What size AC do I need?

Why the square-footage rule oversizes most homes

The rule of thumb, one ton per 400 to 600 square feet, exists because it is fast. A contractor can quote a unit from your home's size without setting foot inside. The trouble is that it was built around older, leakier housing stock, and it ignores everything that actually decides how much cooling a house needs. When engineers run real load calculations on a range of homes, the answers cluster between 600 and 1,000 square feet per ton, not 400 to 600. The lowest figure in one well-known set of measurements was 624 square feet per ton, already above the high end of the rule of thumb. In other words, even the most cooling-hungry homes in that study needed less capacity than the rule would have assigned them.

That gap is not academic. A home the rule calls a 4-ton job is often a true 3-ton load once you account for modern insulation, better windows, and shade. The rule survives anyway because sizing too big rarely produces an obvious complaint, the house does get cold, so few homeowners ever learn they were oversized. The contractor is not usually trying to upsell you either, the bigger unit costs them little more to install and protects them against the one complaint people do notice, a house that will not cool on the hottest day. The result is a quiet bias toward too big. You can pull a rule-of-thumb number yourself with our AC tonnage calculator to see roughly where your home lands, but treat that as the number to question, not the answer to buy.

Why a bigger AC is not better

It is the most common sizing mistake, and it feels safe, so it goes unquestioned. The instinct is that a larger unit must cool better. In practice the opposite happens. An oversized AC cools the air to the thermostat setting fast, then shuts off, before it has run long enough to pull moisture out of the air. So the house hits temperature but stays clammy and humid, and you end up dropping the thermostat to chase a comfort the unit cannot deliver, which costs more.

That start-stop pattern, called short cycling, is also hard on the equipment. The compressor draws its biggest current at startup, so a unit that switches on and off every few minutes wears out faster than one that runs in long steady cycles, and it can shave years off the system's life. An oversized AC delivers worse comfort, a higher bill, and a shorter lifespan, all at once. If your current system already shows this, our guide on the signs an AC is too big walks through confirming it.

What actually decides the right AC size besides square footage?

Two houses of identical size can need very different equipment, and the difference is everything the square-footage rule throws away. Insulation is the big one: a home with a well-insulated attic and sealed ductwork loses far less cooling than a leaky one, so it needs less capacity for the same comfort. Windows matter nearly as much, both how many and which way they face, because afternoon sun through west-facing glass pours heat in that the AC has to remove.

Climate sets the baseline. The same house needs more cooling in Phoenix than in Seattle, which is why a national chart cannot be right for your specific home. Ceiling height changes the volume of air to cool, a two-story foyer or vaulted ceiling adds load a flat square-footage number misses. And how the house is used plays in too: a full house with a busy kitchen generates more heat than a quiet one. None of this is exotic, it is exactly what a proper load calculation measures and a square-footage lookup ignores.

Ductwork deserves its own mention, because it is the one factor that can make an otherwise correctly sized unit underperform. Leaky or undersized ducts lose cooling between the unit and the rooms, which tempts a contractor to compensate with a bigger AC instead of fixing the ducts. That is the wrong trade: a larger unit pushing air through bad ducts still cools unevenly and still short cycles. A good load calculation looks at the distribution system, not just the box, so the right answer is sometimes to seal the ducts and keep the smaller unit rather than upsize around the problem.

What is a Manual J load calculation and do you need one?

A Manual J is the industry-standard method for sizing residential cooling, developed by the Air Conditioning Contractors of America. Instead of guessing from floor area, it adds up the actual heat gain of your specific house: square footage and ceiling height, insulation levels, the number and type and orientation of windows, air leakage, and your local climate. The result is the real cooling load, and the correctly sized unit follows from it.

You do not run this yourself, the contractor does, and the good news is that a reputable one includes it free as part of a replacement quote. So the question is less whether you need one and more whether your contractor will do it. If you are replacing a system, ask for it by name. A company that sizes your new AC off square footage alone, or off the size of your old unit, is not doing the job right. To understand what the calculation involves before you ask, our Manual J calculator shows the inputs that go into a real load number.

Should you just match the size of your old AC?

It is the most tempting shortcut and one of the riskiest. The logic sounds airtight: the old unit was a 3.5-ton, the house has not changed, so replace it with another 3.5-ton. The flaw is the assumption that the old unit was sized right in the first place. A large share of existing systems were themselves oversized off the same square-footage rule, so matching the old size just copies the original mistake into a new unit you will own for the next 15 years.

It is also wrong whenever the house has changed. If you have added attic insulation, replaced windows, sealed ductwork, or finished a basement since the last AC went in, the load is different now, usually lower. Matching the old tonnage ignores all of that. The honest path is the same either way: let a load calculation set the size, and treat the old unit's tonnage as a data point to question, not a target to hit.

How do you make sure a contractor sizes your AC correctly?

Sizing is the part of the job a homeowner can actually police, even without technical training, because the tells are simple. Ask each contractor how they arrived at the tonnage. The answer you want is a load calculation that looked at your insulation, windows, and climate. The answers that should give you pause are a unit sized from square footage alone, a quote that just matches your existing equipment, or a larger size pitched as a margin of safety or room to grow. Bigger is not safer, it is the exact mistake to avoid.

When two quotes come back a half-ton or a full ton apart, that difference usually traces straight back to method: the larger number is the rule of thumb, the smaller one is often the real load. Use the ballpark tonnage as your reference point, and if a quote lands well above it with no load calculation to justify the jump, that is the one to question. The contractor who measures, explains, and lands close to the calculated load is the one sizing your AC for comfort and bill, not for the convenience of a quick quote.