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Grow room parameters

Exhaust, carbon filter, ducting, lamp power and plant count.

Units

Inputs

Room dimensions

cm
cm
cm

Ventilation and ducting

60/h = one full change per minute (standard)

m

Lighting

µmol/m²/s

Plants

Results

Results: Grow area: 1.44 m² Room volume: 2.88 m³ Base airflow: 173 m³/h Minimum exhaust capacity: 323 m³/h Suggested duct diameter: 160 mm Passive intake (min. area): 603 cm² Required PPF output: 1,296 µmol/s Suggested LED power: 518 W Suggested HPS/CMH power: 762 W Power per m²: 360 W/m² Photoperiod plant count: 4 Autoflower plant count: 7

Grow area

1.44m²

Room volume

2.88m³

Ventilation and ducting

Base airflow

60 × 2.88 m³

173m³/h

Minimum exhaust capacity

323m³/h

Suggested duct diameter

160mm

Passive intake (min. area)

603cm²

Installation resistance derates

Carbon filter +25%
Ducting +15%
Bends +30%
Combined derate +87%

Lighting

Required PPF output

1,296µmol/s

Suggested LED power

518W

Suggested HPS/CMH power

762W

Power per m²

360W/m²

Plants

Photoperiod plant count

4

Autoflower plant count

7

The grow room calculator turns three tent dimensions into the numbers you actually buy hardware with: extraction capacity in m³/h, duct diameter, passive intake area, lamp wattage and a realistic plant count.

Formula — Grow room parameters

Extraction starts from room volume and the number of air changes per hour (ACH). 60 ACH means the whole volume is replaced once every minute, which is the standard target for a carbon-filtered cannabis tent. Every restriction in the run is then applied as a multiplier, because a fan curve loses flow to static pressure.

  • volume [m³] = width × depth × height
  • base flow [m³/h] = volume × ACH
  • K = K_filter × K_duct × K_bends × K_silencer
  • K_filter = 1.25 · K_duct = 1 + (metres × loss/m) · K_bend90 = 1 + 0.15 per elbow
  • required fan capacity = base flow × K
  • PPF [µmol/s] = target PPFD × area · watts = PPF ÷ efficacy [µmol/J]
  • Duct loss per metre: flexible 5%, semi-rigid 3.5%, rigid spiral 2%.
  • A 45° bend costs about 7%, a silencer about 10%.
  • Efficacy defaults: LED 2.5 µmol/J, CMH 1.9, HPS DE 1.7, HPS SE 1.45, MH 1.35.
  • Duct diameter is the smallest catalogue size that keeps air velocity at or below 5 m/s; passive intake is sized at three times the duct cross-section.

Worked example — Grow room parameters

A 120 × 120 × 200 cm tent has an area of 1.44 m² and a volume of 2.88 m³. At 60 ACH the base flow is 2.88 × 60 = 173 m³/h. Add a carbon filter (×1.25), 3 m of flexible ducting (1 + 3 × 0.05 = ×1.15) and two 90° elbows (1 + 2 × 0.15 = ×1.30): the combined derate is 1.25 × 1.15 × 1.30 ≈ 1.87. The fan therefore has to be rated for roughly 173 × 1.87 ≈ 323 m³/h, so a 125 mm fan quoted at 350–400 m³/h is the honest choice — a 180 m³/h unit would be strangled by the same installation.

For light, a flowering target of 900 PPFD over 1.44 m² needs 900 × 1.44 = 1296 µmol/s of PPF. At LED efficacy of 2.5 µmol/J that is about 518 W (360 W/m²); a double-ended HPS at 1.7 µmol/J would need about 762 W for the same canopy intensity, which is why the HID column always looks hotter and more expensive.

Typical ranges — Grow room parameters

Treat the wattage results as a photon budget, not a hard limit: a fixture is only as good as its efficacy figure, and stated “equivalent to 1000 W” marketing numbers should be ignored in favour of measured µmol/J.

ParameterUsual range
Air changes30–60/h without CO₂ dosing, 20–30/h with a sealed room and AC
Combined installation derate1.4–2.2× for a filtered tent with flex ducting
Duct diameter100–125 mm up to ~1.5 m², 150–200 mm for 2–4 m²
LED power density250–350 W/m² in flower, 120–180 W/m² in veg
Canopy PPFD200 seedling · 400–600 veg · 700–1000 flower (up to 1400 with CO₂)
Plant spacing0.16 m² SOG · 0.25 m² standard · 0.5 m² SCROG · 1 m² large plants