What AC tonnage is right for my home?

Understanding AC tonnage and why it matters

AC tonnage is a measure of cooling capacity. One ton equals 12,000 BTU per hour (1 ton = 12,000 BTU/h). Choosing the right size is not about picking the biggest unit—it is about matching your home’s heat gain so the system cools efficiently, controls humidity, and runs long enough to clean the air.

Oversized AC short-cycles, leaves rooms clammy, is louder, wastes energy, and can shorten equipment life. Undersized AC runs constantly, struggles during heat waves, can cause coil freeze-ups, and still leaves hot rooms. Correct AC tonnage delivers comfort, air quality, and lower bills.

What drives the right AC size

• Climate and microclimate: Coastal areas with marine layer need different capacity than inland valleys or foothills. In Southern California, a home near the beach may need less tonnage than the same home in the San Fernando or Santa Clarita Valley.
• Home size and layout: Total square footage, ceiling height, open-to-below spaces, and room-by-room loads all matter.
• Insulation and air sealing: Attic R-values, wall insulation, and air leakage (infiltration) can shift your load dramatically.
• Windows and glass: Area, orientation (west/south gain), shading, and window SHGC/U-factor change cooling BTU significantly.
• Ductwork and location: Leaky ducts, long runs, or ducts in a hot attic increase required capacity. Manual D duct design helps keep the selected tonnage effective at the registers.
• Occupants and internal gains: People, cooking, lighting, electronics, and equipment add heat that must be removed.
• Ventilation and IAQ: Fresh air requirements, ERV/HRV systems, and filtration targets (for wildfire smoke, MERV 13) affect fan sizing and sensible/latent balance.

Rules of thumb vs. proper calculation

You will hear rules like “500–700 square feet per ton.” In real homes, the true range can be wider (300–1,000 sq ft/ton) depending on insulation, glazing, and air leakage. The gold standard is a Manual J load calculation, followed by Manual S equipment selection and Manual D duct design. These methods consider your specific home, climate data, and construction details so the chosen AC tonnage is accurate.

In our region, Title 24 and HERS testing also influence duct leakage limits, airflow, and equipment efficiency, which can change the optimal size and the SEER2 rating you pursue.

A cautious at-home estimate (not a substitute for Manual J)

1. Start with conditioned square footage.
2. Pick a baseline factor: 500–700 sq ft per ton for many well-insulated Southern California homes. Tighter, shaded homes may lean higher (more sq ft per ton); older, leaky homes may lean lower.
3. Adjust down if you have low solar gain windows, light-colored cool roof, deep overhangs, and excellent air sealing. Adjust up if you have lots of west-facing glass, minimal shading, or ducts in a hot attic.
4. Add internal gains: 300–600 BTU/h per person typically present, plus notable appliance loads (home office, gaming, or pro-style ranges).
5. Check duct condition: Leaky or undersized ducts can require more capacity or, better, duct upgrades.

Example: A 2,000 sq ft, well-insulated home with modern windows in a warm inland area might land between roughly 2.5 and 3.5 tons. The same size home by the coast could be lower, while one with big west-facing glass could be higher. Only a Manual J confirms the right AC tonnage.

Heat pumps, variable speed, and humidity control

Whether you choose a traditional AC or a heat pump, tonnage still equals cooling capacity. Two-stage and variable-speed systems can better match part-load conditions, improving comfort and dehumidification while reducing short cycling. Proper sizing still matters—staging and modulation are not a cure for being far oversized.

Ductless mini-split and zoning considerations

Ductless mini-split systems and multi-zone setups size each indoor head to the room’s load. This can reduce total tonnage compared to a single central unit because you avoid duct losses and can shut off zones. Still, each zone needs a load calculation for accurate BTU and ton selection.

Why DIY sizing and installs are risky

• Safety: Refrigerants are under high pressure; some blends are mildly flammable. Electrical hazards and brazing risks are real.
• Regulatory: Many jurisdictions require permits and HERS verification. Improper installs can fail inspections.
• Insurance and warranty: Many insurance companies and manufacturers require a licensed technician, documented commissioning, and proof of proper sizing for claims and warranties.
• Performance: Without Manual J/S/D, the chance of oversized or undersized AC skyrockets, especially in homes with lots of glass or unique layouts.

Local context in Southern California

Across Los Angeles County and nearby areas—including Orange County, Ventura County, and Western Riverside County—microclimates vary fast. A design that fits Santa Monica may be off for Pasadena, Glendale, Burbank, Long Beach, Torrance, or the San Fernando and San Gabriel Valleys. Similarly, needs in Anaheim, Irvine, Santa Ana, Fullerton, Oxnard, Ventura, Thousand Oaks, Simi Valley, Riverside, Corona, Temecula, and Murrieta can differ by insulation level, window package, and solar exposure.

Wildfire smoke events also make filtration critical. Planning for MERV 13 filters can affect static pressure and blower selection, which ties back to Manual S/D and the final AC tonnage and airflow.

Efficiency, SEER2, and comfort

Higher SEER2 equipment can trim energy use, but it does not replace proper sizing. A right-sized, well-ducted 15.2 SEER2 system can outperform an oversized, poorly ducted premium unit. Balance tonnage, ventilation, and filtration so you get both comfort and healthy indoor air.

Who we are

#1 AC Guys is a family HVAC business serving Los Angeles County and nearby regions—including Orange County, Ventura County, and Western Riverside County. Our mission is healthy air at home for healthy living. We bring 80+ years of engineering experience as a fourth-generation family of engineers, and our engineers trained at manufacturer facilities of Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China). We specialize in residential and commercial projects, including large and complex systems where precise AC tonnage, duct design, and commissioning are essential.

Key takeaways

• Size by Manual J, select by Manual S, and design ducts by Manual D.
• Consider climate, insulation, windows, ducts, and internal gains.
• Avoid rules of thumb as your final answer; use them only as a rough starting point.
• Permits, HERS testing, and documentation protect safety, insurance, and warranty.
• Right AC tonnage improves comfort, humidity control, air quality, and energy use.