Grow Room Calculator — BTU, AC Size & Monthly Cost

Calculate the exact BTU/hr cooling load, AC unit size, minimum ventilation CFM, air exchange rate, and monthly electricity cost for any grow room or grow tent — covering LED, HID, CMH, and fluorescent lighting, all plant counts, and every climate.

Size my grow room Portable AC calculator

💡 QUICK REFERENCE — Cooling BTU by lighting type

HID / HPS (1,000W): ~3,800 BTU/hr direct heat  ·  LED (600W): ~2,050 BTU/hr  ·  CMH (315W): ~1,075 BTU/hr

Lighting accounts for 70–85% of total grow room cooling load. Use the calculator below for a complete estimate including plant transpiration and equipment. Already know your BTUs? 12K BTU mini split · 12K BTU portable AC · 14K BTU portable AC

Calculator

Enter actual wall-plug wattage for all fixtures combined.

Pumps, fans, CO&sub2; equipment, controllers, etc.

This calculator provides a planning-grade estimate. Actual loads vary with ducting efficiency, reflective surfaces, equipment duty cycles, and local conditions. Always round up to the next available AC unit size.

How grow room cooling load is calculated

Unlike residential HVAC, a grow room’s dominant heat source is artificial lighting, not outdoor temperature. Every watt of electricity consumed by lights becomes heat in the room at a rate of 3.412 BTU/hr per watt. Even “efficient” LEDs convert 100% of their input power to heat — the efficiency gain is that you need fewer watts to achieve the same light intensity, meaning less total heat for the same canopy coverage.

The four components of grow room heat load

Heat Source Typical % of Total Load How It’s Calculated
Lighting (sensible heat) 70–85% Total watts × 3.412 BTU/hr/W × light type factor
Plant transpiration (latent heat) 8–15% Plant count × stage factor × latent BTU/hr per plant
Envelope heat gain 5–15% Room area × insulation/location/climate multiplier
Other equipment 3–8% Pump/fan/controller watts × 3.412 BTU/hr/W

A 15% buffer is applied to the total to account for duct losses, AC cycling inefficiency, and real-world variability.

Light type heat conversion factors

Light Type Sensible Heat Factor Notes
HID / HPS / MH 1.00 — 100% of watts to heat Ballast also generates significant heat; add ballast wattage
LED (quantum board) 0.90 — ~90% of watts to room heat ~10% of LED heat removed via driver/heatsink if remotely mounted
CMH / LEC 0.95 — ~95% of watts to room heat More efficient than HPS but still high heat density
T5 fluorescent / CFL 0.95 — ~95% of watts to room heat Low wattage per fixture; mainly used for seedlings/clones

Ventilation sizing (CFM)

Ventilation serves two purposes in a grow room: (1) removing heat and humidity, and (2) refreshing CO&sub2; for plant respiration. The standard approach is to size the inline fan to exchange the entire room volume at least once per minute during lights-on:

Minimum CFM = Room volume (cu ft) × target air changes per minute

A carbon filter adds 20–25% static pressure resistance — so fans should be rated 25% higher than calculated CFM, or sized using the fan’s rated CFM at 0.25” static pressure rather than free-air rating.

Grow room cooling & ventilation equipment

Mini split systems (recommended for all rooms > 100 sq ft):
9,000 BTU mini split →  ·  12,000 BTU  ·  18,000 BTU  ·  24,000 BTU

Portable AC units (tents & small rooms):
8,000 BTU portable AC →  ·  12,000 BTU  ·  14,000 BTU

Monitoring & control:
Digital hygrometer / thermometer →  ·  Kill-A-Watt energy monitor  ·  Programmable thermostat

Links marked → are affiliate links. We earn a small commission at no extra cost to you.

Grow room AC options compared

AC Type Best Grow Room Size Pros Cons
Mini split (ductless heat pump) Any size — 50 to 5,000+ sq ft Precise temp control; no air exchange (preserves CO&sub2;); can heat during lights-off; very efficient (COP 3–5) Higher upfront cost; requires professional install or DIY kit
Portable AC (single or dual hose) Tents and rooms up to ~200 sq ft No permanent install; works in any room; can be moved between grows Exhausts room air (negative pressure, CO&sub2; loss); dual-hose more efficient than single-hose
Window AC Rooms with window access, up to 300 sq ft Low cost; widely available; simple install Requires window; exhausts room air like portable; not ideal for sealed rooms
In-line duct cooler (chiller) Tents — small spaces only Compact; fits inside tent ducting High cost per BTU; limited capacity; primarily dehumidifies, not cools
Active cooling + water chiller Large commercial rooms Very high capacity; precise zone control High install cost; complex; requires reservoir management

8 tips to reduce grow room heat and electricity cost

💡 Switch to quality LED

A high-efficiency LED (2.5–3.0 µmol/J) can replace a 1,000W HPS with only 600–700W while producing equal or greater yields. This directly cuts heat load by 30–40% and reduces electricity cost by the same margin.

🌙 Run lights at night

Scheduling the lights-on period overnight uses cooler ambient air for ventilation and often benefits from lower electricity rates (time-of-use billing). This is the single easiest and free way to reduce daytime heat load.

🌀 Use a dual-hose portable AC

Single-hose portable ACs pull room air (and CO₂) to exhaust heat — creating negative pressure and requiring make-up air. A dual-hose unit uses outside air for the condenser, maintaining positive or neutral pressure and protecting CO₂ levels.

🔌 Move ballasts and drivers outside

For HID lights, the magnetic or digital ballast can generate significant heat (50–150W each). Moving ballasts outside the grow space removes that heat source entirely. Many LED drivers can also be located outside the tent.

🌡️ Use a VPD chart, not just RH

Vapor pressure deficit (VPD) is the combination of temperature and humidity that drives transpiration. Targeting the correct VPD (0.8–1.2 kPa in flower) reduces over-transpiration, which lowers latent heat load and dehumidifier runtime.

🪟 Insulate exposed walls

Lining walls with 1–2 inch polyiso foam board (R-6 to R-12) dramatically reduces envelope heat gain in attic rooms or exterior spaces. Combined with Mylar or white-painted walls, this also improves light reflection and reduces PPFD hotspots.

💨 Size your inline fan correctly

Oversized fans running at full speed are louder, less efficient, and can over-dry your plants. Use a speed controller and size the fan at 1.5–2× minimum CFM so it runs at 50–70% speed — quieter, longer-lasting, and better for humidity control.

🔋 Monitor your actual power draw

Use a Kill-A-Watt meter on each circuit to verify actual wattage. "600W" LED panels often draw 520–620W; ballasts draw more at startup. Accurate watt readings make this calculator (and your electricity bill) much more predictable.

Frequently asked questions

How many BTU do I need to cool a grow room?

A good starting rule is 3,000–4,000 BTU per 1,000 watts of HID lighting, or 2,000–2,500 BTU per 1,000 watts of LED. Lighting accounts for 70–85% of total cooling load. This calculator also adds plant transpiration, envelope heat gain, and other equipment watts for a complete estimate. Use the calculator above with your exact setup.

How many BTU to cool a 4×4 grow tent?

A 4×4 tent with a 600W LED running 18 hrs/day needs approximately 3,500–5,500 BTU/hr of cooling, depending on climate and insulation. A 5,000–6,000 BTU portable AC or small mini split handles most 4×4 setups. At 1,000W HPS, the same tent needs 6,000–8,000 BTU of cooling.

How many BTU to cool a 10×10 or 10×20 grow room?

A 10×10 room with 2,000W LED typically needs 12,000–18,000 BTU/hr. A 10×20 room with 4,000W HID often needs 24,000–36,000 BTU/hr. These are best served by a 1.5-ton to 3-ton mini split, or multiple portable units. Use the calculator above with your exact wattage and room dimensions for the most accurate figure.

Do I need a dehumidifier in my grow room?

Yes, in most cases. Plants in flower transpire large amounts of water — a 4×4 tent with 4 flowering plants can add 1–2 pints of water per hour to the air. While the AC removes some moisture as part of cooling, a dedicated dehumidifier is often needed in flower to keep RH below 50% and prevent powdery mildew and botrytis. See our dehumidifier calculator for sizing.

What ventilation CFM do I need for a grow room?

The standard rule is to exchange the room’s entire volume at least once per minute: CFM = length × width × height. A 4×4×8 ft tent (128 cu ft) needs at least 128 CFM. Add 25% for carbon filter resistance and buy a fan rated at 160+ CFM. For hot climates or high-wattage setups, target 2 air changes per minute.

Can I use a regular window AC in a grow room?

Yes, but with caveats. A window AC exhausts room air to cool the condenser, creating negative pressure and pulling unfiltered outside air in through gaps. This can introduce pests, pathogens, and temperature-inconsistent air. If you use a window AC, seal all other openings and use a sealed intake with a HEPA filter. For sealed-room CO&sub2; enrichment, a window AC is incompatible — use a mini split instead.

How much does it cost to run a 1,000W HPS grow room per month?

At 16¢/kWh: 1,000W lights × 12 hrs/day × 30 days = 360 kWh = ~$58/month in lighting alone. Add AC (typically 400–600W equivalent draw), fans, pumps, and other equipment and total monthly cost is typically $100–$180/month for a 1,000W HPS setup. Use the calculator above with your actual wattage for a personalized estimate.

← All BTU calculators · Browse equipment by BTU range