 3 – Choose your crop type
What are you growing?
Your light recipe
📊 Full Light Specification
📋 How to use this hydroponic grow light calculator
- Choose your crop type from seedlings, leafy greens and herbs, fruiting vegetables, or strawberries. Each category has different light requirements.
- Enter your grow space size in centimetres or inches. Length times width gives the coverage area your light needs to fill.
- Pick your light type so the calculator can give you the right wattage recommendation for your setup.
- Read your results and get exact wattage, PPFD target, daily light hours, hang height, and DLI all in one place.
❓ Common questions about hydroponic grow lights
💡 What is PPFD and why does it matter?
PPFD stands for Photosynthetic Photon Flux Density and measures how much usable light actually reaches your plants in micromoles per square metre per second. It is a far more accurate measure than lumens or watts. Leafy greens need 150 to 300 PPFD depending on the crop. Fruiting crops like tomatoes need 600 to 900 PPFD. Too low and plants stretch. Too high and leaves bleach.
🌞 What is DLI and how do I use it?
DLI stands for Daily Light Integral and measures the total amount of light a plant receives in a full day. It is calculated by multiplying PPFD by the number of light hours and then by 3600, then dividing by 1,000,000. Leafy greens need a DLI of 8.6 to 17.3 mol/m²/day. Fruiting crops running at high PPFD can reach 38.9 to 58.3. If your PPFD is low, you can compensate by running lights longer.
📏 What is true watts vs equivalent watts for LED?
Equivalent watts (also called replacement watts) is a marketing figure comparing the LED to an older HPS or incandescent bulb. It tells you nothing useful. True watts is how much electricity the LED actually draws from the wall. Always use true watts when calculating coverage. A light labelled 1000W equivalent may only draw 100 true watts.
🔌 How many watts of LED do I need per square foot?
For leafy greens and herbs, plan for 20 to 35 true watts per square foot of grow space. For fruiting crops like tomatoes and peppers, plan for 40 to 50 true watts per square foot. These are starting points. The actual PPFD at canopy level depends on the specific light and hang height, so always check the manufacturer coverage map if available.
🌙 How many hours of light do hydroponic plants need per day?
Most leafy greens and herbs grow well on 16 hours of light and 8 hours of darkness. Fruiting crops like tomatoes do best on 18 hours. Seedlings can handle 18 hours. Running lights 24 hours a day is not recommended as most plants need a dark period for proper metabolic function. Use a simple plug-in timer to automate the cycle. The BN-LINK timer at $9.99 is the easiest solution.
🌡️ Does heat from grow lights affect my hydroponic system?
Yes. Strong lights raise ambient temperature which warms your nutrient solution. Warm water holds less oxygen and increases root rot risk. Keep your reservoir away from direct light, use an opaque container, and monitor solution temperature with a thermometer. If it rises above 24 degrees C consistently, consider adding an inline fan for ventilation or moving the reservoir further from the light.
💡 Can I grow hydroponics with just a window?
For leafy greens on a bright south-facing windowsill in summer, yes. A sunny window provides roughly 500 to 1000 lux on a clear day, which is marginal for lettuce and spinach. Basil and fruiting crops need more than a window can reliably provide indoors. For consistent results year-round, a Mars Hydro TS600 at $59.99 is the best value starting point and covers a 60×60 cm area well.
🔧 How high should I hang my grow light above my plants?
For seedlings, keep LED panels 40 to 60 cm (16 to 24 inches) above the tray to avoid burning tender seedlings. For leafy greens and herbs, 20 to 40 cm (8 to 16 inches) works well. For fruiting crops that need high PPFD, 15 to 30 cm (6 to 12 inches) is common. Always start higher and lower gradually while watching for any bleaching or stretching.
🏠 Should I use a grow tent?
A grow tent is not required but makes a significant difference. The reflective interior walls bounce light back onto your plants, effectively increasing your PPFD without buying a bigger light. A tent also lets you control temperature, humidity, and airflow much more precisely. The VIVOSUN 2×4 tent at $89.98 is a great starter size for herb and vegetable growing.