A grow light calculator helps you determine the Daily Light Integral (DLI) your indoor plants receive and size your lighting setup to match each crop's requirements. Enter your light's PPFD and photoperiod to calculate DLI, or work backwards from a target DLI to find the PPFD you need. This tool also estimates coverage area, light height adjustments, and monthly electricity costs so you can optimize your indoor garden efficiently.
Calculate DLI from PPFD
Required PPFD from Target DLI
DLI Assessment
Light Height Estimator
Estimate how PPFD changes as you move the light closer or farther from the canopy (inverse square law approximation).
Coverage Area Calculator
Estimate how much growing area a light can cover at your target PPFD.
Typical LED: 2.0-3.0 µmol/J
Electricity Cost Estimator
Plant DLI Requirements
| Plant Type | DLI Range | Light Level | Min PPFD @ 16h |
|---|---|---|---|
| Houseplants | 4 – 8 | Low | 70 |
| Orchids | 5 – 12 | Low | 87 |
| Succulents | 8 – 15 | Medium | 139 |
| Microgreens | 10 – 15 | Medium | 174 |
| Seedlings | 10 – 15 | Medium | 174 |
| Lettuce / Herbs | 12 – 17 | Medium | 209 |
| Tomatoes / Peppers | 20 – 30 | High | 347 |
| Cannabis | 30 – 45 | Very High | 521 |
Min PPFD calculated at 16-hour photoperiod. Actual requirements vary by cultivar, growth stage, and CO2 levels.
Light Schedule by Growth Stage
Gentle light, lower PPFD (100-200). Long days promote strong stem growth.
Maximum growth. Higher PPFD (200-600). More light hours = more vegetative mass.
Short days trigger flowering in photoperiod-sensitive plants. High PPFD (400-800+).
Common Grow Light Types
| Light Type | Typical PPFD @ 12" | Efficacy (µmol/J) | Best For |
|---|---|---|---|
| LED Panel | 400 – 800 | 2.0 – 3.0 | All stages |
| T5 Fluorescent | 200 – 400 | 1.2 – 1.8 | Seedlings, herbs |
| HPS (High Pressure Sodium) | 600 – 1000 | 1.5 – 1.8 | Flowering, fruiting |
| CMH (Ceramic Metal Halide) | 400 – 700 | 1.5 – 2.0 | Full spectrum |
| CFL (Compact Fluorescent) | 50 – 150 | 0.8 – 1.2 | Low-light houseplants |
How to Use the Grow Light Calculator
Growing plants indoors requires matching your lighting setup to each plant's photosynthetic needs. Rather than guessing based on wattage alone, this grow light calculator uses Daily Light Integral (DLI) — the gold standard metric for measuring how much usable light a plant receives over an entire day. Understanding DLI helps you avoid the two most common indoor gardening mistakes: too little light (leggy, pale growth) and too much light (bleached leaves and wasted electricity).
Step 1: Calculate Your DLI
In the "PPFD to DLI" mode, enter the PPFD (Photosynthetic Photon Flux Density) from your grow light's spec sheet — this is measured in micromoles per square meter per second. Then set your planned photoperiod in hours per day. The calculator uses the standard formula: DLI = PPFD × hours × 3600 / 1,000,000. Use the light-type presets to quickly fill in typical PPFD values for LED panels, T5 fluorescents, or HPS fixtures at 12 inches.
Step 2: Find Required PPFD for Your Plants
Switch to "Target DLI to PPFD" mode when you know what DLI your plants need but want to find the minimum PPFD required. Select a plant preset (lettuce, tomatoes, cannabis, orchids) or enter a custom DLI target. The calculator determines exactly what PPFD your light must deliver at the canopy level for your chosen photoperiod.
Step 3: Adjust Light Height
PPFD decreases as you raise the light farther from the plant canopy. The light height estimator uses the inverse square law to approximate how intensity changes at different distances. Enter your light's known PPFD at a reference height, then try different heights to find the sweet spot that delivers the PPFD your plants need without risking light burn from being too close.
Step 4: Size Your Coverage Area
The coverage area calculator helps you determine how many square feet a single light can serve at your target PPFD. Enter the light's wattage and efficacy rating (found on the spec sheet in micromoles per joule). Higher-efficacy LED panels cover more area per watt than older HPS or fluorescent fixtures, making them more cost-effective for larger grows.
Step 5: Estimate Electricity Costs
Indoor lighting can significantly impact your electricity bill, especially with high-wattage fixtures running 16 to 18 hours per day. The cost estimator multiplies your light's wattage by the daily runtime and your local electricity rate to show daily, monthly, and annual operating costs. This helps you compare the total cost of ownership between different grow light options — a more efficient LED may cost more upfront but save substantially on electricity over a growing season.
Frequently Asked Questions
Is this grow light calculator really free?
Yes, completely free with no signup required. All calculations run locally in your browser. No data is ever sent to a server, and you can use it as many times as you need.
Is my data safe when using this tool?
Absolutely. Everything runs entirely in your browser using JavaScript. No information is transmitted anywhere. You can even use it offline once the page loads.
What is DLI and why does it matter for indoor plants?
DLI (Daily Light Integral) measures the total amount of photosynthetically active light a plant receives in a day, expressed in mol/m2/day. It is the most accurate way to match your grow light setup to what your plants actually need, rather than relying on wattage alone.
Where do I find the PPFD value for my grow light?
PPFD is listed on the spec sheet or product page of most quality grow lights. It is measured in micromoles per square meter per second. If the manufacturer only lists lux, you can approximate PPFD by dividing lux by 54 for white LEDs, though this is less precise.
How many hours of light do indoor plants need?
Most indoor plants thrive with 12 to 16 hours of light per day. Seedlings and vegetative growth benefit from 16 to 18 hours, while flowering plants like tomatoes and peppers often perform best at 12 to 14 hours to trigger bloom.
Does PPFD really decrease with distance from the light?
Yes. Light intensity drops roughly following the inverse square law, meaning doubling the distance cuts intensity to about one quarter. In practice, reflectors and lens angles modify this, but moving lights closer to plants always increases PPFD significantly.
How do I calculate grow light electricity cost?
Multiply your light's wattage by the hours per day you run it, then multiply by your electricity rate per kWh and by 30 days. For example, a 200W light running 16 hours per day at $0.12/kWh costs about $11.52 per month.
What DLI do tomatoes and peppers need indoors?
Tomatoes and peppers are high-light crops that need a DLI of 20 to 30 mol/m2/day for healthy growth and fruit production. This typically requires a grow light delivering at least 400 PPFD for 16 hours per day.