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Equipment · Beginner · 3 min read

Grow-room calculations: size lights, cooling, dehumidifier, CO₂, fans and power properly

Size a grow in this order: light first (PPFD target × area ÷ efficacy), then the heat it makes (every watt is 3.412 BTU/h), the water the plants transpire (about what you water), the airflow or AC to remove both, CO₂ only if the room is sealed and bright, and finally the circuits to power it all at 80% of the breaker. Each step has a guide with formulas in metric and imperial and worked examples for 2×4, 4×4, 5×5 and 4×8 ft tents and a 10×10 ft room.

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Short answer, in plain words. Every part of a grow room depends on the part before it. Light decides the heat. Heat and water decide the cooling, the drying and the airflow. All of it decides the wiring. So work it out in order, write the numbers down, and measure once it's running. Each guide below gives the formulas in metric and imperial and worked examples for 2 × 4, 4 × 4, 5 × 5 and 4 × 8 ft tents and a 10 × 10 ft room. Supported.

Interactive versions: grow-room calculators, including a whole-tent planner.

The order to work in

  1. Light. PPFD target × canopy area ÷ 0.85 ÷ fixture µmol/J = wall watts. House recommendation: about 50 W/ft² of dimmable LED. → Lighting sizing
  2. Heat. Every watt of every device = 3.412 BTU/h [1][2]. → AC and cooling sizing
  3. Water. Plants transpire about what you water, minus runoff. → Dehumidifier sizing
  4. Air. Exhaust CFM for heat or one air change a minute, whichever is bigger; passive intakes 2–3× the duct area; a carbon filter rated for the real airflow. → Ventilation and carbon filters
  5. CO₂. Only for sealed, air-conditioned rooms with high light, and only with a monitor. → CO₂ calculations and safety

  6. Power. Amps = watts ÷ volts; keep continuous loads to 80% of the breaker [3]. → Electrical load

  7. Feed and water. Mix to an EC target, dilute with C₁V₁ = C₂V₂, size the reservoir for 3–5 days of water use. → Nutrient dilution and reservoir math (and EC vs PPM for the 500 and 700 scales)

Grow-room calculationsSize a grow room in this order
  1. LightPPFD × area ÷ 0.85 ÷ µmol/J
  2. HeatEvery watt × 3.412 BTU/h
  3. WaterAbout what you water
  4. Air and coolingCFM or AC
  5. CO₂Sealed rooms only
  6. Power80% of the breaker

Source: Guides linked above [1][2][3].

The five spaces at a glance

Flower, about 900 µmol/m²/s with a top-tier 3.0 µmol/J LED, 12/12, water use 4 L/m²/day; sealed-room CO₂ at 1,200 ppm. Rounded; every guide shows its assumptions.

2 × 4 tent 4 × 4 tent 5 × 5 tent 4 × 8 tent 10 × 10 room
Canopy 8 ft² (0.74 m²) 16 ft² (1.49 m²) 25 ft² (2.32 m²) 32 ft² (2.97 m²) 100 ft² (9.29 m²)
Volume 48 ft³ 104 ft³ 163 ft³ 208 ft³ 800 ft³
LED at 900 µmol/m²/s ~260 W ~525 W ~820 W ~1,050 W ~3,280 W
House rec. 50 W/ft² (dimmable) 400 W 800 W 1,250 W 1,600 W 5,000 W
Heat added (lights, fans, dehumidifier) ~1,310 BTU/h ~2,610 BTU/h ~4,000 BTU/h ~5,090 BTU/h ~15,650 BTU/h (1.3 tons)
Water to remove ~3 L (6 pt)/day ~6 L (13 pt)/day ~9 L (20 pt)/day ~12 L (25 pt)/day ~37 L (79 pt)/day
Exhaust CFM (heat, 10 °F rise) ~95 ~190 ~290 ~370 ~1,130 (usually sealed + AC instead)
Passive intake (2–3× duct) 4 in duct: 25–38 in² 6 in duct: 57–85 in² 6 in duct: 57–85 in² 6–8 in duct: 57–150 in² AC room: per design
CO₂ per 12 h day, sealed ~0.09 lb ~0.19 lb ~0.29 lb ~0.37 lb ~1.2 lb
Amps at 120 V (lights, fans, dehumidifier) ~5 A ~8 A ~11 A ~13 A ~36 A (or ~18 A at 240 V)

Calculators vs articles

The articles explain why and show worked examples. A calculator is better when you're plugging in your own tent size, fixture efficacy, temperatures and water logs, or when one change should ripple through every other number (bigger lights → more heat → bigger fan, AC and circuits). That's what the whole-tent planner is for.

Questions this article answers

How do I plan a grow tent setup?

Plan a grow tent in this order. First the light: target PPFD × canopy area ÷ fixture µmol/J gives wall watts. Then the heat: every watt is 3.412 BTU/h. Then the water: plants transpire about what you water, which sets the dehumidifier. Then the airflow or AC to remove heat and moisture, CO₂ only for sealed, bright rooms, and last the electrical circuits at 80% of the breaker.

What equipment do I need for a 4x4 grow tent?

For a 4×4 ft tent in flower, plan about 525–800 W of good LED (the house recommendation is about 50 W/ft² you can dim), an exhaust fan delivering about 190–280 CFM through a matching carbon filter, passive intakes about 2–3 times the duct area, a dehumidifier rated about 12–24 pints a day, and one 15 A GFCI-protected circuit. The guides show the math for each.

Sources

  1. Thompson A, Taylor BN. 2008. NIST Special Publication 811, Appendix B (1 W = 3.412 Btu/h). nist.gov/pml/special-publication-811
  2. ASHRAE. 2021. ASHRAE Handbook: Fundamentals (load calculations, psychrometrics, duct design). ashrae.org/technical-resources/ashrae-handbook
  3. NFPA. 2023. NFPA 70, National Electrical Code (continuous loads at 80%). nfpa.org/codes-and-standards/nfpa-70-standard-development/70

Related

pH and ECSupportedIntermediate · Draft

Calculate it properly: nutrient dilution, EC targets and reservoir size

Mix to an EC target, not a bottle dose: measure your water's EC, add nutrient in steps, and read the EC each time. Dilution follows C1 × V1 = C2 × V2, so to bring a 100 L reservoir from EC 2.4 down to 1.8 with EC 0.2 water you add about 38 L of water. Size a reservoir to hold 3–5 days of the plants' water use (about 30 L for a 4×4 ft tent in flower) and change it at least when you've topped up about one reservoir's worth.

SupportedIntermediate · Draft

Calculate it properly: how much grow light your tent or room needs

Size grow lights from the light the plants need, not from watts. Pick a PPFD target (about 900 µmol/m²/s for flower at ambient CO₂), multiply by canopy area, divide by about 0.85 for photons that miss the canopy, then divide by the fixture's µmol/J. A 4×4 ft tent needs about 525 W of a 3.0 µmol/J LED, 605 W at 2.6, 685 W at 2.3 or about 925 W of double-ended HPS. Then hang it to get even light, checked with a PAR meter.

SupportedIntermediate · Draft

Calculate it properly: AC and cooling load for a grow tent or room

Your cooling load is every watt you plug in, times 3.412 BTU/h, plus heat through walls and from people. Lights, drivers, fans, pumps and the dehumidifier all count. Plants don't add heat; they turn some of it into humidity, and a dehumidifier turns it back into heat. A sealed 10×10 ft room with about 3,300 W of LED needs roughly 16,000 BTU/h (1.3 tons) of cooling with dehumidification, before a 10–20% margin. Prefer a mini-split to a single-hose portable unit.

SupportedIntermediate · Draft

Calculate it properly: dehumidifier size for a grow tent or room

Plants transpire nearly all the water you give them, so a dehumidifier must remove roughly what you water each day, minus runoff. Measure it in litres or US gallons, convert to pints (1 L = 2.11 pints), and add 25% for lights-off peaks. A 4×4 ft tent in flower using 4.5–9 L a day needs about 12–24 pints a day of rated capacity. Since 2019, US labels are tested at 65 °F instead of 80 °F, so the same machine now shows a smaller number.

EnvironmentSupportedAdvanced · Draft

Calculate it properly: CO₂ for a grow tent or room (and staying safe)

Raising a sealed 10×10×8 ft room from about 420 to 1,200 ppm takes only about 33 g (0.07 lb) of CO₂; holding it is the real cost, about 1.2 lb a day for the plants plus leaks. With an exhaust fan doing one air change a minute, the same room would throw away about 50 lb a day, so CO₂ only pays in a sealed room with high light (about 900+ µmol/m²/s). Burners add heat and water; tanks don't. CO₂ is dangerous: workplace limits are 5,000 ppm over 8 hours and 40,000 ppm is immediately dangerous to life.

SupportedIntermediate · Draft

Calculate it properly: exhaust fan CFM, intake size and carbon filters

Size an exhaust fan for the bigger of two numbers: one air change a minute (CFM = tent volume in ft³) or the heat it must carry out (CFM = BTU/h ÷ (1.08 × temperature rise in °F)). For a 4×4×6.5 ft tent that's 104 CFM for air changes but about 190 CFM to carry 600 W of heat out at a 10 °F rise. Then allow for the carbon filter, ducting and bends using the fan's curve. Use passive intakes about 2–3 times the exhaust duct area for slight negative pressure.

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