What Size AC Unit Do I Need? Sizing by Square Footage

Square footage gives you a fast ballpark for AC size, and that's exactly what it is: a ballpark. The quick rule is about 20 BTU of cooling per square foot, which puts most homes somewhere between 2 and 5 tons. Use the tool below to get a starting estimate, then read on for the tonnage chart and the reasons a real load calculation almost always beats the rule of thumb.

The rule of thumb

The US Department of Energy's own guidance uses roughly 20 BTU per square foot of living space. Air conditioners are rated in tons, and one ton equals 12,000 BTU per hour. Put those two facts together and you get a one-line formula:

tons = (square feet × 20 BTU) / 12,000

So a 1,500 sq ft home works out to (1,500 × 20) / 12,000 = 2.5 tons. A 1,800 sq ft home lands at 3 tons. That's the whole rule of thumb. It's popular because it's easy and it's usually in the right neighborhood, and it appears on nearly every HVAC sizing page, including LearnMetrics' tonnage calculator.

Before you plug in a number, measure the conditioned space correctly. Only heated and cooled floor area counts, so leave out the garage and any unconditioned space. If you're not sure of your square footage, run the rooms through the house calculator or the pillar guide on measuring square footage first.

AC tonnage by square footage chart

Here's the rule of thumb laid out as a table. Ranges overlap on purpose, because the exact break point depends on your house, not just its size.

Home size	Cooling load	AC size
600 to 1,000 sq ft	12,000 to 20,000 BTU	1 to 1.5 ton
1,000 to 1,200 sq ft	20,000 to 24,000 BTU	2 ton
1,200 to 1,500 sq ft	24,000 to 30,000 BTU	2.5 ton
1,500 to 1,800 sq ft	30,000 to 36,000 BTU	3 ton
1,800 to 2,100 sq ft	36,000 to 42,000 BTU	3.5 ton
2,100 to 2,400 sq ft	42,000 to 48,000 BTU	4 ton
2,400 to 3,000 sq ft	48,000 to 60,000 BTU	5 ton

Two of the most searched sizes are the 2.5 ton and 3 ton units. A 2.5 ton unit (30,000 BTU) covers roughly 1,200 to 1,500 sq ft under the rule of thumb, and a 3 ton unit (36,000 BTU) covers roughly 1,500 to 1,800 sq ft. Central AC comes in half-ton steps, so if the math lands between sizes, you don't get to split the difference; you pick one and the choice matters.

Why a Manual J load calculation beats the rule of thumb

The 20-BTU rule assumes an average house in an average climate. Real houses aren't average. Cooling load depends on a stack of variables the square footage number can't see:

Climate. A 2,000 sq ft house in Phoenix has a very different load than the same house in Portland.
Insulation and air sealing. A tight, well-insulated home needs far less cooling than a drafty one of identical size.
Windows. Number, size, orientation, and glass type. West-facing glass in the afternoon is a big heat gain.
Ceiling height. The rule counts floor area, but you're cooling volume. Nine or ten foot ceilings add load a square-foot count ignores.
Sun exposure and shading. A house under mature trees loads differently than one baking in full sun.
Occupancy and internal gains. People, appliances, and a hot kitchen all add heat.

The industry answer to all of this is a Manual J load calculation, the ACCA standard that adds up gains and losses room by room. HVAC engineers are blunt about the gap between the two methods. Energy Vanguard's writeup, Air Conditioner Sizing: Load Calculations vs Rules of Thumb, documents cases where a proper Manual J came in well under what a contractor's square-foot rule would have specified, and ACCA makes the same argument in its own Manual J vs rules of thumb piece. The Department of Energy flatly recommends a load calculation over a rule of thumb for exactly this reason.

Trade forums say it even more plainly. In a long-running HVAC sizing thread on the Mike Holt forums, the "how many square feet per ton" question gets the same answer over and over from the pros: it depends, the number swings from roughly 400 to 700 sq ft per ton depending on the house and region, and anyone quoting one flat figure is guessing. That range, 400 to 700 sq ft per ton, is a useful reality check. If a contractor sizes purely off square footage and skips the load calc, that's a yellow flag.

Don't oversize: the short-cycling problem

Most people assume a bigger unit is safer. It isn't. An oversized air conditioner cools the air fast, hits the thermostat setpoint, and shuts off before it has run long enough to pull moisture out of the air. Then it turns back on a few minutes later. That's called short cycling, and it causes real problems:

The house feels cold and clammy because humidity never gets removed.
Rooms cool unevenly, since the unit never runs long enough to mix the air.
Compressor wear goes up from all the stopping and starting, shortening the equipment's life.
Efficiency drops, so you pay more to be less comfortable.

A right-sized unit runs longer, quieter cycles and dehumidifies while it cools. That's why bumping up "just to be safe" is the wrong instinct. When the load calc says 2.5 tons, a 3 ton unit is not a free upgrade, it's a comfort and humidity downgrade.

Worked example

Say you've got a 1,700 sq ft single-story house. The rule of thumb gives:

(1,700 × 20) / 12,000 = 2.83 tons

That rounds to a 3 ton unit. Reasonable starting point. Now adjust for reality. If the house is in a hot, humid climate with a lot of west-facing glass and older insulation, the load might push toward 3 tons or a hair over. If it's a newer, tightly built house in a mild climate with good shading, the true load could come in nearer 2.5 tons. Same square footage, one size apart, and only a Manual J tells you which. Use the rule of thumb to sanity-check the contractor's number, not to replace the calculation.

FAQ

What size AC unit do I need for my house?

Start with the rule of thumb: about 20 BTU per square foot, and 12,000 BTU per ton. A 1,500 sq ft home points to roughly 2.5 tons, and an 1,800 sq ft home to about 3 tons. Treat that as a starting estimate, then get a Manual J load calculation, because climate, insulation, windows, and ceiling height can move the real answer up or down by a half ton or more.

How many square feet does a 3 ton AC unit cover?

A 3 ton unit is 36,000 BTU, which covers roughly 1,500 to 1,800 sq ft under the 20-BTU rule. In practice the range is wider, since HVAC pros commonly figure 400 to 700 sq ft per ton depending on the house and climate. A well-insulated home in a mild climate can push a 3 ton unit further than a leaky home in a hot one.

How many square feet does a 2.5 ton AC unit cover?

A 2.5 ton unit is 30,000 BTU and covers roughly 1,200 to 1,500 sq ft using the rule of thumb. Tighter, shadier, better-insulated homes stretch that further, while hot climates and lots of west-facing glass pull it back. A load calculation is the only way to know if 2.5 tons or the next size up is right for your specific house.

How many BTU per square foot do I need for cooling?

The common figure is about 20 BTU per square foot of conditioned living space. Multiply your square footage by 20 to get total BTU, then divide by 12,000 to convert to tons. This is a rough guide only; actual BTU per square foot varies with climate, insulation, and sun exposure.

Is it bad to buy an AC unit that's too big?

Yes. An oversized unit short cycles, meaning it cools fast and shuts off before removing humidity, which leaves the house cold and clammy. It also wears out the compressor faster and runs less efficiently. Right-sizing, ideally from a Manual J calculation, keeps the unit running longer, gentler cycles that dehumidify properly.

Sources

Related tools: house square footage calculator, room calculator, the pillar guide on measuring square footage, and the homepage for every square footage tool.