Heat Pumps
What Size Air Conditioner Do I Need?
By Tom Moor•July 13, 2025
Thinking about getting a new air conditioner? Whether you’re replacing an old unit or installing one for the first time, figuring out the right size can feel like a bit of a puzzle. At HVAC.com, we’re here to simplify it. In this guide, we’ll explain what “AC tonnage” really means, why proper sizing matters, and how to make sure your new system is the perfect fit for your home.
What Is AC Tonnage?
AC tonnage doesn’t refer to how much an air conditioner weighs. Instead, it’s a measure of its cooling capacity and how much heat it can remove from your home in an hour.
Air conditioners are rated in BTUs, or British Thermal Units. One ton of cooling equals 12,000 BTUs per hour.
Why the Right Size Air Conditioner Matters
Getting the size right air conditioner isn’t just about comfort; it’s about efficiency and cost, too.
- Too small? Your system will run constantly and struggle to cool your home, leading to higher energy bills and wear and tear.
- Too big? It’ll cool your home too fast. These short, frequent cycling (referred to as short cycling) can result in reduced dehumidification, increased energy costs, and uneven cooling.
Getting a properly sized unit keeps things cool and dry, and it helps your system last longer.
Rule of Thumb: Square Footage vs. AC Tonnage
A good starting point is to use the 20 BTUs per square foot rule. While this gives you a ballpark idea, remember that home layout, insulation, and climate also play a big role.
Here’s a quick sizing guide:
Home Size (sq. ft.) | Approx. BTUs Needed | Recommended AC Tonnage |
| 600-1,000 | 12,000-18,000 | 1-1.5 tons |
| 1,000-1,500 | 20,000-30,000 | 1.5-2.5 tons |
| 1,500-2,000 | 30,000-36,000 | 2.5-3 tons |
| 2,000-2,500 | 36,000-48,000 | 3-4 tons |
| 2,500-3,000 | 48,000-60,000 | 4-5 tons |
For hotter climates or poorly insulated homes, consider rounding up a half ton. For cooler climates or newer, well-insulated homes, you might round down.
How to Calculate What Size Air Conditioner You Need
Want to do the math manually? Here’s a simplified formula:
- [Home Square Footage] × 20 = Approximate BTUs Needed
- Then divide by 12,000 to convert to tons.
Example:
If your home is 2,000 sq. ft.:
- 2,000 × 20 = 40,000 BTUs
- 40,000 ÷ 12,000 = 3.33 tons
- You’d likely round up to a 3.5-ton system – or maybe 4 tons if you’re in Florida or Texas.
There are plenty of online AC size calculators that can do this for you. Just enter your square footage and answer a few simple questions about your insulation, ceiling height, and climate to receive an instant recommendation.
Other Factors That Affect AC Sizing
While square footage is a crucial piece of the overall puzzle when determining the right size air conditioning system, there are several other important factors that many HVAC technicians will take into account when they perform an HVAC load calculation:
- Ceiling height: Higher ceilings mean more air to cool.
- Insulation quality: Poor insulation lets cool air escape.
- Number of windows: Windows can let in a lot of heat, especially if they’re older or face the sun.
- Shade and sun exposure: Shaded homes need less cooling power than sun-drenched ones.
- Ductwork condition: Leaky or unbalanced ducts can affect system performance.
- Humidity: If you live somewhere muggy, your system needs to manage both temperature and moisture.