Formula used
DLI = PPFD × Light Hours × 3,600 ÷ 1,000,000 Required Light Hours = Remaining DLI × 1,000,000 ÷ PPFD ÷ 3,600 PPF = Fixture Watts × Photon Efficacy × Fixture Count Average PPFD ≈ PPF × Utilisation Factors ÷ Canopy Area Energy = Total Watts × Daily Hours ÷ 1,000 Heat Load = Effective Watts × 3.412 BTU/h
Natural and artificial DLI values are added together. Loss factors adjust practical canopy delivery. Measured PAR data provides the strongest accuracy.
How to use this calculator
- Choose a plant preset and current growth stage.
- Enter the growing area or individual plant spacing.
- Describe available daylight and the window conditions.
- Enter grow-light specifications, coverage, height, and schedule.
- Adjust advanced losses when reliable values are available.
- Calculate and review DLI, coverage, heat, and costs.
- Confirm important decisions using a calibrated PAR meter.
Example plant light reference table
| Plant group | Typical PPFD | Typical DLI | Common schedule |
|---|---|---|---|
| Low-light foliage | 40–120 µmol/m²/s | 2–6 mol/m²/day | 10–14 hours |
| Medium houseplants | 100–250 µmol/m²/s | 5–12 mol/m²/day | 12–16 hours |
| Herbs | 200–500 µmol/m²/s | 12–25 mol/m²/day | 14–18 hours |
| Leafy vegetables | 250–550 µmol/m²/s | 14–28 mol/m²/day | 14–18 hours |
| Fruiting vegetables | 450–900 µmol/m²/s | 22–40 mol/m²/day | 12–18 hours |
| Succulents and cacti | 300–700 µmol/m²/s | 18–35 mol/m²/day | 12–16 hours |
These ranges are planning estimates. Species and cultivars can differ. Acclimate plants gradually after major lighting changes.
Understanding PPFD, DLI, lux, and foot-candles
PPFD measures useful photon intensity reaching plant surfaces. DLI totals those photons across one day. Both values guide more dependable plant-light planning.
Lux and foot-candles describe human-visible brightness. Plant responses depend more directly upon photon delivery. Spectrum changes every lux-to-PPFD conversion significantly.
Natural light varies with weather, glass, and distance. Fixture output also changes across the canopy. Direct measurements improve setup decisions and consistency.
Frequently asked questions
What is a good PPFD for houseplants?
Many houseplants perform between 100 and 250 µmol/m²/s. Low-light species often need less intensity. Observe new growth and adjust gradually.
What does DLI measure?
DLI measures total daily photosynthetic photon exposure. It combines intensity with lighting duration. Higher intensity needs fewer operating hours.
Can lux replace a PAR meter?
Lux can support rough comparisons under similar spectra. It cannot measure plant-usable photons directly. PAR measurements are better for important decisions.
How far should grow lights remain?
Distance depends on fixture power and beam shape. Manufacturer maps provide a useful starting point. Watch for stretching, bleaching, and excessive heat.
Can plants receive too much light?
Excessive light can cause bleaching and leaf stress. Heat can intensify those visible symptoms. Reduce intensity or increase fixture distance.
Do plants need darkness?
Most plants benefit from a regular dark period. Darkness supports normal metabolic and signalling processes. Avoid continuous lighting without species-specific evidence.
How accurate is natural-light estimation?
Window estimates remain approximate because daylight changes constantly. Glass, clouds, seasons, and obstructions reduce intensity. A meter provides stronger local data.
Why does fixture coverage matter?
Poor coverage creates bright centres and dim edges. Plants then develop at noticeably different rates. Overlap footprints for better canopy uniformity.
How is electricity cost calculated?
The calculator multiplies effective watts by operating hours. It then applies the entered energy rate. Actual bills may include other charges.
Why estimate ventilation?
Lighting electricity eventually becomes heat inside enclosed spaces. Ventilation removes heat and supports temperature control. Humidity management may require additional airflow.
Should seedlings use adult plant targets?
Seedlings usually need lower intensity than mature plants. Excess light can slow early development. Increase intensity as roots and leaves expand.
Accuracy and safety note
This calculator provides planning estimates, not guaranteed horticultural outcomes. Electrical equipment must remain dry and properly rated. Follow fixture instructions and local electrical requirements.