What size AC for a 1,800 sq ft house?

Sizing by climate

Equipment comes in half-ton steps, so round to the nearest available size — and size on a load calculation rather than on floor area alone before you buy.

Get closer than a rule of thumb

Floor area ignores everything that actually decides the load: ceiling height, glazing, orientation, insulation and how many people are in the house. Work through it room by room with the load calculator — that also tells you how the load splits between rooms, which is what duct sizing and zoning depend on. For a permit or a final equipment order, have a full ACCA Manual J done.

FAQ

How many BTU for 1,800 sq ft?

Roughly 36,000 to 54,000 BTU/h for a whole house of that size — 3 to 4.5 tons. The spread is climate and construction: 20 BTU per square foot in a mild climate with good insulation, 30 in a hot climate or an older, leakier house.

What tonnage AC does a 1,800 sq ft house need?

Commonly 3–3.8 tons. Contractors also quote it the other way round as square feet per ton: 500–600 sq ft per ton in mild climates, closer to 400 where summers are severe.

Should it be one system or several?

Above roughly 5 tons, residential equipment usually gives way to two systems or a zoned setup — and even below that, a two-storey house often works better zoned than on a single thermostat. Room-by-room load matters here as much as the total.

Why not just use a room formula?

Because per-room methods assume every wall faces outdoors and every room peaks at once. Across a whole house, interior rooms carry almost no envelope load and the rooms do not peak simultaneously, so a room formula scaled up overstates the load badly. For anything you will buy equipment from, get a Manual J.