# 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.

- Canonical: https://genetixbay.com/knowledge/equipment/grow-room-calculations.html
- Category: Equipment · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review
- Stage: Any stage · Systems: Any system
- Updated: 2026-11-01 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How do I plan a grow tent setup?** Plan a grow tent in this order. First the light: target [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) × [canopy area](https://genetixbay.com/knowledge/glossary/canopy.md) ÷ fixture [µmol/J](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) gives wall watts. Then the heat: every watt is 3.412 BTU/h. Then the water: plants [transpire](https://genetixbay.com/knowledge/glossary/transpiration.md) about what you water, which sets the [dehumidifier](https://genetixbay.com/knowledge/glossary/dehumidifier.md). 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](https://genetixbay.com/knowledge/glossary/led.md) (the house recommendation is about 50 W/ft² you can dim), an [exhaust fan](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) delivering about 190–280 [CFM](https://genetixbay.com/knowledge/glossary/cfm.md) through a matching [carbon filter](https://genetixbay.com/knowledge/glossary/carbon-filter.md), 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.

**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](https://genetixbay.com/knowledge/tools/), including a [whole-tent planner](https://genetixbay.com/knowledge/tools/#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](https://genetixbay.com/knowledge/equipment/calc-lighting-sizing.md)
2. **Heat.** Every watt of every device = 3.412 BTU/h [1][2]. → [AC and cooling sizing](https://genetixbay.com/knowledge/equipment/calc-cooling-ac-sizing.md)
3. **Water.** Plants transpire about what you water, minus [runoff](https://genetixbay.com/knowledge/glossary/runoff.md). → [Dehumidifier sizing](https://genetixbay.com/knowledge/equipment/calc-dehumidifier-sizing.md)
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](https://genetixbay.com/knowledge/equipment/calc-ventilation-and-carbon-filters.md)
5. **CO₂.** Only for sealed, air-conditioned rooms with high light, and only with a monitor. → [CO₂ calculations and safety](https://genetixbay.com/knowledge/environment/calc-co2-enrichment.md)
6. **Power.** Amps = watts ÷ volts; keep continuous loads to 80% of the breaker [3]. → [Electrical load](https://genetixbay.com/knowledge/equipment/calc-electrical-load.md)

7. **[Feed and water](https://genetixbay.com/knowledge/glossary/fertigation.md).** Mix to an [EC](https://genetixbay.com/knowledge/glossary/ec.md) target, dilute with C₁V₁ = C₂V₂, size the reservoir for 3–5 days of water use. → [Nutrient dilution and reservoir math](https://genetixbay.com/knowledge/ph-ec/calc-nutrient-dilution-reservoir.md) (and [EC vs PPM](https://genetixbay.com/knowledge/ph-ec/ec-ppm-scales.md) for the 500 and 700 scales)

**Size a grow room in this order.** Light: PPFD × area ÷ 0.85 ÷ µmol/J → Heat: Every watt × 3.412 BTU/h → Water: About what you water → Air and cooling: CFM or AC → CO₂: Sealed rooms only → Power: 80% 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](https://genetixbay.com/knowledge/glossary/12-12.md), 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](https://genetixbay.com/knowledge/glossary/fan-leaf.md), 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](https://genetixbay.com/knowledge/tools/#planner) is for.

## Sources

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