What Size AC Unit Do I Need? A 1.5–5 Ton Sizing Guide

One ton of cooling capacity equals 12,000 BTU per hour — a unit of measurement for heat removal, not weight, left over from an era when cooling capacity was compared to melting a ton of ice in 24 hours. From there, the common sizes scale directly:

The rule of thumb most people find first — roughly 400 to 600 square feet per ton — is a reasonable starting estimate for a fairly average house in a moderate climate with decent insulation. It's not a load calculation; it's an average of averages, which is exactly why it gets some houses right and a meaningful number of others wrong in either direction.

A 1,800-square-foot ranch with good attic insulation and shaded, north-facing windows might genuinely need a 2.5-ton system. A 1,800-square-foot house with a vaulted great room, west-facing glass, and a duct system that leaks 20% of its air before it reaches the vents might need a 3.5-ton system to feel the same — same square footage, a full ton apart.

The instinct to size up "just to be safe" is common, and it backfires — a bigger system doesn't mean more efficient cooling, it means one that short-cycles instead of running the way it was designed to. ENERGY STAR's own guidance on right-sizing is built around exactly this tradeoff: comfort and efficiency both come from a system that runs full cycles, not a bigger one that shuts off early.

The mechanism is short-cycling: an oversized unit cools the air fast enough to satisfy the thermostat before it's run long enough to properly dehumidify the space, so it shuts off, then kicks back on minutes later. The house ends up cold and clammy at the same time, running through more start-up cycles — the least efficient part of a compressor's operation — than a correctly sized system ever would. It's the same mechanical pattern that shows up as ice on the coil or line set when airflow or runtime gets thrown off.

An undersized system doesn't fail the way an oversized one does — it just runs constantly during peak heat and still doesn't fully catch up on the hottest days, which reads as "the AC is broken" when it's actually the AC doing exactly what an undersized unit does. Constant runtime also means more total operating hours per season, which shortens the compressor's working life even though no single cycle is doing anything wrong.

A Manual J load calculation — the ACCA-developed standard ENERGY STAR points to for correct residential sizing — replaces the square-footage guess with the actual variables that move a home's cooling load:

The air handler is the indoor unit that moves conditioned air through your ductwork — the blower and coil that pair with the outdoor condenser to actually deliver the cooling that condenser produces. Buying the right tonnage of condenser but pairing it with a mismatched air handler (wrong airflow rating, or ductwork sized for a different tonnage) undercuts correct sizing just as much as guessing the tonnage wrong in the first place. This is why a full AC installation sizes the whole system together — condenser, air handler, and ductwork — rather than swapping one component in isolation.

There's no single national price per ton, and any chart that gives you one is guessing the same way a square-footage sizing chart is. A full AC installation with a real Manual J calculation, old equipment haul-away, and a commissioning report starts from $3,800 — where a specific quote lands past that starting point depends on tonnage, ductwork condition, and efficiency tier together, which is exactly why a written quote follows a home visit, not a phone call.

Sizing off the old unit's label gets riskier the older and more varied a city's housing stock is. Aurora, IL is a real example — Downtown Aurora alone has more pre-1939 housing than 97.5% of neighborhoods nationwide, alongside new construction on the outer edges of the same city. A tonnage chart built around one 'typical' house doesn't hold up across that range; a system sized for a postwar subdivision home and dropped into a pre-1900 house with different insulation and window exposure is exactly the mismatch a real Manual J calculation is built to catch.

A square-footage chart will get you into the right neighborhood of tonnage — somewhere between 1.5 and 5 tons for most homes — and not much closer than that. Insulation, window exposure, ceiling height, and duct condition move the real number more than square footage alone, in both directions. The only way to know your actual size, rather than guess at it, is a Manual J load calculation performed on your specific house.