Lighting calculators for cannabis grows
PPFD, DLI, wattage sizing, hang distance, light scheduling, and fixture efficiency. The lighting decisions that determine whether your cannabis plants build mass in veg and stack flowers in bloom — or sit at sub-optimal light intensity wondering why yields disappoint.
Cannabis lighting toolkit
Every lighting calculator in the cannabis suite. Green cards are live now; grey cards are in active build. Each tool runs with cannabis stage targets by default — clone, veg, early flower, mid flower, late flower — so the output matches what your specific stage actually needs.
PPFD & DLI Calculator
Convert PPFD intensity to daily light integral, check stage targets, get adjustment recommendations. Replaces older PPFD-only and DLI-only tools — one calculator handles both, with cannabis stage targets and CO₂ adjustments built in.
Fixture Sizing & Placement Tool
Two answers in one calculation. Input your tent size and stage, get the right fixture wattage. Add a fixture you already own and get the optimal hang height. Combines wattage sizing and hang-distance math because they're the same decision asked in two directions.
PAR Map Visualizer
Visualize PPFD distribution across your tent footprint. Surfaces hot spots in the center and cold corners that single-point PPFD readings miss. Critical for ScrOG and SOG layouts where canopy uniformity matters.
Light Schedule Generator
Build a photoperiod schedule for autoflowers (typically 18/6 or 20/4) and photoperiods (18/6 veg, 12/12 flower). Includes ramp-up/down patterns to avoid stress at stage transitions.
Power Load Calculator
Calculate total electrical draw across all fixtures to size circuits, breakers, and surge protection. Critical for multi-tent setups or rooms approaching the 80%-of-circuit threshold.
Light is one piece of the system
Cannabis lighting questions
What's the most important lighting metric for cannabis?
PPFD at canopy. Wattage tells you what your light draws from the wall; PPFD tells you what your plants actually receive. Two fixtures rated the same wattage can deliver wildly different PPFD depending on diode quality, hang height, and reflector design. The PPFD/DLI Calculator is where to start — it tells you whether your current setup is hitting cannabis stage targets, and what to adjust if it's not.
How many watts of LED do I need per square foot for cannabis?
For modern LED measured at the wall: 20–35W per sq ft for veg-only, 30–50W for full-cycle (veg + flower in one tent), 35–50W for flower-only rooms. A 4×4 cannabis flowering tent (16 sq ft) wants roughly 640–800W at the wall to hit 700–1000 PPFD targets. Old "equivalent HPS" wattage marketing on LED boxes is meaningless — measure or estimate at-the-wall draw and use that.
Can I use a phone app to measure PPFD instead of buying a quantum meter?
Not reliably. Phone apps measure lux (visible light weighted toward green, where the human eye sees best) and convert to PPFD using a single fixed factor — usually one calibrated for sunlight. That factor is wrong for LEDs (off by 20-40%) and very wrong for old purple LEDs (off by 50%+). A real quantum meter (Apogee SQ-500 or LI-COR) costs $350–$450 and gives you actual PAR readings. For most home growers, the meter pays for itself the first time it catches an under-spec'd fixture before bleaching damage.
What photoperiod should I run for cannabis?
For photoperiod genetics: 18/6 in veg (some growers run 20/4 for slightly faster veg at higher electricity cost), then trigger flower with a switch to 12/12. The 12/12 trigger is universal — anything significantly different (10/14 or 14/10) confuses the plant. For autoflowers: photoperiod doesn't trigger flower so 18/6 or 20/4 throughout works. Some autoflower growers run 24/0 but the electricity savings of 18/6 usually outweighs the marginal yield gain.
How high should my LED hang above the cannabis canopy?
Depends on fixture wattage and stage. Quick guide for modern LED: seedlings 24–36" from canopy, veg 18–24", early flower 12–18", late flower 12–18". Higher-wattage fixtures (600W+ true draw) need to start at the high end of each range. The PAR Map Visualizer (in development) will help map this; in the meantime, the rule of thumb is: too close = bleached top colas with white/yellow leaves; too far = stretchy growth with weak side branches. Adjust 2–3" at a time and watch for plant response over 48 hours.
Should I add CO₂ supplementation to push higher PPFD?
Only if you're already hitting 800+ PPFD at canopy and your tent is sealed. CO₂ shifts the photosynthesis ceiling — at ambient CO₂ (400 ppm), plants stop benefiting from PPFD past about 1000 µmol/m²/s; at 1200–1500 ppm CO₂, that ceiling shifts to about 1500 PPFD. But CO₂ requires a sealed room (no exhaust during lights-on), tighter temp control (rooms can run 85°F+ with CO₂), and higher VPD targets. For most home growers running standard exhaust ventilation, just hit cannabis stage targets without CO₂ first; CO₂ is a commercial-tier optimization, not a beginner upgrade.