Most vegetable and flower seedlings need 14 to 18 hours of bright light per day. The exact duration depends on the plant species, light intensity (PPFD), and growth stage. For example, tomato seedlings thrive with 16–18 hours, while lettuce and leafy greens do well with 14–16 hours. Always provide at least 6 hours of uninterrupted darkness for proper respiration. Use this calculator to find the optimal duration for your specific setup.

DLI (Daily Light Integral) measures the total amount of photosynthetically active light a plant receives in one day, expressed in mol/m²/day. It combines both light intensity (PPFD) and duration. Seedlings typically need a DLI of 10–20 mol/m²/day. Too little light causes weak, leggy stems; too much can burn tender leaves. This calculator estimates both PPFD and DLI so you can dial in the perfect light schedule.

Both work well, but they have different strengths. LED grow lights are energy-efficient, produce less heat, and last longer (50,000+ hours). They're excellent for seedlings when placed 12–24 inches above the canopy. T5 fluorescent lights are affordable, widely available, and provide even light coverage—ideal for seed starting racks when placed 2–6 inches above plants. LEDs cost more upfront but save money over time from lower electricity usage. Avoid incandescent bulbs; they're inefficient and produce too much heat.

Light intensity follows the inverse square law: doubling the distance reduces intensity to 25% of the original. If your LED light produces 400 µmol/m²/s at 12 inches, moving it to 24 inches drops intensity to about 100 µmol/m²/s. For seedlings, keep lights close enough to prevent stretching but far enough to avoid heat stress. As a rule of thumb: LEDs at 12–24 inches, T5 fluorescents at 2–6 inches, and HID lights at 18–36 inches above the canopy. Watch your plants—if they look pale or curled, increase distance; if they're tall and thin, decrease distance.

No, 24-hour lighting is not recommended for most plants. Plants need a dark period for respiration—a critical process where they convert stored sugars into energy for growth. Without darkness, many plants develop nutrient deficiencies, reduced vigor, and delayed flowering. Most seedlings do best with 6–8 hours of darkness daily. Some short-day plants actually require longer dark periods to trigger flowering. Stick to 14–18 hours of light and let your plants rest.

Watch for these telltale signs: Insufficient light causes tall, thin, leggy stems; pale or yellowing leaves; and seedlings leaning toward the light source. Too much light results in curled, crispy leaf edges; bleached or brown spots; and stunted growth. Healthy seedlings have compact, sturdy stems with deep green leaves. If you notice issues, adjust either the light duration or distance using this calculator as a guide. A good rule: the stem should be thick enough to support the seedling without staking.

Running seedling grow lights is surprisingly affordable. A 100W LED light running 16 hours per day at the US average electricity rate ($0.12/kWh) costs approximately $5.76 per month or about $69 per year. A 54W T5 fluorescent costs roughly $3.10/month. Even a larger 300W setup only costs about $17/month. Use the electricity cost estimator in this calculator with your actual rate for a precise figure. Compared to buying mature plants from a nursery, starting seeds indoors with grow lights saves significant money.

Yes, absolutely. A timer is one of the most important accessories for indoor seed starting. Consistent light cycles are crucial for healthy plant development. Mechanical timers are inexpensive ($5–$15) and reliable. Smart plugs ($15–$30) offer app control, scheduling flexibility, and even sunrise/sunset simulation. Set your timer based on this calculator's recommendation, and your seedlings will receive perfectly consistent light every day without any manual effort.