AC Sizing Calculator

A free, open-source tool to help homeowners estimate the correct air conditioning tonnage for their home. Properly sized AC units run more efficiently, last longer, and keep your home comfortable year-round.

An oversized AC short-cycles (turning on and off too frequently), wastes energy, and fails to dehumidify properly. An undersized unit runs constantly and can't keep up on hot days. This calculator helps you find the sweet spot.

How It Works

The calculator uses Manual J load calculation principles (simplified for homeowners) to estimate cooling capacity in BTU/h and AC tonnage based on:

• Square footage of your home
• Climate zone (1–5, from mild to extreme heat)
• Ceiling height
• Insulation quality (poor, average, good, excellent)
• Number and type of windows
• Sun exposure (shaded, partial, full sun)
• Number of occupants

Quick Start

# Clone the repo
git clone https://github.com/shift-ai007/ac-sizing-calculator.git
cd ac-sizing-calculator

# No dependencies required — pure Python 3.6+
python3 calculator.py

The interactive mode walks you through each input. You can also pass arguments directly:

python3 calculator.py --sqft 2000 --zone 5 --insulation average --windows 12 --sun full

Example Output

╔══════════════════════════════════════════════════════╗
║           AC SIZING ESTIMATE                        ║
╠══════════════════════════════════════════════════════╣
║  Home: 2,000 sq ft | Zone 5 (Hot & Humid)          ║
║  Insulation: Average | Sun: Full Exposure           ║
╠══════════════════════════════════════════════════════╣
║  Base Load:          24,000 BTU/h                   ║
║  Climate Adjustment: +30%                           ║
║  Window Load:        +4,800 BTU/h                   ║
║  Sun Exposure:       +15%                           ║
║  Insulation Factor:  +0%                            ║
║  Occupant Load:      +2,400 BTU/h                   ║
╠══════════════════════════════════════════════════════╣
║  TOTAL COOLING LOAD: 41,880 BTU/h                   ║
║  RECOMMENDED SIZE:   3.5 Tons                       ║
║  ACCEPTABLE RANGE:   3.0 – 4.0 Tons                 ║
╚══════════════════════════════════════════════════════╝

Climate Zones

Zone	Description	Base BTU/sq ft	Examples
1	Mild	10	Pacific Northwest, Northern California
2	Moderate	12	Mid-Atlantic, Tennessee Valley
3	Warm	14	Texas, Georgia, Carolinas
4	Hot	16	Arizona, Nevada, inland California
5	Hot & Humid	18	South Florida, Gulf Coast, Hawaii

South Florida homeowners: You're in Zone 5. Humidity adds significant load — your AC does double duty cooling AND dehumidifying. Proper sizing is critical here.

Why Sizing Matters

Problem	Undersized AC	Oversized AC
Energy bills	Very high (runs 24/7)	High (inefficient cycling)
Comfort	Can't reach set temp on hot days	Uneven temps, hot/cold spots
Humidity	Decent (long run times help)	Poor — short cycles skip dehumidification
Lifespan	Shortened (constant wear)	Shortened (frequent start/stop stress)
Cost	Cheaper upfront, expensive long-term	Expensive upfront AND long-term

Files

├── calculator.py          # Main calculator (interactive + CLI)
├── sizing_engine.py       # Core calculation logic (importable module)
├── climate_zones.py       # Zone definitions and regional data
├── docs/
│   ├── methodology.md     # Calculation methodology explained
│   └── when-to-call-pro.md # When to get a professional Manual J
├── LICENSE
└── README.md

Using as a Python Module

from sizing_engine import calculate_cooling_load

result = calculate_cooling_load(
    square_feet=2000,
    climate_zone=5,
    ceiling_height=9,
    insulation="average",
    num_windows=12,
    window_type="double",
    sun_exposure="full",
    occupants=4
)

print(f"Recommended: {result['recommended_tons']} tons")
print(f"BTU/h: {result['total_btu']}")

Limitations

This calculator provides a rough estimate suitable for planning purposes. A proper Manual J load calculation performed by a licensed HVAC contractor accounts for additional factors:

• Ductwork condition and layout
• Attic and crawlspace conditions
• Kitchen and appliance heat loads
• Local building codes
• Equipment efficiency ratings (SEER2)

For accurate sizing before purchasing a new AC system, always get a professional load calculation.

Need a professional AC sizing assessment in South Florida? AC Repair Today provides free in-home estimates with Manual J calculations — call (305) 850-6810 for same-day appointments.

Contributing

Contributions welcome! Please open an issue or PR for:

• Additional climate zone data
• Regional adjustment factors
• Improved calculation accuracy
• Bug fixes

License

MIT License — see LICENSE for details.

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Built and maintained by AC Repair Today — Licensed Florida HVAC contractor (CAC1824118) serving Miami-Dade, Broward, and Palm Beach counties.