Your AC runs all afternoon, but the house still feels warm and sticky. In Houston heat near 98°F with heavy humidity, that is more than annoying. It often points to one thing: a system that is the wrong size for your home.

Knowing what size AC unit your home needs starts with a few simple numbers. The right fit matters most here, where summers stay long, hot, and humid. A unit that is too small never catches up. One that is too big cools fast but leaves your air damp.

We have sized and replaced cooling systems across Houston since 2003. Below, we walk you through the quick square-footage rule, the trouble with a wrong-size unit, and the one calculation that gets your size exactly right.

Start with a quick estimate. Most homes need about 20 BTUs of cooling for each square foot. Multiply your home's square footage by 20. Then divide that number by 12,000 to get the size in tons.

Here is a simple example. A 2,000 square foot home needs roughly 3 tons, or about 36,000 to 40,000 BTUs.

This gives you a starting point, not a final answer. Insulation, ceiling height, windows, sun exposure, and Houston humidity all change the real number. The most accurate way to find your size is a Manual J load calculation done by a licensed pro.

Ready to replace an aging system? We can help with your AC replacement for an exact, no-guess sizing.

AC size means cooling power, not how big the unit looks. Two numbers describe that power: BTUs and tons.

A BTU is the amount of heat your AC removes in one hour. The higher the BTU number, the more heat it pulls out. One ton equals 12,000 BTUs of cooling per hour.

Most home systems fall between 1.5 and 5 tons. Sizes go up in half-ton steps, like 2, 2.5, and 3 tons. Many Houston homes land somewhere between 2 and 5 tons.

So when we talk about a "3-ton" AC, we mean its cooling capacity. We do not mean its weight or shape.

Want a fast estimate before you call anyone? Use the square-footage rule. Plan for about 20 BTUs of cooling per square foot of living space.

Here is the formula:

• Square footage × 20 = BTUs needed
• BTUs ÷ 12,000 = size in tons

A 2,000 square foot home works out to about 3 tons. Use this quick chart as a guide:

• 1,200–1,500 sq ft: about 2 to 2.5 tons
• 1,500–2,000 sq ft: about 2.5 to 3 tons
• 2,000–2,500 sq ft: about 3 to 3.5 tons
• 2,500–3,000 sq ft: about 4 to 5 tons

Treat these numbers as a starting point only. In our Houston service calls, square footage alone rarely tells the whole story. Our humidity changes the math, so two same-size homes can need different units.

A wrong-size AC costs you comfort and money. Both directions cause problems, just in different ways.

A unit that is too small never keeps up:

• It runs almost all day.
• Your home still feels warm.
• Energy bills climb higher.

A unit that is too big causes its own trouble:

• It cools fast, then shuts off too soon.
• It cannot pull enough humidity from your air.
• Your home feels cold but damp and clammy.

That stop-and-start pattern is called short cycling. It wears parts out faster and shortens the life of your system. A worn system may need more frequent AC repair before it finally quits. In our humid Houston summers, poor moisture control is a real comfort problem.

Many people think a bigger AC is the safer pick. It is not. The right size always beats the biggest size.

Square footage gets you close, but Houston changes the math. Several local factors raise or lower the size your home really needs.

Here is what matters most around here:

• Heat and humidity. Long, sticky summers push your cooling load higher.
• Sun exposure. West-facing rooms and big windows soak up afternoon heat.
• Insulation. Older Houston homes often lose cool air through weak insulation.
• Ceiling height. Tall ceilings hold more air, so add about 10% per extra foot.
• Occupants. More people in the home means more heat to remove.

These factors explain why two homes of the same size can need different units. The ENERGY STAR guide on home cooling can also help you understand how efficiency ratings affect the load your system carries.