How to Size an Air Conditioner: BTU Guide for Every System Type

The 20 BTU Rule (and When to Break It)

The standard starting point for AC sizing is 20 BTU per square foot of living space. A 1,500 square foot home needs roughly 30,000 BTU, which equals a 2.5-ton central air system (12,000 BTU = 1 ton).

But 20 BTU per square foot is just the baseline. Real-world sizing adjusts for at least five factors that can swing the number 30% in either direction.

Factor 1: Climate Zone

A home in Phoenix needs 15-20% more cooling capacity than the same home in Seattle. The outdoor design temperature (the hottest temperature your system must handle) directly determines how hard the AC works. HVAC contractors use ACCA Manual J calculations that reference local design temperatures, but the simplified version is: hot climates add 10-20% above the baseline, and mild climates subtract 10-15%.

Factor 2: Insulation Quality

Insulation slows heat transfer into your home. A well-insulated home with double-pane windows needs less cooling than a drafty older home with single-pane windows. Poor insulation adds 15-20% to your BTU requirement. Excellent insulation (spray foam, new construction) reduces it by 10-20%.

Factor 3: Sun Exposure and Windows

South and west-facing windows receive the most direct sunlight and add the most heat load. A room with large south-facing windows in direct afternoon sun can need 10-15% more cooling than a north-facing room of the same size. Trees, awnings, and window treatments help significantly.

Factor 4: Ceiling Height

The 20 BTU rule assumes 8-foot ceilings. Higher ceilings mean more air volume to cool. A 10-foot ceiling adds roughly 20% to the cooling requirement. A vaulted 12-foot ceiling adds 35%. This is one of the most commonly overlooked factors.

Factor 5: Heat Sources

Kitchens generate significant heat from cooking appliances. Each person in a room adds about 600 BTU of body heat. Electronics, lighting, and south-facing windows all add thermal load. The simplified adjustment: add 4,000 BTU if the space includes a kitchen, and 600 BTU per person beyond the first two occupants.

Why Oversizing Is Worse Than Undersizing

This is counterintuitive, but an oversized AC is worse than a slightly undersized one. An oversized unit cools the air quickly but shuts off before it adequately dehumidifies. The result is a cold, clammy house that feels uncomfortable even at the right temperature. The unit also short-cycles (turns on and off frequently), which wastes energy and wears out the compressor faster.

A slightly undersized unit runs longer, which actually improves dehumidification and provides more even temperatures throughout the house. It might struggle on the absolute hottest day of the year, but those days are rare, and the tradeoff is better comfort the other 360 days.

Central Air vs Mini-Split vs Window Unit

Central air is best for cooling an entire home with existing ductwork. Typical cost: $3,000-$7,000 installed. If your home has ducts, this is the default choice.

Mini-splits (ductless) are ideal for homes without ductwork, room additions, or zones that need independent temperature control. They're 20-30% more efficient than central air and allow you to cool only the rooms you're using. Typical cost: $1,500-$5,000 installed depending on how many indoor heads you need.

Window units are the cheapest option for single rooms ($150-$500, no installation). They're loud and block part of the window, but they cool one room effectively and cost nothing to install. For a rental or a room that only needs occasional cooling, they're hard to beat on value.