Cannabis Tool

Fixture Sizing & Placement

How many watts does your tent need, and how far should you hang the light? Two answers from one input set: total wattage sized to tent + stage + light type + growing style, plus a full hang-height schedule from seedling through flower. No more "just hang it 18 inches" rules of thumb that ignore wattage.

Plan your light setup in Grōbot →
Calculator

Size and place your fixture

Pro adjustments

Pro mode notes

Budget LEDs (the $180 blurple/white panels) need 25% more wattage than modern Samsung-diode LEDs to hit equivalent PPFD. If you own a specific fixture already, enter your watts and get just the hang-height schedule. Canopy height lets the tool flag clearance issues before you buy.

About your light type

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Pick a light type to see guidance.

Common light sizing & placement mistakes

  • Measuring advertised watts, not wall watts. A "1000W LED" that draws 240W at the wall is a 240W fixture for our purposes. The advertised number is marketing — use the real draw on the power label or spec sheet.
  • Hanging too close in flower. Bleaching happens slowly — a day or two of overexposure yellows the top cola irreversibly. Err 2" higher than the recommendation; you can always lower later.
  • Forgetting to dim in veg. A fixture sized for flower hung at flower height will bleach young plants. Use the veg and seedling hang heights in the schedule — or dim the driver to 40–60% during veg.
  • Ignoring light footprint vs tent footprint. An oversized fixture in a small tent dumps too much light on the canopy edges; undersized means starved corners. Match the fixture's rated coverage to your tent area, not just the wattage.

Recommended wattage

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W / sq ft: —

Your fixture spec

⭐ Fixture class

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nominal watts

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Flower hang height

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inches from canopy

operating spec

Hang height by stage

Seedling

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inches

Veg

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inches

Flower ⭐

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inches

Coverage match

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Alternative fixture classes

Detailed breakdown

Tent area —
Wattage range —
Watts per sq ft —
Primary fixture class —
Seedling hang —
Veg hang —
Flower hang —

Wattage note

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Hang height note

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How to use

From tent to light spec in 4 steps

1

Pick tent size and stage focus

Tent area drives wattage target. Stage focus changes the watts-per-sq-ft multiplier: veg-only grows (mothers, clones, short cycles) need less intensity, full-cycle is the default for most home grows, flower-only rooms push maximum PPFD for yield. Most home growers pick full-cycle because the same tent runs veg and flower on a rotation.

2

Pick light type

LED is the modern default — best watts-to-PPFD ratio, runs cool, 5+ year fixture life. HPS is traditional flower lighting — cheaper per watt upfront but hot, wider spectrum, bulbs replaced yearly. CMH splits the difference — ceramic metal halide runs cooler than HPS with better spectrum. The tool adjusts wattage recommendation: HPS needs ~30% more watts than LED, CMH needs ~15% more.

3

Pick growing style

Personal / hobby targets gentler intensity — lower electricity cost, reduced nutrient burn risk, yields 15–20% below commercial. Standard home grow is the balanced middle — good yield without requiring CO₂ supplementation. Commercial pushes maximum photons at the canopy and assumes you're running CO₂ at 1200+ ppm — without CO₂, plants can't use more than ~45 W/sq ft anyway, so the extra wattage just becomes heat.

4

Read the spec, shop, hang

Output is three things: total wattage (plus min/max range), a fixture class recommendation (e.g., "600W LED class") with 1–2 alternatives, and a hang-height schedule from seedling through flower. Wattage is for shopping; the schedule is for operating. Set your initial hang height for the stage you're starting at, then raise or lower as plants transition. Most light hangers use rope ratchets for 1-inch adjustments — keep your hang-height notes handy during transitions.

Why the hang height changes across the cycle

Plants tolerate different light intensities at different stages. Seedlings are fragile — high PPFD stresses them, causes pale foliage and stunted growth. Hang the light 8" higher than flower height. Veg tolerates more intensity but still benefits from a gentler ramp — 2" higher than flower. Flower is the maximum-intensity stage; this is your operating hang height. The schedule assumes the fixture is sized correctly — if you're running a way-oversized fixture, hang higher or dim the driver. The tool's coverage analysis (green/orange badge near the cards) flags if your fixture and tent don't match.

Advertised watts vs wall watts

Nearly every LED marketed on Amazon lists a misleading "equivalent" wattage — "1000W LED" that actually draws 150W at the wall is typical. Our calculator always means wall watts — the real power draw from the outlet, shown on the power adapter label or the spec sheet. If you're shopping a fixture and the listing says "1000W" without a wall-watt number, treat it as suspicious and check the spec sheet. Look for "actual power draw" or "true wattage" or the efficacy rating (µmol/J). A 2.7 µmol/J fixture at 240W wall draws actually outperforms most "1000W equivalent" budget panels.

The science

Photons, heat, and canopy geometry

Indoor cannabis yield is driven by photons hitting the canopy, not watts consumed. Wattage is a proxy we use because it's what fixtures advertise and what electricity bills charge for — but the real thing plants care about is PPFD (photons per second per square meter). The fixture that delivers the most PPFD per watt wins. The Grow Lights guide in the Learning Center covers lighting fundamentals; this section covers why specific wattage and hang-height numbers produce better grows.

2.5+

µmol/J — modern LED target

Photon efficacy. Modern Samsung/Osram LEDs hit 2.5–2.9 µmol/J. Budget panels run 1.8–2.0. HPS tops out around 1.7.

~900

PPFD target — flower

Photosynthetic photon flux density at canopy. Below 600 you lose yield; above 1100 you bleach without CO₂ supplementation.

3.412

BTU/hr per watt

Thermodynamic constant. Every watt consumed by a fixture becomes heat. A 600W fixture produces 2,047 BTU/hr — climate equipment sizing follows.

Why wattage isn't the same as light output

A fixture's wattage tells you how much electricity it consumes, not how much light it produces. The critical metric is µmol/J (micromoles of PAR photons per joule) — how efficiently the fixture converts electrical energy into usable photons. A 240W fixture at 2.7 µmol/J produces more canopy PPFD than a 400W fixture at 1.5 µmol/J, despite drawing 40% less power. This is why modern high-efficiency LEDs have displaced HPS for serious growers: better light per watt, lower heat load, cheaper electricity per harvest.

Our wattage recommendations assume modern LED efficacy (2.5+ µmol/J). The budget-LED adjustment in Pro mode bumps wattage by 25% to compensate for sub-2.0 µmol/J fixtures hitting the same canopy PPFD — those $180 "1000W LED" panels are producing about 750 µmol/s at the canopy, same as a 300W high-efficiency fixture, just with more electricity cost and heat.

PPFD curves by stage

Stage Target PPFD DLI (mol/m²/day) Photoperiod (hr)
Seedling / clone 100–300 10–18 18–24
Early veg 300–500 22–32 18
Late veg 500–700 28–40 18
Early flower 700–900 30–40 12
Mid flower 800–1000 35–43 12
Late flower 700–900 30–40 12

PPFD above ~1100 without CO₂ supplementation causes light-saturation: plants hit a photosynthetic ceiling where extra photons become heat stress instead of sugar production. Top leaves bleach white-yellow (photoinhibition), edges curl, and terpene production drops. This is why commercial cannabis rooms run CO₂ at 1200+ ppm — it raises the light-saturation ceiling, letting plants use 1200–1500 PPFD productively.

Why HPS hangs higher than LED at same wattage

Two reasons. First, radiant heat. HPS emits ~30% of its energy as infrared radiation directly from the bulb surface — this is absorbed by whatever's underneath it. Canopies directly under HPS heat up 5–10°F warmer than ambient even with proper exhaust. Hanging higher spreads that radiant heat over more area, preventing local scorching. Second, spot intensity. HPS produces a concentrated beam directly under the bulb; LED distributes more evenly across a wider area. Hanging HPS closer concentrates PPFD too aggressively in a hot spot under the bulb while leaving the corners light-starved.

The rule: HPS at the same wattage as LED hangs 6–10" higher. A 400W HPS flower hangs at ~26"; a 400W LED flower hangs at ~22–24". The tool's curves encode this — same wattage input returns different hang heights for each light type.

Fixture coverage: why tent-to-fixture match matters

Matched

Fixture sized for tent

Fixture's rated coverage area overlaps your tent area. Even PPFD across the canopy, expected yield, no hot spots.

Oversized

Fixture too big for tent

Fixture rated for more sq ft than you have. Wastes photons spilling past tent walls; risks bleaching unless you dim the driver to 60–70%.

Undersized

Fixture too small for tent

Fixture rated for less sq ft than your tent. Corners and edges starve for light. Yield drops 20–30%; consider two fixtures or stepping up.

Wattage density vs growing style

The calculator's growing-style multiplier (0.85× / 1.0× / 1.15×) reflects how hard growers push their canopies. A personal/hobby grow at 30 W/sq ft produces good yield with forgiving conditions — recovers easily from minor mistakes, tolerates cheaper nutrients, doesn't demand perfect climate. A commercial-style grow at 50+ W/sq ft demands high CO₂, aggressive feeding, tight VPD control, and harvest-quality genetics to see yield returns. Going from standard to commercial wattage typically adds 15–25% yield but doubles the complexity of environmental control. Most home growers land at standard and are happier for it.

Where the numbers come from

Wattage per sq ft values synthesized from lighting manufacturer (Fluence, Gavita, AC Infinity, Spider Farmer, Mars Hydro) spec sheets and PPFD maps, cross-referenced with r/microgrowery documented grows and industry resources like Bruce Bugbee's horticultural lighting research. Light-type multipliers (LED baseline, HPS 1.3×, CMH 1.15×) reflect the efficacy gap in µmol/J — modern LEDs outperform HPS by ~60% in photon delivery per watt. Hang height curves blend fixture manufacturer recommendations with community post-harvest analysis. PPFD targets per stage are well-established in cannabis horticulture research and consistent across multiple independent sources.

FAQ

Common lighting questions

Should I buy LED, HPS, or CMH for a home grow?

LED for almost every home grower. Modern LEDs (2.5+ µmol/J, Samsung or Osram diodes) produce more canopy PPFD per watt than HPS or CMH, run dramatically cooler (less climate equipment needed), last 5+ years without replacement, and have dimmable drivers for easy intensity control. HPS is still viable if your ambient climate is cold enough to need the extra heat (garage grows in winter) or if you're buying used commercial gear — a $200 used 1000W HPS kit can out-yield a $400 budget LED in a 4×4. CMH is niche — great spectrum but limited wattage options (315W and 630W) and the glass bulbs are fragile. For first grows, pick a 240–320W LED in the $250–400 range.

Why is advertised wattage different from wall wattage?

Pure marketing. Amazon is full of "1000W LED" panels that draw 150–240W at the wall. The "1000W" is supposed to mean "equivalent to a 1000W HPS" but it's an apples-to-oranges comparison — a 240W modern LED actually does equal a 1000W HPS in canopy PPFD, but only because modern LEDs are 60% more efficient. The misleading part is when a budget 1000W LED panel claims the same equivalence despite having 1.8 µmol/J efficacy — it's producing less than half the PPFD of a real 1000W HPS. Check the power label on the adapter or the spec sheet for the actual "wall draw" or "input power." That number is what our calculator means by wattage.

How do I know if my light is too close to the plants?

Three signs. Upper leaf yellowing without matching lower-leaf symptoms is light bleach — the top of the canopy is photosaturated while the rest is fine. Leaves cupping upward (tacoing) means heat stress; leaves trying to minimize surface area facing the light. Pale green or white spots on the highest cola is advanced bleaching — already yield-damaging at that point. Easy preventive check: put your hand at canopy height, palm up, for 60 seconds. If it's uncomfortably warm, the light is too close. With LED you should barely feel heat; with HPS you'll feel warmth but it shouldn't be painful. Raise the light 2" and recheck after a day.

Can I use one light for both veg and flower?

Yes, and it's the default for most home grows — "full-cycle" fixtures (the full-cycle setting in this calculator) sized for flower intensity work fine for veg if you either hang higher or dim the driver. During veg, hang 6–8" above the flower position OR dim to 50–60% output if the driver supports it. The schedule in the calculator output gives you the exact hang heights. Dedicated veg/flower rooms only make sense for perpetual harvests where plants rotate weekly — the efficiency gain from specializing each room doesn't matter for a single-tent home grow running 1–2 cycles per year.

Should I buy the biggest light I can afford?

No. Oversized fixtures are a common mistake. A 600W LED in a 4×4 with no CO₂ supplementation will bleach your top colas — plants can't use PPFD above ~1100 without CO₂, and more watts just create more heat. The calculator sizes to your actual tent and growing style so you're not wasting money on watts you can't use productively. If your budget exceeds the recommendation, invest in: better meters (Bluelab over Apera), better climate control (real exhaust fan, Cloudline-tier), or better genetics (premium seed banks) instead of oversized lighting.

How often do I need to adjust hang height?

Three main adjustments per cycle: seedling → veg transition (around week 2 from seed), veg → stretch/early flower (first week of 12/12), and stretch → mid flower (around week 3 of flower when plants stop stretching). Between those transitions, you'll want minor 1–2" adjustments as plants grow — a weekly check is usually sufficient. Use rope ratchets for easy height changes (about $10 per pair). If your plants are growing fast and you're approaching the minimum safe distance, a daily check for the last week before a transition is wise. The calculator's schedule gives target heights; the weekly check confirms you're actually at them.

What's a good µmol/J number to target when shopping LEDs?

2.5 µmol/J is the "good modern LED" threshold. 2.7+ is excellent (Samsung LM301H diodes, Gavita CT series). 2.9+ is commercial/top-shelf (Fluence SPYDR, high-end Gavitas). Below 2.0 µmol/J is budget — works but requires 25% more watts for the same canopy PPFD. Most manufacturers list this number in their spec sheets under "efficacy" or "PPF per watt." If a fixture doesn't list it, assume it's under 2.0. For a first grow, 2.3–2.5 µmol/J fixtures from AC Infinity, Spider Farmer, or Mars Hydro in the $250–400 range are the sweet spot — good enough performance at reasonable prices.

Does hang height affect light footprint?

Yes, significantly. Hanging a light closer concentrates photons in a smaller hot spot directly underneath; hanging it further spreads photons over a wider area with lower peak intensity. For a 4×4 tent with a 4-bar LED, hanging at 18" gives intense PPFD in a 3×3 central area and dim corners; hanging at 28" gives more uniform 4×4 coverage at lower peak intensity. The trick is matching hang height to your training method: SCROG and manifold growers want even 4×4 coverage at 26–30" for their flat canopies; SOG growers want concentrated intensity and hang shorter at 20–22" to maximize peak PPFD on the dense canopy top. A PAR meter ($100–200) is the tool that actually shows you the footprint at different hang heights; without one, trust the calculator's schedule.