Grow Calculator
Light Coverage: PPFD & DLI
Find out whether your LED actually covers your tent — before you waste a whole run on light-starved larf.
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estimated PPFD (µmol/m²/s)
DLI: 32.7 mol/m²/day
Aim for 25-40 in veg and 35-55 in flower.
Target for flower: 700-1000 µmol/m²/s
Canopy area 16 sq ft · 28 W per sq ft.
Watts are marketing. PPFD is physics.
Fixture listings love "1000W equivalent" claims. What matters is the actual power draw at the wall and the fixture's efficacy in micromoles of usable light per joule of electricity. Budget LEDs land near 2.0 umol/J; good quantum boards hit 2.5-2.8; the best fixtures reach 3.0 or better.
This calculator spreads your fixture's photon output evenly over your canopy footprint to estimate PPFD, then multiplies by your photoperiod to get DLI — the total daily light dose that actually drives yield.
Reading your result
- Well under target: buds form loose and airy, stems stretch, lower growth turns to larf
- In range: the plant converts light into flower efficiently
- Well over target: leaves bleach white at the tops, edges fold up, and you burn power for no extra yield unless you are running CO2
Remember distribution. One fixture in the middle of a 5x5 leaves the corners far below the center reading. Two smaller fixtures usually beat one big one.
Common questions
- How many watts do I need for a 4x4 grow tent?
- For a 4x4 (16 sq ft) canopy, a quality LED drawing 400-500 actual watts hits flowering PPFD of roughly 700-900 umol/m2/s. Ignore inflated 'equivalent' wattage on the box and use the real draw at the wall.
- What PPFD does cannabis need?
- About 150-300 umol/m2/s for seedlings, 400-600 in vegetative growth, and 700-1000 in flower. Going above roughly 1000 only pays off if you are also supplementing CO2.
- What is DLI and why does it matter?
- Daily Light Integral is the total number of photons a plant receives in a day, in mol/m2/day. It combines intensity and photoperiod, so an 18-hour veg day at moderate intensity can deliver the same DLI as a 12-hour flower day at high intensity. Cannabis generally wants 25-40 mol/m2/day in veg and 35-55 in flower.
- Is this as accurate as a PAR meter?
- No. This is a planning estimate based on average LED efficacy of about 2.5 umol/J and even distribution across your canopy. Real fixtures have hotspots and edge falloff. Use it to size a light before you buy, then verify with a quantum meter or a good PAR app.