AC Sizing Calculator
Calculate the correct air conditioner size for your home based on square footage, climate zone, and insulation quality.
Enter your home details above to calculate the ideal AC system size.
How AC Sizing Works: BTU and Tonnage Explained
Air conditioner sizing is measured in BTUs (British Thermal Units) per hour. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. As a general rule of thumb, you need about 20 BTUs per square foot of living space, but this varies significantly based on your climate, insulation quality, ceiling height, window count, and sun exposure.
AC tonnage refers to the cooling capacity: 1 ton equals 12,000 BTU/hr. A typical 2,000 sq ft home in a moderate climate needs a 3-3.5 ton system (36,000-42,000 BTU). Oversizing is just as problematic as undersizing -- an oversized unit short-cycles, failing to properly dehumidify and wearing out faster.
Understanding SEER Ratings
SEER (Seasonal Energy Efficiency Ratio) measures how efficiently an AC converts electricity into cooling. It is calculated as total cooling output (BTU) divided by total electrical input (watt-hours) over a cooling season. The federal minimum SEER rating is 14 in northern states and 15 in southern states (as of the 2023 efficiency standard update). High-efficiency units range from 18 to 26+ SEER. A higher SEER unit costs more upfront but saves significantly on electricity -- upgrading from SEER 14 to SEER 18 can reduce cooling costs by about 22%.
Climate Zone Impact on AC Sizing
Climate zones dramatically affect sizing needs. In Zone 1 (hot-humid areas like Florida and southern Texas), homes need 30% more cooling capacity than the baseline. In Zone 7 (very cold areas like Alaska), homes need 20% less cooling capacity because the AC runs far fewer hours per year. Cooling degree days (CDD) measure how much cooling a location needs annually -- Florida averages 3,500+ CDD while Minnesota averages around 500 CDD.
Frequently Asked Questions
How many BTU do I need per square foot?
The standard estimate is 20 BTU per square foot, adjusted for climate zone and insulation. In hot-humid climates (Zone 1-2), plan for 24-26 BTU/sq ft. In cooler climates (Zone 5-7), 16-18 BTU/sq ft is often sufficient. Factors like cathedral ceilings, large windows, or heavy sun exposure can add 10-20% to the requirement.
What happens if my AC is too big or too small?
An oversized AC cools too quickly without removing enough humidity, leading to a cold but clammy house and higher energy bills from short-cycling. An undersized AC runs constantly without reaching the set temperature, also driving up bills and wearing out prematurely. Proper sizing is critical for comfort, efficiency, and equipment longevity.
How does insulation quality affect AC sizing?
Poor insulation can increase your cooling needs by 30% compared to a well-insulated home. If your home has single-pane windows, minimal attic insulation (below R-30), or noticeable drafts, select "Poor" insulation. Homes with modern double-pane windows, R-38+ attic insulation, and sealed ductwork qualify as "Good" or "Excellent."
What is a good SEER rating?
For most homeowners, SEER 16-18 offers the best balance of upfront cost and energy savings. In hot climates where AC runs 6+ months per year, investing in SEER 20+ can pay back within 5-7 years. In cooler climates where AC runs only a few months, the payback period for high-SEER units is longer, making SEER 14-16 more cost-effective.