What size AC do I need for 1,500 square feet?

Roughly 2.5 to 3 tons in a moderate climate, using about 20 BTU per square foot. Hot, humid regions may need 3 tons or more; a well-insulated or shaded home may need less. Confirm with a Manual J calculation before buying.

How do I convert tons to BTU?

Multiply tons by 12,000. A 3-ton system is 36,000 BTU per hour, and a 2.5-ton system is 30,000 BTU. Central systems are sold in half-ton steps from 1.5 to 5 tons.

Is it better to oversize or undersize an AC?

Neither. Oversizing causes short-cycling and a humid, clammy home; undersizing means the unit runs constantly and cannot keep up on hot days. Right-sizing with a load calculation gives the best comfort and efficiency.

Can I just match the size of my old system?

Not reliably. The old unit may have been sized wrong, and changes like new windows or added insulation shift the load. A fresh Manual J calculation is the accurate way to size a replacement.

What Size Air Conditioner Do I Need? A Simple Sizing Guide

Updated Jul 30, 2026 • 2 min read

Written by ClearChoiceRadar Editorial

The quick answer

A common rule of thumb is one ton of cooling, which equals 12,000 BTU per hour, for every 400 to 700 square feet, or roughly 20 BTU per square foot. By that math a 1,600 square foot home in a moderate climate often needs about 2.5 to 3 tons, and a 2,400 square foot home about 3.5 to 4 tons.

It is only a starting point. Enter your square footage and climate in our free HVAC sizing calculator for a tighter estimate, then read on for what the rule of thumb misses.

Sizing by climate

Climate is the biggest single factor, because the same house needs more cooling capacity in Phoenix than in Portland:

Hot and humid (South, Southwest): about 25 to 35 BTU per square foot, roughly one ton per 400 to 500 square feet.
Mixed (most of the country): about 18 to 30 BTU per square foot, roughly one ton per 500 to 600 square feet.
Cool or mild (Northwest, upper Midwest): about 16 to 22 BTU per square foot, roughly one ton per 600 to 700 square feet.

Why an oversized AC is a problem

The instinct to size up to be safe backfires. An oversized air conditioner cools the air fast but shuts off before it can pull humidity out, leaving the house cold and clammy. The constant on-off short-cycling also wears the compressor and wastes energy. An undersized unit has the opposite problem: it runs nonstop and never quite catches up on the hottest days. The right size, not the biggest, is the goal.

What the rule of thumb misses

Square footage alone ignores things that change the load by a full ton or more:

Insulation and air sealing: a tight, well-insulated home needs less.
Windows and orientation: large west-facing glass adds serious afternoon heat gain.
Ceiling height: vaulted ceilings mean more air to condition than the floor area suggests.
Ductwork: leaky or undersized ducts undercut even a correctly sized unit.

When only a Manual J will do

A Manual J load calculation is the industry-standard method that accounts for all of the above, plus local climate data, to size the system precisely. A quality contractor performs one before recommending equipment. If a bidder sizes your system on the spot from square footage alone, or simply matches your old unit, which may itself have been wrong, ask why and get a second opinion.

Use our sizing calculator for a ballpark and the cost calculator for a price range, then let the contractor's load calculation settle the final size.