Choosing the right AC size is about more than matching square footage. Undersized equipment runs nonstop and still misses your setpoint on hot days. Oversized systems short-cycle, leave rooms clammy, waste energy, and wear out components. The correct size is the result of a professional load calculation that considers your home’s unique heat gains and losses—not just a rule of thumb.

BTU, tonnage, and what they really mean

AC capacity is measured in BTU per hour or in tons. One ton equals 12,000 BTU/h. A typical U.S. home might need anywhere from 1.5 to 5 tons, but the exact requirement depends on insulation, windows, air leakage, duct design, occupancy, appliances, and local climate. Your comfort also depends on humidity control and airflow, not only raw BTUs.

Rules of thumb vs. Manual J

You may hear rules like “500–600 sq ft per ton.” Those shortcuts ignore critical variables. The industry standard is ACCA Manual J, a room-by-room heat gain/loss calculation that accounts for orientation, shading, window U-factor/SHGC, ceiling height, infiltration, duct location, and more. Manual S then matches equipment to the load, and Manual D ensures ducts can deliver the required airflow. Skipping these steps risks poor comfort, higher bills, noise, and premature failures.

Southern California factors that change the load

Even within one region, microclimates matter. Coastal zones get marine layer, inland valleys see higher peaks, and foothill homes face larger nighttime swings. West-facing glass can add thousands of BTU/h in the afternoon. Attic insulation, radiant barriers, cool roofs, and low-e windows can reduce tonnage needs, sometimes dramatically. Older LA bungalows with leaky ducts in hot attics often need more capacity than a newer, tight home of the same size in Irvine or Thousand Oaks.

A careful, quick estimation (with caution)

1. Measure conditioned square footage and ceiling heights (volume matters).
2. Note window area, direction, shading, and glass specs (U-factor, SHGC if known).
3. Check insulation levels (attic R-value, wall construction) and air sealing.
4. Count occupants (about 230–300 BTU/h per person as a starting point).
5. List internal gains (lighting, cooking, home office equipment).
6. Consider climate zone and design temps for your location.
7. Assess ducts (location, leakage, and static pressure potential).

For a ballpark only: a tight, well-shaded 2,000 sq ft SoCal home might land near 2.5–3 tons; the same home with poor insulation, large west glass, and attic ducts might need 3.5–4 tons. These are not design numbers—perform a Manual J for accuracy.

Ducts, airflow, and efficiency

Capacity is useless without airflow. Most systems need about 350–400 CFM per ton. Undersized or leaky ducts, high static pressure, and clogged filters slash delivered BTUs and can cause coil icing. Right-sized equipment paired with balanced, well-sealed ducts and a proper return path usually outperforms oversized systems with poor ductwork. Variable-speed blowers and two-stage or inverter condensers can widen comfort and humidity control—even in dry climates.

Ductless and multi-zone sizing

Mini-split and multi-zone systems are sized per room or zone. Diversity (not all rooms peaking at once) allows careful downsizing of the outdoor unit, but each head still needs accurate room loads. Line length, elevation, and coil sizing affect performance. Our engineers maintain factory training from Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China), with deep experience on complex residential and commercial layouts.

Insurance, permits, and safety

Many jurisdictions and insurers expect permitted installations and qualified sizing documents. Refrigerant handling, electrical work, and structural changes (e.g., new roof penetrations) have safety and code implications. Improper sizing or installation can void warranties and may create liability issues. When in doubt, get a documented Manual J, S, and D along with commissioning data.

Examples for common home types

• Classic 1,200 sq ft bungalow with original single-pane windows and attic ducts: often 2–2.5 tons after duct sealing/insulation upgrades; more if windows and envelope remain leaky.
• Newer 2,400 sq ft two-story with good insulation but big west-facing glass: roughly 3–4 tons depending on shading, glazing, and duct design.
• Coastal 1,800 sq ft home with frequent marine layer: latent control can matter; 2–3 tons with careful airflow and staging to avoid short cycling.
• Inland 2,000 sq ft ranch in a hotter valley: 3–3.5 tons if envelope is average; improvements can bring that down.

These scenarios illustrate trends, not prescriptions. Real answers come from measurements and calculation.

Who we are and where we work

#1 AC Guys is a family HVAC business in Southern California, serving Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County (for example: Los Angeles, Pasadena, Glendale, Santa Monica, Long Beach, Anaheim, Irvine, Costa Mesa, Huntington Beach, Ventura, Oxnard, Thousand Oaks, Riverside, Corona, Temecula, and more). We are a fourth-generation family of engineers with 80+ years in engineering, focused on residential and commercial projects—especially large and complex systems. Our mission: healthy air at home for a healthy life.

What to do next: a homeowner sizing checklist

• Gather floor plans, room dimensions, orientation, and ceiling heights.
• Document window types, sizes, shading, U-factor/SHGC if available.
• Verify insulation levels, air sealing, and duct location/leakage.
• Decide comfort targets (summer setpoint, desired indoor humidity).
• Request a Manual J, S, and D plus a duct static/airflow report.
• Confirm equipment SEER2/EER2/HSPF2 ratings and staging type.
• Ensure permits, code compliance, and commissioning data (CFM/ton, superheat/subcool) are provided.

Right-sizing is a process, not a guess. With a proper load calculation and attention to ducts and airflow, your system will be quieter, more efficient, and far more comfortable.