Hydroponic Grow Light Calculator
Choosing the wrong grow light is, in fact, 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, crop type, and light type and tells you exactly how many true watts you need, the right PPFD and DLI targets, daily light hours, and how high to hang the panel. It still works for LED panels, clip-on bars, 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, microgreens, mint, parsley and chives, leafy greens and basil, woody herbs, fruiting vegetables, or strawberries. Each group has a different light appetite, and mint wants roughly half of what a tomato does.
- Enter your grow space size in centimeters or inches. Length times width, essentially, gives the coverage area your light needs to fill.
- Pick your light type so the calculator can tell you whether that hardware can realistically hit the target, because a clip-on bar and a 200W panel are not interchangeable.
- Read your results and get true wattage, PPFD, DLI, photoperiod, hang height, and coverage in one place. Finally, copy or print the plan and keep it near the shelf.
Common questions about hydroponic grow lights
What is PPFD and why does it matter?
PPFD, in short, stands for Photosynthetic Photon Flux Density and measures how much usable light actually reaches your plants in micromoles per square meter per second. It is far more accurate than lumens or watts, both of which describe the bulb rather than what arrives at the leaf. Mint, parsley and chives do well at 200 to 320 PPFD. Leafy greens and basil, meanwhile, need 200 to 400, with cilantro and dill toward 250 to 350. Microgreens, by contrast, need about 180 to 320. Fruiting crops, overall, need 400 to 600. Too low and plants stretch. Too high and leaves turn pale.
What is DLI and how do I use it?
DLI, in short, stands for Daily Light Integral and measures the total light a plant receives across a full day, which matters more than intensity at any single moment. First multiply PPFD by light hours, then by 3600, then divide by 1,000,000. Leafy greens and herbs need a DLI of 10.1 to 20.2 mol/m²/day on the standard 14-hour photoperiod. Fruiting crops running 16 hours, however, reach 23.0 to 34.6. If your PPFD is low you can partially compensate with a longer photoperiod, though not on most herbs and greens, where longer days push cilantro and spinach toward bolting.
What is true watts vs equivalent watts for LED?
Equivalent watts, sometimes called replacement watts, is instead a marketing figure comparing the LED to an older HPS or incandescent bulb. So it tells you nothing useful. True watts, instead, is how much electricity the LED actually draws from the wall. Always use true watts when calculating coverage. A light advertised as 1000W equivalent may only draw 100 true watts, and the listing will usually bury that number in the specification table rather than the title.
How many watts of LED do I need per square foot?
For microgreens, overall, 10 to 20 true watts per square foot is enough. For leafy greens and herbs, however, plan for 20 to 35 true watts per square foot. Strawberries and fruiting crops, meanwhile, both sit at 30 to 45 watts per square foot. As a countertop rule of thumb, a 2-foot footprint wants 20 to 36 real watts, which is why a single mid-range panel handles most apartment setups. The actual PPFD at canopy still depends on the specific light and hang height, so check the manufacturer coverage map where one exists.
How many hours of light do hydroponic plants need per day?
14 hours on and 10 hours off is the standard for most herbs and leafy greens, including seedlings and microgreens. Fruiting crops like cherry tomatoes and dwarf peppers, however, may run up to 16 hours, and strawberries, shiso, stevia and nasturtiums do best at 16. Spinach is the one to watch, because it wants 12 hours and its bolting range starts around 13. Running lights 24 hours a day is not recommended, since plants need a dark period for proper metabolic function. The BN-LINK timer automates the cycle and is the easiest solution.
How much light do microgreens need?
Microgreens, by contrast, need very little light compared to other crops. A PPFD target of about 180 to 320 is, overall, plenty, with a 14-hour photoperiod and the panel 10 to 12 inches above the tray. Even a basic clip-on bar LED still works well for a single tray. Microgreens are harvested before they need to fuel significant new growth, so high-intensity lighting is unnecessary and will wash out tender cotyledons. Spend the money on airflow instead, since mold is the real microgreen risk.
Does heat from grow lights affect my hydroponic system?
Yes, in fact, and this is the connection most beginners miss. Strong lights, however, raise ambient temperature, which warms your nutrient solution, and warm water holds less dissolved oxygen. That oxygen drop is what actually starts root rot, rather than the pathogen arriving from somewhere. Keep the root zone at 65 to 75°F (18 to 24°C), which is the safe target for every crop on this site. So keep your reservoir out of direct light, use an opaque container, and add a fan if the number climbs.
Can I grow hydroponics with just a window?
For leafy greens on a bright south-facing windowsill in summer, however, yes. A sunny window is, however, marginal for lettuce and spinach even at its best, and it drops off sharply in autumn and winter just as you are most likely to want fresh greens. Basil and fruiting crops, however, need more than a window can reliably provide indoors. For consistent results year-round, the VIPARSPECTRA P700 is a fanless, silent starting point, or the Mars Hydro TS600 if you want more output.
How high should I hang my grow light above my plants?
Pick the row that matches your stage rather than using a loose range, since a vague number is how people end up bleaching seedlings. Seedlings before true leaves want 10 to 12 inches. Leafy herbs and greens want 6 to 8 inches. Woody herbs like oregano and thyme want 4 to 6 inches. Fruiting crops want 8 to 12 inches. Microgreens, by contrast, sit at 10 to 12 inches. Start at the top of your row and lower gradually while watching for bleaching or stretching.
Should I use a grow tent?
A grow tent is not required but makes a real difference. The reflective interior walls bounce light back onto your plants, so you effectively increase PPFD without buying a bigger panel. A tent also lets you control temperature, humidity, and airflow much more precisely, and it stops light spilling across a studio at 11pm. The VIVOSUN 2×4 tent is a good starter size for herb and vegetable growing.