Quick answer: Calculate light from canopy area and target PPFD, not from a fixed number of watts per plant. Measure the lit canopy, choose a PPFD range for the growth stage, check the fixture's PPFD map at the intended hanging height, and confirm the daily light integral (DLI) from the lighting hours.
If you searched how much grow light do cannabis plants need or how to calculate the light required for weed, the useful answer starts with square metres. Plant count changes with pot size and training, but canopy area is what the fixture must cover.
Grow-light calculator: four inputs
- Canopy length and width: multiply them to get square metres.
- Growth stage: seedlings, vegetative growth and flowering need different intensity.
- Lighting hours: the same PPFD produces a different DLI at 12, 18 or 20 hours.
- Fixture coverage: use a real PPFD map at the planned height, not the model name alone.
Step 1: measure the canopy, not the room
| Example canopy | Area | What to check |
|---|---|---|
| 60 × 60 cm | 0.36 m² | Uniform centre-to-edge PPFD and enough dimming for young plants |
| 90 × 90 cm | 0.81 m² | PPFD map should cover the full square at the intended height |
| 120 × 120 cm | 1.44 m² | Edge intensity, bar spacing, heat and access around the canopy |
| 120 × 240 cm | 2.88 m² | Two fixtures may give better uniformity than one central hotspot |
Step 2: choose a practical PPFD target
| Stage | Starting PPFD range | Typical use |
|---|---|---|
| Seedlings and clones | 100–300 µmol/m²/s | Rooting and early establishment |
| Vegetative growth | 300–600 µmol/m²/s | Building a healthy, even canopy |
| Flowering without added CO₂ | 600–900 µmol/m²/s | High-light flowering while watching leaf temperature and stress |
These are starting ranges, not a yield guarantee. Cultivar, temperature, CO₂, watering, nutrition and leaf health change how much light a crop can use.
Step 3: calculate DLI from PPFD and hours
DLI (mol/m²/day) = PPFD × lighting hours × 3,600 ÷ 1,000,000.
For example, 700 µmol/m²/s for 12 hours is about 30.2 mol/m²/day. At 18 hours, the same PPFD is about 45.4 mol/m²/day. This is why intensity and photoperiod must be planned together.
Step 4: use the PPFD map to choose a fixture
A 90 × 90 cm canopy is 0.81 m². At an average target of 700 µmol/m²/s, the canopy receives roughly 567 µmol/s. Fixture PPF will need to be higher because walls, hanging height and distribution create losses. The reliable check is the manufacturer's PPFD grid—or your own measurements—at the full canopy, not only the centre reading.
Why watts per plant is a weak shortcut
Two LEDs with the same wall draw can have different efficacy, spectrum and distribution. Likewise, one trained plant can fill the same canopy as several smaller plants. Use watts only as a power and heat sanity check after you have matched PPFD and coverage.
Hanging height, dimming and uniformity
- Start higher or dimmer for young plants, then adjust from measurements and plant response.
- Do not chase a high centre PPFD while the edges remain underlit.
- Keep the canopy level with training so more leaves sit inside the useful intensity band.
- Watch for upward-curling leaf edges, bleaching, excess heat or stalled growth.
Estimate the electricity cost in South Africa
Daily energy use is actual wall draw in kW × operating hours. Multiply that by your tariff in rand per kWh. Use actual wall draw rather than an “equivalent wattage” name. For a worked example, see the grow-light power-use and monthly-cost guide.
Still choosing a tent? Match the light and ventilation together with our grow tent size guide.
Match the light to the canopy
A complete tent kit reduces sizing mistakes, while a smaller dimmable board suits compact propagation or low-intensity garden setups. Check the current coverage information before buying.
View the 90 × 90 cm grow tent kit View the 120W quantum-board LED
Frequently asked questions
How many watts of grow light do I need per plant?
There is no dependable fixed number because plant count does not define canopy area. Calculate from square metres, target PPFD, photoperiod and the fixture's coverage map.
Can a phone lux app measure PPFD?
It can show relative bright and dark areas, but spectral response and calibration limit accuracy. Use a calibrated quantum sensor when decisions depend on exact PPFD.
Is more light always better?
No. Excess intensity can waste electricity and increase heat or stress when the environment cannot support it.
Cultivate only where lawful and keep any cannabis activity within the private-use rules that apply to you.