What Size Air Conditioner Do I Need?

Sizing is the decision that determines whether a system is comfortable or merely cold. And the industry's default method — square footage — is the reason so many homes have systems that are too big.

The rule of thumb, and its limits

The common shortcut is roughly 20 BTU per square foot:

A ton is 12,000 BTU/h — from the cooling effect of melting a ton of ice over a day.

Useful for a sanity check. Useless as a design method, because it ignores everything that actually varies:

Insulation. A 1960s house with minimal wall insulation and a 2020 house with a tight envelope have very different loads at the same square footage.

Windows. Area, glazing type, and especially orientation. West-facing glass in the afternoon is an enormous solar gain; the same window facing north is not.

Air leakage. Infiltration can be a large share of load in a leaky house and nearly nil in a tight one.

Ceiling height. Volume, not floor area, is what gets conditioned.

Climate. Phoenix and Seattle are not the same problem at the same square footage.

Internal gains. Occupants, cooking, lighting, and electronics all add heat.

Two identical-footprint houses can genuinely differ by a factor of two.

Manual J

The ACCA Manual J load calculation is the standard. It works room by room and accounts for all of the above, plus local design temperatures.

Its output is a sensible load (temperature) and a latent load (moisture) — and the latent number is why sizing matters for humidity, not just temperature.

Companion standards: Manual S selects equipment against the calculated load, Manual D sizes the ductwork, and Manual T places the registers.

Ask which method was used. If the answer is "Manual J", there should be a document. If it's square footage or "same as the old one", that's the finding.

Why is oversizing the common error?

Nobody complains that their house cools too fast. They do complain if it can't keep up on the hottest day. So the defensive choice is bigger, and the costs are invisible to the contractor:

Short cycling. Satisfies the thermostat quickly, shuts off, restarts minutes later. Compressor starts are what wear equipment out, and startup is the least efficient part of the cycle.

Poor dehumidification. This is the one you feel. Moisture removal requires time with a cold coil. An oversized unit hits setpoint before much moisture condenses, leaving the house cold and clammy. People respond by lowering the setpoint, which costs more and doesn't fix it.

Uneven temperatures. Short cycles don't run long enough to distribute air to distant rooms.

Higher cost, upfront and ongoing.

See oversized HVAC problems.

Why is replacing like-for-like a trap?

"We'll match what's there" perpetuates whatever error was made before — and the old system was probably oversized too.

Worse, if the house has since had insulation added, windows replaced, or air sealing done, the correct capacity is now smaller than the original. Matching the old tonnage means buying an oversized system and losing the benefit of the envelope work you paid for.

Sanity-checking a quote

You can't do a Manual J yourself, but you can tell whether the number is plausible.

1. Get a rough estimate from the mini split sizing calculator or heat pump sizing calculator.
2. Compare. If the contractor's number is far above the estimate, ask why. There may be a good reason — a west-facing wall of glass, a poorly insulated attic — and a Manual J will name it.
3. Ask what the calculation assumed about insulation and windows. If the answer is generic, it wasn't a calculation.
4. Be suspicious of round numbers that exactly match the old system.

Signs yours is wrong now

Oversized:

• Cycles under ~10 minutes in moderate weather
• Cold but humid — the clammy feeling
• Noticeable temperature swings between cycles

Undersized:

• Runs continuously and still can't reach setpoint in moderate weather
• Struggles only in extreme heat — this one is often correct behavior, not undersizing

Correctly sized: long steady runs, near-continuous operation on design days, comfortable humidity.

That last point surprises people. A correctly sized system running almost constantly on the hottest day is doing exactly what it should.

Room units

For window and portable units, the same logic applies at smaller scale, with additional adjustments for sun exposure, kitchens, and occupancy. See window AC sizing guide.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• ACCA Manual J — residential load calculation
• ACCA Manual S — selecting equipment to the calculated load

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Oversized HVAC problems
• Central AC installation cost
• Home humidity control

More in our heating and cooling guides.