What Is BTU and How to Size an Air Conditioner

BTU explained in plain language

BTU stands for British Thermal Unit. In cooling, we talk about BTU per hour (BTU/h), which is the rate at which an air conditioner can remove heat. One ton of cooling equals 12,000 BTU/h. When you see a system rated at 24,000 BTU/h, that is a 2-ton unit. BTU tells you capacity, not efficiency. Efficiency is measured by SEER2 (seasonal efficiency) and EER2 (steady-state efficiency at a specific test condition).

Cooling is not only about temperature. Air conditioners remove both sensible heat (what the thermostat measures) and latent heat (moisture). In dry coastal mornings and hot inland afternoons in Southern California, the sensible vs latent split changes. Proper sizing protects comfort, humidity control, energy costs, and indoor air quality.

How to size an AC the right way

The gold standard is a professional Manual J load calculation (room-by-room), followed by Manual S equipment selection and Manual D duct design. These methods consider your home’s exact conditions rather than a guess based on square footage. Many insurance companies and municipal code officials expect permitted work and documented calculations for compliance and liability. DIY shortcuts can void warranties, fail inspections, or create safety and moisture risks.

Why rules of thumb fall short

Square-foot rules (like “X BTU per square foot”) ignore local climate, insulation, windows, and solar exposure. In Southern California, a tight, shaded 1,800 sq ft home near the coast might need far less capacity than a similar-size inland home with large west-facing windows. Even within Los Angeles County, cooling loads vary widely.

Key factors a proper load calculation considers

• Climate data and design temperatures for your exact location
• Home size, layout, and ceiling height
• Insulation levels and air tightness
• Window area, U-factor, SHGC, orientation, and shading
• Duct location and leakage (attic vs conditioned space)
• Occupants, appliances, lighting, and plug loads
• Ventilation and filtration targets (for example, MERV 13)

These inputs determine the BTU/h your system must deliver at peak conditions. Then Manual S aligns that load with real equipment performance at your design temperature, not just the headline tonnage.

Ton vs BTU and what it means for comfort

Remember: 1 ton = 12,000 BTU/h. Two 3-ton systems can behave very differently. One may short-cycle and struggle with humidity; the other may modulate smoothly and keep rooms even. Look at sensible capacity, latent capacity, and part-load performance, not just tonnage. Also check SEER2 and EER2 for efficiency in our climate.

Common signs your system is the wrong size

• Short cycling: frequent on/off, uneven temperatures, and poor humidity control
• Hot and cold spots from room to room
• High energy bills despite a “big” system
• Excessive noise or drafts
• Persistent dust, odors, or wildfire smoke intrusion due to inadequate filtration and ventilation strategy

Southern California considerations

Across Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County, we see microclimates and a wide range of building types. Coastal marine layers, inland heat spikes, and wildfire seasons affect AC sizing and filtration. A right-sized system with a clear ventilation and filtration plan (often MERV 13 or better) improves indoor air quality and helps maintain healthy air at home. In many homes, heat pumps sized with accurate Manual J are ideal, delivering efficient cooling in summer and dependable heating during mild winters.

Title 24 and permits

California Title 24 energy standards, local codes, and utility programs may require documentation of load calculations, duct testing, and airflow verification. Insurers and manufacturers often require licensed installation and compliance records to protect coverage and warranties.

Ducted or ductless? Choosing the right approach

Ducted systems are great for whole-home distribution and can be designed for quiet, efficient operation with proper duct sizing (Manual D). Ductless mini splits shine when you need zoning, additions, ADUs, or retrofit solutions without major ductwork. Either way, the correct BTU/h per room and proper airflow balance are critical for comfort and energy savings.

Estimating vs doing it right

As an educational starting point, some homes land roughly between 15–25 BTU/h per square foot in our region, but that range can be off by thousands of BTU/h if your windows, insulation, or sun exposure differ from “average.” That is why a measured Manual J is the safe path—especially when permits, warranty, and insurance are on the line.

Large and complex projects

For commercial spaces, multifamily, and large custom homes, accurate BTU planning becomes even more important. Mixed-use loads, ventilation codes, high-occupancy spaces, internal heat from equipment, and complex zoning demand deep engineering and careful commissioning to ensure comfort and compliance.

Who we are

#1 AC Guys is a fourth-generation family of engineers with 80+ years of experience in engineering. Our mission is healthy air at home for healthy living. Our engineers have factory training from Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China). We serve Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County. From residential to commercial, we specialize in large and complex projects where precision sizing, Manual J/S/D, and code compliance matter.

Bottom line

BTU tells you how much heat your AC can remove; correct sizing ensures real-world comfort, efficiency, and air quality. Because sizing touches safety, codes, warranties, and insurance, many situations call for a qualified HVAC professional to perform Manual J/S/D and manage permitting. Getting BTU right upfront saves money, prevents moisture and comfort issues, and keeps your system performing as designed.